Chandra Frontiers in Time-Domain Science

Presentation Abstracts

    Wednesday, October 7th
    Observational Progress in Identifying and Characterizing Tidal Disruption Events Brad Cenko (NASA/GSFC)

    I present an overview of observational efforts across the electromagnetic spectrum to identify and study tidal disruption events (TDEs), when a star wanders too close to a super-massive black hole and is torn apart by tidal forces. I describe insights from growing samples of these sources, as well as a number of open puzzles: the origin of the luminous UV/optical emission, the ubiquity of outflows ranging from speeds of 100 km/s to near the speed of light, and the unique host galaxy population. Finally I discuss prospects in the near-future for addressing some of these issues with discoveries from eROSITA and the Vera Rubin Observatory.

    Invited Talk in Tidal Disruption Events and Transients I on Wednesday, Oct 7th 11:00–12:30 EDT
    Formation of a temporary accretion disk on GX 301-2 during a pulsar spin-up episode Hannah McCall (Washington University in St. Louis) Co-author(s): Manel Errando (Washington University in St. Louis), Logan Press (Washington University in St. Louis)

    The accretion-powered high-mass X-ray binary GX 301-2 is composed of a pulsar accreting from a B1-type stellar companion, and is one of the brightest X-ray sources in the sky. Sources like GX 301-2 are important to our understanding of the physics underlying accretion and mass-transfer mechanisms. Especially useful are simultaneous measurements of the flux and the change in spin frequency, which can be tested against models of angular momentum transfer under different accretion scenarios. In the case of GX 301-2, NICER and Swift-XRT observations across two orbital periods between MJD 58469 and 58552 during a spin-up episode measured by Fermi GBM allow for testing for the formation of a transient accretion disk. Using this data, we examine the correlation between the spin frequency and flux based on accretion torque models and calculate how much of the observed luminosity results from the mass transfer. We determine that even though the contribution to the luminosity from the temporary accretion disk is negligible compared to the luminosity produced by wind accretion, the presence of an accretion disk is indicated by changes in the pulsed fraction of the emission.

    Lightning Talk in Tidal Disruption Events and Transients I on Wednesday, Oct 7th 11:00–12:30 EDT
    The Heraklion Extragalactic Catalogue (HECATE): a tool for transient astrophysics Konstantinos Kovlakas (University of Crete, IA/FORTH) Co-author(s): Andreas Zezas (University of Crete), Jeff Andrews (Northwestern University), Antara Basu-Zych (University of Meryland Baltimore County), Tassos Fragos (University of Geneva), A. Hornschemeier (NASA-GSFC), Konstantinos Kouroumpatzakis (University of Crete), Bret Lehmer (University of Arkansas), Andrew Ptak (NASA-GSFC)

    We present the HECATE, a catalog of 205K galaxies with D<200Mpc, providing sizes, distances, star-formation rates, stellar masses, metallicities & AGN classifications. We discuss the identification of transient sources, as well as, the prioritization of host galaxy candidates in follow-up searches of gravitational-wave sources & gamma-ray bursts, based on their position & astrophysical properties.

    Lightning Talk in Tidal Disruption Events and Transients I on Wednesday, Oct 7th 11:00–12:30 EDT
    A New Extragalactic Population of Faint, Fast X-ray Transients W. Niel Brandt (Penn State University) Co-author(s): F.E. Bauer (Pont Univ Catolica de Chile), B. Luo (Nanjing Univ), J. Quirola-Vasquez (Pont Univ Catolica de Chile), Y.Q. Xue (USTC), G. Yang (Texas A&M)

    Recently, two faint X-ray transients have been discovered in the Chandra Deep Field-South. Both lasted a few hours and are extragalactic with z = 0.74 (spectroscopic) and z ~ 2.1 (photometric), implying large total energy release. The first has been proposed to be a magnetar-powered X-ray transient resulting from a binary neutron-star merger, while the nature of the second is less clear. These findings demonstrate that a population of similar transients should exist in archival X-ray observations. We have thus recently set systematic rate constraints on such transients based on 19 Ms of Chandra surveys data. Rapid searching of incoming Chandra and XMM-Newton observations, using our methodology, should allow discovery of additional such transients for prompt follow-up. Future large-grasp X-ray missions such as Athena and Einstein Probe are needed to open the faint-fast X-ray transient discovery space fully.

    Contributed Talk in Tidal Disruption Events and Transients I on Wednesday, Oct 7th 11:00–12:30 EDT
    Determining the nature of the Fast Blue Optical Transients Deanne Coppejans (Northwestern University)

    Fast Blue Optical Transients (FBOTs) are a recently identified new class of transients. To date, only two of the few dozen known FBOTs have been observed at X-ray wavelengths. In one FBOT the X-ray emission indicated the presence of a central engine. I will outline what X-ray emission has taught us about FBOTs and why X-ray observations are crucial to determine their nature.

    Contributed Talk in Tidal Disruption Events and Transients I on Wednesday, Oct 7th 11:00–12:30 EDT
    Tidal Disruption of Stars by Supermassive Black Holes Enrico Ramirez-Ruiz (University of California, Santa Cruz)

    The art of modeling the tidal disruption of stars by massive black holes forms the main theme of my talk. Detailed simulations should tell us what happen when stars of different types get tidally disrupted, and what radiation a distant observer might detect as the observational signature of such events.

    Invited Talk in Tidal Disruption Events and Transients II on Wednesday, Oct 7th 14:00–15:30 EDT
    An Energy Inventory of Tidal Disruption Events Brenna Mockler (UC Santa Cruz & DARK, NBI) Co-author(s): Enrico Ramirez-Ruiz (UC Santa Cruz & DARK)

    Tidal disruption events (TDEs) offer a unique opportunity to study a single super-massive black hole (SMBH) under feeding conditions that change over timescales of days or months. Despite the increasing number of observed TDEs, it is unclear whether most of the energy in the initial flare comes from accretion near the gravitational radius or from circularizing debris at larger distances from the SMBH. We use the MOSFiT transient fitting code to calculate the conversion efficiency from mass to radiated energy, and find that, for many events, it is similar to efficiencies inferred for active galactic nuclei. However, for some events, it is also similar to stream collision efficiencies. The systematic uncertainties in the measured efficiency must be reduced before we can definitively resolve the emission mechanism of individual TDEs with this method. While the early light curve is generally dominated by optical and UV emission from a large photosphere, observations of late time emission at x-ray frequencies suggest the possibility of directly observing the accretion disk, motivating the need for late-time Chandra observations to further constrain the emission mechanism of TDEs.

    Lightning Talk in Tidal Disruption Events and Transients II on Wednesday, Oct 7th 14:00–15:30 EDT
    Rapid state transitions in a supermassive black hole's accretion flow Dheeraj R Pasham (MIT) Co-author(s): Thomas Wevers (University of Cambridge), Sjoert van Velzen (University of Maryland College Park), James Miller-Jones (Curtin University), P. Uttley (University of Amsterdam), Keith Gendreau (NASA/GSFC), Ron Remillard (MIT), Zaven Arzoumanian (NASA/GSFC), Mike Loewenstein (NASA/GSFC), Ani Chiti (MIT)

    The structures through which long-term persistent accretion occurs in supermassive black holes evolve secularly over hundreds to thousands of years. Different accretion geometries lead to changes in emission properties, observed as different accretion states. Stellar disruptions by supermassive black holes experience a wide range of accretion rates, which could provide opportunities to study the formation/evolution of accretion structures over timescales of just months to years. However, such structural changes have not been observed hitherto. I will describe our recent discovery where we identify three distinct accretion states following a tidal disruption event by a supermassive black hole, with properties strikingly similar to stellar-mass black holes as they evolve through their outbursts. Our findings demonstrate the scale invariance of accretion processes over seven orders of magnitude in black hole mass. This result demonstrates that tidal disruption events can be used to study accretion states in individual supermassive black holes, removing limitations inherent to commonly used ensemble studies.

    Lightning Talk in Tidal Disruption Events and Transients II on Wednesday, Oct 7th 14:00–15:30 EDT
    Mysterious Outburst of a Be/X-ray Binary in the LMC Helena Treiber (Amherst College, Yale University) Co-author(s): G. Vasilopoulos (Yale University), C. Bailyn (Yale University), G.K. Jaisawal (DTU Space), P.S. Ray (NRL), F. Haberl (MPE), K. Gendreau (NASA/GSFC), A. Udalski (University of Warsaw), C. Maitra (MPE)

    Be/X-ray binaries can go into periodic outburst at periastron, when mass is transferred from an O or B star’s disk of ejected material onto a compact object. Characterizing these outbursts informs our understanding of magnetospheric accretion onto objects with extreme magnetic fields, as well as the resulting feedback between the high-mass star and the orbiting compact object, which is most commonly a pulsar. In June 2020, eROSITA detected an outburst from LXP 69.5, a BeXRB system in the Large Magellanic Cloud. We triggered ToO observations from NICER and Swift. We found a spin period of 68.68 ± .01 seconds. The resulting spectral and temporal X-ray properties of the system call into question the correlation between pulse profile morphology and luminosity. The OGLE V and I band light curves, which stopped in March 2020, exhibit a changing flare periodicity, best modeled by three epochs with period values of 149, 171, and 200 days, respectively. Archival Swift data and our ToO observations suggest that the optical and X-ray flares of the system do not coincide. Thus, the behavior of this system challenges established understanding of accretion in BeXRBs.

    Lightning Talk in Tidal Disruption Events and Transients II on Wednesday, Oct 7th 14:00–15:30 EDT
    Long-term Monitoring of Two X-ray Tidal Disruption Events Dacheng Lin (University of New Hampshire)

    Here I present our long-term multiwavelength follow-up campaign of two very special X-ray tidal disruption events. One is a decade-long tidal disruption event that showed the spectral state transition from super-Eddington accretion state of quasisoft X-ray spectra to the thermal state of supersoft X-ray spectra. Recently we also observed the cooling of the thermal state spectra. The other event is associated with an off-center intermediate-mass black hole of few 10^4 solar mass. Recently we confirmed that it resides in a massive star cluster through the HST imaging and that the X-ray spectra cool as expected for a black hole of such a mass.

    Contributed Talk in Tidal Disruption Events and Transients II on Wednesday, Oct 7th 14:00–15:30 EDT
    Ongoing Monitoring of the Tidal Disruption Event Swift J1644+57 Yvette Cendes (Center for Astrophysics | Harvard & Smithsonian) Co-author(s): Tarraneh Eftekhari (Center for Astrophysics | Harvard & Smithsonian), Edo Berger (Center for Astrophysics | Harvard & Smithsonian)

    Swift J164449.3+573451 (Sw J1644+57) is a tidal disruption event (TDE) where a star became unbound after getting too close to a supermassive black hole and got torn apart by tidal forces. Sw J1644+57 was the first TDE discovered in 2011, and to date is the only TDE where the launch and subsequent turnoff of a relativistic jet has been observed in detail. In this talk, I will give an overview of almost a decade of Sw 1644+57 observations, from its initial discovery by Swift to its transition to a sub-relativistic phase. I will also provide an update of the TDE from recent Chandra and Very Large Array observations in the X-ray and radio as the shockwave continues to expand and interact with the black hole's circumnuclear environment. Finally, I will explain how Sw J1644+57 fits into the broader picture of TDE studies, and how it will continue to provide a benchmark for these transient phenomena for years to come.

    Contributed Talk in Tidal Disruption Events and Transients II on Wednesday, Oct 7th 14:00–15:30 EDT
    Thursday, October 8th
    Chandra synergies with ALMA Maria Diaz Trigo (ESO)

    The Atacama Large Millimeter/submillimeter Array (ALMA), located at an altitude of 5000 m in the Atacama desert, is the first of the large facilities of the 2020s landscape to be operational. Inaugurated in 2013, it provides unprecedented performance capabilities in the millimeter/submillimeter range for exploration of the Universe. Since the start of its early operations in 2011, it has revolutionised our view on the process of planet formation or how the first stars and galaxies in the Universe were born. In this talk I will present some of the scientific topics that can only be investigated by exploiting the synergies between Chandra and ALMA. I will also discuss some millimeter/X-ray synergy topics that will need of future X-ray facilities such as Athena to match the ALMA capabilities.

    Invited Panelist in Synergies Panel & Discussion on Thursday, Oct 8th 09:00–10:30 EDT
    Synergies of Chandra, SRG/eROSITA, and Athena in X-ray Time Domain Studies Arne Rau (Max Planck Institute for extraterrestrial Physics, Garching, Germany)

    In this presentation I will address possible synergies between Chandra and eROSITA with an outlook to Athena. The eROSITA instrument on-board the Russian/German SRG mission is currently performing its second of eight consecutive X-ray imaging all-sky surveys. Over the course of four years, eROSITA will probe transient and variable X-ray phenomena on timescales of seconds to years uncovering a rich sample of extra-galactic (e.g., Tidal Disruption Events, Gamma-ray Burst afterglows, AGN releated phenomena) and Galactic (e.g., X-ray binaries, novae, cataclysmic variables, stellar phenomena) transients and variables. Athena will be next large X-ray observatory in the ESA large mission program to be launched in the early 2030s. Equipped with a large effective area optics, thanks to the novel Silicon Pore Optics technology, and two technology development pushing instruments, a very high-resolution micro-calorimeter (X-IFU) and a wide-field camera (WFI).

    Invited Panelist in Synergies Panel & Discussion on Thursday, Oct 8th 09:00–10:30 EDT
    Catching Compact AGN Jets at Cosmic Noon in Radio Time Domain Surveys Kristina Nyland (NRC postdoctoral fellow, resident at NRL)

    Active galactic nuclei (AGN) rank among the most powerful sources in the universe and are capable of dramatically altering their environments through energetic feedback. In the prevailing view, feedback associated with large-scale jets with extents of 10’s or 100’s of kpc is the primary means by which AGN regulate galaxy evolution. However, the role of feedback by compact, sub-galactic jets on the interstellar medium, particularly at “cosmic noon” (z = 1-3), remains poorly understood. New multi-epoch radio surveys offer a promising means of identifying compact jets based on radio variability. I present a sample of currently radio-loud AGN identified in the on-going Very Large Array Sky Survey (VLASS) that were radio-quiet just 1-2 decades ago based on previous radio surveys. These newly radio-loud AGN may be associated with young jets, thus providing new insights into jet triggering and the link between feedback driven by compact jets and galaxy evolution. I discuss the unique and important role of Chandra observations in conjunction with new multi-epoch radio surveys for measuring the accretion states and host galaxy environments of powerful AGN with young jets.

    Invited Panelist in Synergies Panel & Discussion on Thursday, Oct 8th 09:00–10:30 EDT
    Tailoring X-ray follow-up observations of high-energy gamma-ray targets Elizabeth Ferrara (University of Maryland College Park, NASA GSFC, CRESST)

    In working to determine the astrophysical source of high-energy gamma rays, a prime impediment is the lack of sub-arcminute localizations. Multi-wavelength observations, primarily in the X-ray and radio regimes, are important to unlocking the secrets of the gamma-ray sky. For X-ray follow-up, a key decision is the choice of which observatory should be used, and must take into account the likely nature of the source. I will describe a few of the more common follow-up methods for Fermi-LAT sources, and discuss how Chandra and other current facilities fit best into this process.

    Invited Panelist in Synergies Panel & Discussion on Thursday, Oct 8th 09:00–10:30 EDT
    Friday, October 9th
    Dust scattering echoes through cosmic time Lia Corrales (University of Michigan) Co-author(s): Sebastian Heinz (University of Wisconsin-Madison), Brianna Mills (University of Virginia), Frits Paerels (Columbia University)

    Dust particles scatter X-ray light, producing a diffuse halo image around X-ray point sources. In our own Galaxy, spectacular dust scattering echoes from X-ray variable sources can be used to map the 3D spatial distribution of interstellar dust. I will review the legacy of dust scattering echoes imaged by Chandra, and discuss how future X-ray telescopes will soon become dust echo imaging factories. Finally, I will show how dust scattering halos around X-ray bright quasars could be used to probe the abundance and grain sizes of dust inhabiting the intergalactic medium. Future telescope missions with Chandra-like resolution – such as Lynx – provide an opportunity to image extragalactic scattering echoes, directly probing the abundance of dust in galaxy halos and absorption systems.

    Invited Talk in Synergies I (Microlensing, Dust, Multiwavelength) on Friday, Oct 9th 09:00–10:30 EDT
    Simultaneous optical and X-ray variability in Young Stellar Objects from the CSI2264 project Mario Giuseppe Guarcello (INAF - Osservatorio Astronomico di Palermo) Co-author(s): E. Flaccomio (INAF - Osservatorio Astronomico di Palermo), G. Micela (INAF - Osservatorio Astronomico di Palermo), C. Argiroffi (INAF - Osservatorio Astronomico di Palermo/Università degli Studi di Palermo), S. Sciortino (INAF - Osservatorio Astronomico di Palermo), L. Venuti (NASA Ames Research Center), J. Stauffer (Spitzer Science Center), L. Rebull (Spitzer Science Center), A. M. Cody (Bay Area Environmental Research Institute)

    Pre main-sequence stars are variable sources. In stars with protoplanetary disks, this variability is mainly due to disk-related phenomena. Accreting gas heats the stellar atmosphere and create hot spots whose energetic emission is modulated by stellar rotation. Accretion itself is a non-stationary process. Disked stars can also experience variable extinction due to the material in the inner disk and accretion funnels. The main source of variability in stars without disks is due to the magnetic activity, which is orders of magnitude more intense than in main sequence stars. Flares, spots, faculae, and coronal active regions are intrinsically variable, and their emission is modulated by stellar rotation. These phenomena can be studied with simultaneous multi-band observations, which are rare and technically challenging. I will present the results of the CSI2264 project, based on simultaneous multi-wavelength observations of young stars (in this case, the members of the cluster NGC2264). In particular, I will show how simultaneous optical (CoRoT) and X-rays (Chandra) observations deeply probed the phenomena associated with the inner disks, accretion, and magnetic activity.

    Contributed Talk in Synergies I (Microlensing, Dust, Multiwavelength) on Friday, Oct 9th 09:00–10:30 EDT
    MICROLENSING A QUASAR'S ACCRETION DISK George Chartas (College of Charleston) Co-author(s): Henric Krawczynski (Washington University in St. Louis), Matthew Emery (College of Charleston), Andrew West (Washington University in St. Louis)

    We present X-ray and optical monitoring observations and simulations showing how gravitational microlensing is used to infer the structure near the event horizons of supermassive black holes, constrain the spin of the black holes and test general relativity in the strong-gravity regime.

    During microlensing events, magnification caustics cross the accretion disk revealing the gradual change in the energy of Mg, Si, S and Fe fluorescence lines arising form the accretion disk. The change in the energy of these lines is the result of special and general relativistic effects. The distribution of the energy shifts of the lines provide constraints on the innermost stable circular orbit and spin of the black hole. The change in the relative strengths of the disk lines as the caustic moves across the disk provides insight into the ionization structure of the disk.

    We also show how these microlensing observations can be expanded to a statistically large sample of z = 0.5-5 lensed quasars with the predicted discovery by Vera C. Rubin Observatory of > 10,000 additional gravitationally lensed systems and with a next generation X-ray telescope.

    Invited Talk in Synergies I (Microlensing, Dust, Multiwavelength) on Friday, Oct 9th 09:00–10:30 EDT
    New Techniques in X-Ray Dust Tomorgraphy Sebastian Heinz (University of Wisconsin-Madison) Co-author(s): Lia Corrales (University of Michigan-Ann Arbor), Randall Smith (Smithsonian Astrophysical Observatory), Morgan Adams (Florida State University)

    I will discuss a new 2D deconvolution algorithm to analyze x-ray dust echoes. The technique allows joint analysis of multiple temporally distinct observations of dust scattering echoes and will enable more flexible future observing strategies.

    Lightning Talk in Synergies I (Microlensing, Dust, Multiwavelength) on Friday, Oct 9th 09:00–10:30 EDT
    Capturing the Galactic Center through Chandra’s Unique Lens Daryl Haggard (McGill University, McGill Space Institute)

    Multi-wavelength, time domain observations of the Galactic Center have opened a completely new view of the dynamic environment around our closest supermassive black hole. I will discuss Sgr A*’s unique variability alongside other time domain phenomena in the Galactic Center, traced out over more than 20 years of observations from Chandra and coordinated multi-wavelength campaigns. I will also briefly explore how we can continue to push this frontier with existing and next-generation observatories.

    Invited Talk in Synergies II (Galactic Center, Multiwavelength) on Friday, Oct 9th 11:00–12:30 EDT
    Luminous X-ray Emission From a Calcium-rich Transient Wynn Jacobson-Galan (Northwestern University) Co-author(s): Raffaella Margutti (Northwestern University), Charlie Kilpatrick (UC Santa Cruz), Daichi Hiramatsu (UC Santa Barbara)

    We present multi-wavelength observations of the first calcium-rich transient, SN 2019ehk, with a luminous X-ray detection. Our panchromatic observations of supernova (SN) 2019ehk begin 10 hours after explosion and continue for ~400 days. Short-lived X-ray emission observed by Swift-XRT is coincident with both an optical "flare" at ~3 days after explosion and "flash-ionized" Hydrogen and Helium emission lines in the SN spectrum. Combining these observations provides, for the first time, direct evidence of a dense, compact shell of circumstellar material surrounding a calcium-rich SN progenitor. We will discuss the implications of these observations with respect to the phase space of X-ray transients and explore how X-ray production in calcium-rich transients can be applied as a novel probe of progenitor mass-loss history.

    Contributed Talk in Synergies II (Galactic Center, Multiwavelength) on Friday, Oct 9th 11:00–12:30 EDT
    Periodic X-ray Sources in the Galactic Center/Bulge -- Application of the Gregory-Loredo Algorithm Tong Bao (Nanjing University) Co-author(s): Zhiyuan Li (Nanjing University)

    The Gregory-Loredo period searching algorithm, which employs Bayes’s theorem to the phase-folded light curve and is well-suited for irregularly sampled X-ray data, is applied to deep Chandra observations of the inner Galactic bulge (the Limiting Window field) and the Nuclear Star Cluster (NSC). This leads to dozens of newly detected periodic sources, most of which are classified as magnetic CVs including polars and IPs. Under reasonable assumptions about the geometry of CVs and a large set of simulated X-ray light curves, we estimate the fraction of magnetic CVs in the inner bulge to be <~23%, which is similar to that in the solar neighborhood. There is an apparent lack of long-period (>3.3 hours) CVs contrasted with the field CVs, which may be understood as an age effect.

    Contributed Talk in Synergies II (Galactic Center, Multiwavelength) on Friday, Oct 9th 11:00–12:30 EDT
    Extragalactic Fast X-ray Transient Candidates Hidden in the Chandra Source Catalog Jonathan Quirola-Vásquez (Pontificia Universidad Católica de Chile) Co-author(s): Franz Bauer (Pontificia Universidad Católica de Chile), W.N. Brandt (Penn State University)

    We report the detection of 8 candidate extragalactic fast X-ray transients (FXRTs) from a parent sample of 214,701 sources in the Chandra Source Catalog Release 2.0 (160.96 Ms over 592.4 deg2). We characterize their X-ray light curves and spectra. Two candidates have visible counterparts in archival imaging, allowing us to assess weak photometric redshift probability distributions (z~0.3-5.2) and host properties. Four FXRTs show a plateau in their light curves and a softening trend in their hardness ratio (HR), implying a possible relation with CDF-S XT2 and GRBs. We compute the local event rates and investigate a possible relation with a central engine scenario, driven by a proto-magnetar emission.

    Contributed Talk in Synergies II (Galactic Center, Multiwavelength) on Friday, Oct 9th 11:00–12:30 EDT
    Wednesday, October 14th
    Predicting the Past to Understand the Future: The Context of Time in Considering Habitability Rachel Osten (Space Telescope Science Institute, Johns Hopkins University)

    Flares are a fact of life for stars on or near the main sequence, and they are the most energetic releases that occur over pretty much all of the star's main sequence lifetime. The launch of the Chandra X-ray Observatory occurred only a few years after the discovery of the first extrasolar planets, and there is a link between important stellar astrophysics questions and the associated implications for habitability. For instance, the factors that control the coronal emission of stars and their brightenings are important not just for understanding energy balance in the corona, but also for understanding planetary atmosphere irradiation. In this context, it is important to understand not just the details of how flares work but also how these extrasolar space weather or habitability impacts might change with time. In this talk I'll detail what we've learned with Chandra about the types of flaring stars, the details of flares, and how stars can impact their environment, both for forming planets and for ones already formed. I'll then turn to what the future might hold as far as Chandra's continued contribution to understanding flares and planetary habitability.

    Invited Talk in Stars, Disks, and Planets I on Wednesday, Oct 14th 09:00–10:30 EDT
    Variable X-ray Sources in Intermediate-Age Open Clusters Steven Chen (George Washington University) Co-author(s): Oleg Kargaltsev (George Washington University), Hui Yang (George Washington University), Igor Volkov (George Washington University), Jeremy Hare (NASA GSFC), Blagoy Rangelov (Texas State University)

    We investigate the variability properties of X-ray sources located within several intermediate age (20-300 Myrs) clusters using different variability metrics. We use multi-wavelength properties from various catalogs to classify these variable Chandra X-ray sources and explore the variability for different classes of sources. We further explore trends between spectral and variability properties.

    Lightning Talk in Stars, Disks, and Planets I on Wednesday, Oct 14th 09:00–10:30 EDT
    Cycles and the Seven Dwarfs Thomas Ayres (University of Colorado (CASA))

    Chandra has been charting the magnetic heartbeats of late-type stars via high-contrast coronal X-rays. Goal is to characterize the stellar Dynamo, whose internal workings remain elusive. The Sun's high-energy modulations play an important "Space Weather" role in our heliosphere, as do stellar counterparts for their exoplanets. The nearby pair Alp Cen A (G2 V) and B (K1 V) has been followed by Chandra semiannually since 2005; adding to 1990’s ROSAT/HRI and, since 2003, XMM-Newton. AB show clear coronal cycles, 19 yr and 8 yr, respectively; bracketing the solar 11-year cycle by significant amounts, despite the only small differences in stellar masses. Several years ago, nearby bright Procyon (F5 IV-V) was added to the program. Despite Procyon’s high coronal intensity, the slightly evolved F star has displayed a very flat X-ray light curve. More recently, two new promising visual binaries, Xi Boo (G8 V + K4 V) and 70 Oph (K0 V + K5 V), were added to "Cycles." A key question involves the origin of diverging branches in a diagram pitting rotational period versus cycle duration, where the Sun's iconic 11-yr example sits in the middle, possibly in a transitional state.

    Lightning Talk in Stars, Disks, and Planets I on Wednesday, Oct 14th 09:00–10:30 EDT
    X-ray Flares from T-Tauri Stars Drive Dynamic Chemical Evolution in Protoplanetary Disks Abygail Waggoner (University of Virginia) Co-author(s): L. Ilsedore Cleeves (University of Virginia)

    Observations have shown that young stars are highly X-ray variable. What does that mean for the properties and chemistry of their circumstellar planet forming disks? We present a theoretical study connecting disk chemical evolution to stellar X-ray flaring events. We find that X-ray flares drive chemistry on observationally relevant time scales and have a long-term (decades) cumulative impact.

    Lightning Talk in Stars, Disks, and Planets I on Wednesday, Oct 14th 09:00–10:30 EDT
    Illuminating the Chemistry of Planet Formation with X-ray Flares Ilse Cleeves (University of Virginia) Co-author(s): Abygail Waggoner (University of Virginia), Chunhua Qi (SAO), David Wilner (SAO), Karin Öberg (Harvard University), Bradford Snios (CfA), Ed McClain (Chandra X-ray Observatory), Edwin Bergin (University of Michigan), Ryan Loomis (NRAO)

    The chemistry of protoplanetary disks sets the initial composition of newly formed planets and may also regulate the efficiency by which planets form. Disk chemical abundances typically evolve over timescales spanning thousands if not millions of years. Consequently, it was a surprise when ALMA observations taken over the course of a single year showed significantly variable molecular emission in H13CO+ relative to the otherwise constant thermal dust emission in the IM Lup protoplanetary disk. HCO+ is a known X-ray sensitive molecule, and one possible explanation is that stellar activity is perturbing the chemical "steady state" of the disk. If confirmed, simultaneous observations may provide a new tool to measure (and potentially map) fundamental disk parameters, such as electron density, as the light from X-ray flares propagates across the disk.

    Invited Talk in Stars, Disks, and Planets II on Wednesday, Oct 14th 11:00–12:30 EDT
    High-energy processes in Young Stellar Objects: simultaneous X-ray and radio observations Jan Forbrich (University of Hertfordshire)

    While X-ray variability and flaring of Young Stellar Objects (YSOs) has been known for some time, suitable and systematic complementary (nonthermal) centimetric radio observations are now becoming available. I will illustrate the complexity emerging from simultaneous radio and X-ray observations of hundreds of YSOs in the Orion Nebula Cluster on timescales of minutes and outline the next steps.

    Contributed Talk in Stars, Disks, and Planets II on Wednesday, Oct 14th 11:00–12:30 EDT
    X-ray emission from pre-main sequence stars Hans Moritz Günther (MIT)

    Pre-main sequence stars (PMS) are young stars with bright and frequent magnetic activity. In some cases, the flare loops are so long that they can connect the star and its surrounding protoplanetary disk. PMS also show X-ray variability that is unique to this evolutionary stage: An important component of their soft X-ray emission originates in an accretion shock which might change over a rotation period or when blobs of material fall into the star. As the accretion stream or disk warps rotate into and out of the line-of-sight that emission is more or less absorbed and spectral changes allow us to probe the dust content of the absorber. As this dust, pebbles, or even proto-planets fall into the star, we can watch the elemental composition the hot plasma change in the X-ray spectrum over a few years. Chandra was instrumental in many of these discoveries due to the high spatial resolution that allows us to separate close sources in crowded star forming regions. I close this review with a list of open questions on variability from PMS that future X-ray observatories can address.

    Invited Talk in Stars, Disks, and Planets II on Wednesday, Oct 14th 11:00–12:30 EDT
    Thursday, October 15th
    Time Domain studies of Solar System X-ray emission Dennis Bodewits (Auburn University)

    Many objects in our solar system emit X-rays through a variety of physical processes. Their emission is often highly variable in time as they respond to local space weather conditions, on solar flares, or on inherent properties of the observed object. Solar system observations are often TOO or rapid follow up observations as some of the most interesting events are unpredictable.

    Invited Talk in Solar System on Thursday, Oct 15th 09:00–10:30 EDT
    Analysis of charge exchange emission from Comet 46P/Wirtanen Emanuele Bonamente (Auburn University)

    We present the results from contemporary Swift and Chandra observation of Jupiter family Comet 46P/Wirtanen during its 2018 apparition. Water production rate and charge exchange emission were measured during three different epochs over 1.5 months allowing for a comparison with the variability of the solar wind.

    Contributed Talk in Solar System on Thursday, Oct 15th 09:00–10:30 EDT
    Revealing the Processes that Produce Planetary X-ray Pulsations William Dunn (Mullard Space Science Laboratory, UCL) Co-author(s): W. R. Dunn* (University College London), Z. H. Yao* (Chinese Academy of Sciences), E. E. Woodfield (British Antarctic Survey), A. H. Sulaiman (University of Iowa), W. S. Kurth (University of Iowa), D. Grodent (Université de Liège), S. Elliott (University of Iowa), G. Hospodarsky (University of Iowa), M. Imai (University of Iowa & Niihama College), D. Weigt (University of Southampton), S. Kotsiaros (NASA Goddard Space Flight Center), A. Wibisono (University College London), G. Branduardi-Raymont (University College London), G. Clark (Johns Hopkins University), B. Bonfond (Université de Liège), K. Haewsantati (Université de Liège), I. J. Rae (Northumbria University), H. Manners (Imperial College London), G. R. Gladstone (Southwest Research Institute), P. Rodriguez (European Space Astronomy Centre), J-U. Ness (European Space Astronomy Centre), E. McClain (Smithsonian Astrophysical Observatory), B. Snios (Smithsonian Astrophysical Observatory), C. M. Jackman (University of Southampton), R. Kraft (Smithsonian Astrophysical Observatory), B. H. Mauk (Johns Hopkins University), R. W. Ebert (Southwest Research Institute), D. X. Pan (Chinese Academy of Sciences), B. B. Ni (Wuhan University), R. L. Guo (Université de Liège), F. Allegrini (Southwest Research Institute), R. T. Desai (Imperial College London), S. J. Bolton (Southwest Research Institute)

    Since the first Chandra observations of Jupiter in 2000, the planet has been observed to produce mysterious quasi-periodic X-ray flares on timescales of 10s of minutes. The unprecedented X-ray campaigns that are accompanying the Jupiter-orbiting Juno spacecraft have enabled us to decipher the entire chain of physical processes that produce these clockwork-like pulsations. These campaigns reveal that Jupiter’s X-ray aurora pulses in time with periodic magnetic field fluctuations called compressional mode waves. These waves interact with the plasma population through cyclotron resonances causing periodic flows of keV-MeV particles towards the planet. When these particles collide with Jupiter’s atmosphere they cause X-ray bursts. Further observations reveal that this is not unique to the X-rays: the X-rays pulse in time with radio and UV flares, which are all synchronised through compressional mode waves. We close by showing videos of Jupiter’s aurora demonstrating that the term X-ray ‘hot spot’ (Gladstone+2002; Dunn+2017) is misleading for Jupiter’s since it is not produced by a single coherent process but several distinct processes, of which the pulsations are only one component. * W.R.D and ZHY contributed equally to this work

    Invited Talk in Solar System on Thursday, Oct 15th 09:00–10:30 EDT
    SmartNET & Chandra: Community self organization for simultaneous observing Gregory Sivakoff (University of Alberta) Co-author(s): Enrico Bozzo (University of Geneva), Poshak Gandhi (University of Southampton), Matt Middleton (University of Southampton), and the SmartNET community (worldwide)

    Time domain astronomy can require that observers use multiple facilities simultaneously. I will discuss the Simultaneous Multiwavelength Astronomy Research in Transients NETwork (SmartNET), an open networking tool designed to help astronomers organize such observations. I will present some of the recent SmartNET campaigns and focus on how SmartNET and Chandra can combine to tackle new frontiers.

    Invited Panelist in Communications & Coordination (Panel & Discussion) on Thursday, Oct 15th 11:00–12:30 EDT
    Coordinating observations between ground- and space-based facilities Erik Kuulkers (ESA/ESTEC, The Netherlands) Co-author(s): Matthias Ehle (ESA/ESAC), Aitor Ibarra (Spain)

    The emergence of time-domain multi-messenger (astro)physics asks for new and more efficient ways of interchanging information, as well as collaboration. Many space- and ground-based observatories have web pages dedicated to showing information about the complete observations and planned observation schedule. The aim is to standardise the exchange of information about observational schedules and set-ups between facilities and in addition, to standardise the automation of visibility checking for multiple facilities. To reach this, we propose to use the VO protocols (ObsTAP-like) to write services to expose these data to potential client applications and to develop cross facilities visibility servers.

    Relevant link:

    Invited Panelist in Communications & Coordination (Panel & Discussion) on Thursday, Oct 15th 11:00–12:30 EDT
    Communication and Co-ordination: A GROWTH and ZTF perspective Mansi M. Kasliwal (Caltech) Co-author(s): On behalf of the GROWTH and ZTF collaborations (Numerous)

    I plan to start with some examples of scientific rewards of effective communication in time-domain astronomy. Next, I plan to highlight tools that facilitate effective co-ordination. I will conclude with some challenges in this context before opening the floor for discussion.

    Invited Panelist in Communications & Coordination (Panel & Discussion) on Thursday, Oct 15th 11:00–12:30 EDT
    Planning Coordinated Observations with Chandra Joshua Wing (Chandra X-ray Center) Co-author(s): Jan Vrtilek (Chandra X-ray Center), Scott Randall (Chandra X-ray Center), Daniel Castro (Chandra X-ray Center), Ed McClain (Chandra X-ray Center), Tara Dowd (Chandra X-ray Center), Julia Zachary (Chandra X-ray Center)

    Multi-wavelength observations offer astronomers a great deal of information that can help to interpret the astrophysics of a source. For time-variable sources, coordinating these multi-wavelength observations in time can be crucial. Over the first 21 years of the mission, Chandra has offered proposers the opportunity to coordinate their Chandra observations with observations at other observatories. We continue to do so as the mission enters its third decade of service and have opened this opportunity up to coordinations with any observatory, space- or ground-based. We discuss how Chandra has accomplished this in the past and how we will do so moving forward.

    Invited Panelist in Communications & Coordination (Panel & Discussion) on Thursday, Oct 15th 11:00–12:30 EDT
    Friday, October 16th
    X-ray observations of supernovae and supernova remnants Manami Sasaki (Friedrich-Alexander-University Erlangen-Nuernberg)

    This talk gives a short summary on X-ray observations of supernovae and young supernova remnants (SNRs) carried out in the last decades, with particular focus on the results obtained with Chandra. X-ray emission of supernovae helps to constrain the supernova type, explosion mechanism, and the properties of the circumstellar medium. X-ray studies of SNRs allow us to understand the distribution of the ejecta, expansion of the shocks, and the interaction of a SNR with its environment. I will report on the recent results on SN 1987A, Kepler's SNR, Tycho's SNR, and others, which have provided us with new interesting facts about the evolution of supernovae and SNRs.

    Invited Talk in Massive and Evolved Stars I on Friday, Oct 16th 09:00–10:30 EDT
    The Metamorphosis of SN 2014C: A Strongly Interacting Supernova Daniel Brethauer (CIERA, Northwestern University) Co-author(s): Raffaella Margutti (CIERA)

    We present an analysis of five years of coordinated CXO and NuSTAR observations showing a strongly interacting and peculiar supernova, SN 2014C. This is the first broad-band X-ray monitoring of an extragalactic SN over six years of evolution in both the hard and soft X-rays. Our analysis of the bright thermal bremsstrahlung radiation reveals that SN 2014C, initially a Type Ib SN, metamorphosed into a Type IIn as a result of interaction with a hydrogen-rich circumstellar medium (CSM) consisting of 2-3 M_☉, located 5.5 x 10^{16} cm from the explosion site. This H-rich CSM shell of material has a density profile that goes as between R^{-2.5} and R^{-4.3} , which clearly deviates from wind-like density profiles (R^{-2}) that are expected around massive stars. These findings require updates to our understanding of mass loss in massive stars that are approaching core-collapse.

    Lightning Talk in Massive and Evolved Stars I on Friday, Oct 16th 09:00–10:30 EDT
    The exceptional X-ray evolution of SN 1996cr in high resolution Jonathan Quirola-Vásquez (Pontificia Universidad Católica de Chile) Co-author(s): Franz Bauer (Pontificia Universidad Católica de Chile), Vikram Dwarkadas (The University of Chicago)

    We present X-ray spectra spanning 18 years of evolution for SN1996cr, one of the five nearest (~4 Mpc) SNe detected in the modern era. Chandra-HETG observations allow us to resolve spectrally the velocity profiles of Ne, Mg, Si, S, and Fe emission lines and monitor their evolution as tracers of the ejecta-circumstellar medium (CSM) interaction. To explain the diversity of X-ray line profiles, we explore several possible geometrical models. Based on the highest signal-to-noise 2009 epoch, we find that a polar geometry with two distinct opening angle configurations and internal obscuration can successfully reproduce all of the observed line profiles. We extend this model to seven further epochs with lower S/N ratio and/or lower spectral-resolution between 2000-2018, yielding several interesting evolutionary trends.

    Lightning Talk in Massive and Evolved Stars I on Friday, Oct 16th 09:00–10:30 EDT
    Characterization of a Supergiant Fast X-ray Transient in the LMC by eROSITA Georgios Vasilopoulos (Yale University) Co-author(s): C. Maitra (Max-Planck-institute for extraterrestrial physics (MPE)), F. Haberl (MPE), L. Ducci (Institut für Astronomie & Astrophysik), K. Dennerl (MPE)

    Supergiant Fast X-ray Transients (SFXTs) are a class of supergiant high mass X-ray binary systems, characterised by extreme variability in the X-ray domain. Current models attribute flares to the clumpy nature of the stellar wind coupled with gating mechanisms involving the spin and magnetic field of the neutron star. eROSITA is the primary instrument on-board the Russian-German "Spectrum-Roentgen-Gamma" mission, with a mission is to perform an imaging all-sky survey within two years. The location of the Large Magellanic Cloud makes it an ideal laboratory to detect transient systems with eROSITA and follow them with multi-wavelength campaigns. We performed a detailed temporal and spectral analysis of the eROSITA and XMM-Newton data of XMMU J053108.3−690923. We confirm the putative pulsations previously reported from the source certifying its nature as a neutron star in orbit with a supergiant companion. We identified fast flares in the eROSITA light curves, while the long term flux exhibits variability with a dynamic range of >1000 confirming its nature as an SFXT. Further, an estimate of the clumpiness of the medium and the magnetic field of the neutron star.

    Lightning Talk in Massive and Evolved Stars I on Friday, Oct 16th 09:00–10:30 EDT
    Deciphering the stellar winds in high mass X-ray binaries with Chandra: A time domain science case Victoria Grinberg (Institute for Astronomy and Astrophysics (IAAT), University of Tübingen)

    Supergiant high mass X-ray binaries (HMXBs) offer a unique chance to directly probe the highly structured, clumpy winds of O/B stars. In these systems, a compact object (black hole or neutron star) accretes matter from the stellar wind of the companion. The wind's variability drives changes in the accretion and thus the system’s X-ray emission. The interaction of this emission with the wind material is used to study the wind itself and thus to constrain the mass loss processes in the most massive stars. The systems are highly dynamic, with time scales ranging from day-long orbital cycles to minutes and below when the passage of individual wind clumps is observed.
    Chandra, with its superb spectral resolution, has opened a new window onto the the wind structure in HMXBs. It has, for example, revealed the onion-like structure of the wind clumps in Cygnus X-1, and the complex multi-phase wind accretion flow in Vela X-1, similar to what has been hinted onto in simulations. In this talk, I will discuss these and other Chandra results and argue that Chandra is ideally posed to continue its crucial contribution to our understanding of HMXBs and massive stars.

    Invited Talk in Massive and Evolved Stars I on Friday, Oct 16th 09:00–10:30 EDT
    X-rays as tracers of shocks and burning in novae Jeno Sokoloski (Columbia, LSSTC)

    Over the past decade, the discovery that many, if not most, novae produce GeV gamma-rays has revolutionized our understanding of these common major stellar eruptions. Powerful shocks are now thought to play a key role in numerous aspects of nova eruptions, from helping eject the white dwarf's envelope, to powering the optical emission, to possibly triggering catastrophic cooling and dust formation. And X-rays from shock-heated gas are vital for diagnosing these shocks. In this talk, I will briefly review Chandra's past contributions to nova research as well as some current investigations of gamma-ray producing shocks. Looking forward, I'll argue that working ever more closely with other observatories will be crucial for Chandra to continue to make ground-breaking contributions to the study of novae and other related stellar transients.

    Invited Talk in Massive and Evolved Stars II on Friday, Oct 16th 11:00–12:30 EDT
    The Colliding Winds of WR 25 in High Resolution X-rays Pragati Pradhan (MIT Kavli Institute for Astrophysics and Space Research) Co-author(s): DAVID P. HUENEMOERDER (Massachusetts Institute of Technology), RICHARD IGNACE (East Tennessee State University), ANDREW POLLOCK (University of Sheffield), JOY S. NICHOLS (Harvard-Smithsonian Center for Astrophysics)

    WR 25 is a colliding-wind binary star system comprised of a very massive O2.5If*/WN6 primary and an O4 secondary in a 208-day period eccentric orbit. These hot stars have strong, highly-supersonic winds which interact to form a bright X-ray source from wind collision- shocks whose conditions change with stellar separation. Different views through the WR and O star winds are afforded with orbital phase as the stars move about each, allowing for exploration of wind structure in ways not easy or even possible for single stars. We have analyzed an on-axis Chandra/HETGS spectrum of WR 25 obtained shortly before periastron at maximum light. From the on-axis observations, we constrain the line fluxes, centroids, and widths of various emission lines, including He-triplets of Si XIII and Mg XI. We have also been able to include several serendipitous off-axis HETG spectra from the archive and study their flux variation with phase. This is the first report on high-resolution spectral studies of WR 25 in X-rays.

    Contributed Talk in Massive and Evolved Stars II on Friday, Oct 16th 11:00–12:30 EDT
    WZ Sge: a magnetic propeller white dwarf system? Maria Georganti (University of Southampton) Co-author(s): Prof. Christian Knigge (University of Southampton), Noel Castro-Segura (University of Southampton)

    WZ Sge is one of the best-known dwarf novae due to its spectacular super-outbursts every ~30 years. These eruptions are characterised by a primary burst, followed by a decline phase that includes a sharp “dip” and multiple “echo outbursts”. As these decline features are not yet fully understood, we investigate one recently proposed interpretation that the switches between these states represent transitions into and out of a magnetic propeller state. If this is the case, the distinctive UV spectrum observed in the prototypical magnetic propeller system AE Aqr may provide us with a definitive observational signature of the process in WZ Sge. In this talk, I present time-resolved, high-resolution UV spectroscopic observations taken with STIS/HST just before, during and after the dip in WZ Sge’s 2001 super-outburst. We construct time-averaged and RMS spectra for all states during the decline phase and we test whether the spectroscopic signature of a propeller is present and limited to the faint states during which the propeller is thought to operate. Finally, I discuss the broader implications of our findings for our understanding of the disk instability model and dwarf nova eruptions.

    Contributed Talk in Massive and Evolved Stars II on Friday, Oct 16th 11:00–12:30 EDT
    Timing Analysis with ACIS HETG spectra Joy Nichols (Harvard & Smithsonitan CfA)

    Several approachs to timing analysis with ACIS-S HETG spectra of stellar sources are discussed, including light curves, fourier period determination, and statistical studies. Issues and benefits of HETG data in pursuing timing studies are explored. A specific example of the early O star zeta Pup is presented, with coordinated observations at other wavelengths.

    Contributed Talk in Massive and Evolved Stars II on Friday, Oct 16th 11:00–12:30 EDT
    Tuesday, October 20th
    Variability and pulsations above the Eddington limit Felix Fuerst (European Space Agency, ESAC) Co-author(s): D. Walton (IoA Cambridge), M. Heida (ESO Garching), M. Bachetti (Osservatorio Cagliari), M. Brightman (Caltech), H. Earnshaw (Caltech), F. Harrison (Caltech)

    Ultra-Luminous X-ray Sources (ULXs) are ideal laboratories to study the effects of super-Eddington accretion, in particular in those powered by accreting neutron stars. These systems were identified only recently through the discovery of coherent pulsations. This discovery lead to a paradigm change in the field, i.e., it completely changed our picture of the make-up of the ULX population, their evolution, and their impact on the environment. While Chandra did not yet see pulsations from ULXs, its superb angular resolution has helped to disentangle sources and monitor their long-term variability. It is likely that strong variability, including off-states, is a tell-tale sign of ULX pulsars, driven the by the strong magnetic field of the neutron star. This strong magnetic field might be measured directly through cyclotron lines, as Chandra as done for M51 ULX8. Chandra has also discovered the first X-ray bubble around a ULX, highlighting the profound impact these sources have on their environment. I will try to summarize the current state-of-the-art on ULX pulsars and their variability and outline how Chandra has and will continue to contribute to this exciting field.

    Invited Talk in Ultraluminous X-ray Sources on Tuesday, Oct 20th 09:00–10:30 EDT
    Long and Short-Term Variability of Globular Cluster Hosted UItraluminous X-ray Sources Kristen Dage (McGill Space Institute, Michigan State University) Co-author(s): Stephen Zepf (Michigan State University), Erica Thygesen (Michigan State University), Arunav Kundu (Eureka Scientific), Arash Bahramian (Curtin University), Daryl Haggard (McGill Space Institute), Tom Maccarone (Texas Tech University)

    Currently, 17 ultraluminous X-ray sources (ULXs) with globular cluster (GC) counterparts have been identified. ULXs in the old GC environment represent a new population of ULXs, and ones likely to be black holes. These sources show a diverse behaviour with regards to temporal variability, both on long (16 years) and short (~hours) timescales, in both the X-ray and optical wavelengths. These sources can switch on or off over the course of many years, or remain at a constant luminosity. Some sources exhibit a long-term change in their luminosity with no discernible variability within the other observations, Other sources show a stunning long-term variability while also demonstrating variability on the timescale of around four hours. I will undertake a comprehensive comparison of the temporal variability of the zoo of currently known GC ULXs and discuss the possible origins of some of the extreme variability observed.

    Contributed Talk in Ultraluminous X-ray Sources on Tuesday, Oct 20th 09:00–10:30 EDT
    Probing the long-term variability of ULXs with Chandra monitoring Hannah Earnshaw (Caltech) Co-author(s): Murray Brightman (Caltech), Fiona Harrison (Caltech), Marianne Heida (European Southern Observatory), Tim Roberts (Durham University), Rajath Sathyaprakash (Durham University), Xiaoxi Song (University of Manchester), Dominic Walton (University of Cambridge)

    Some ultraluminous X-ray sources, particularly those identified as neutron star accretors, demonstrate extreme long-term variation in flux of over an order of magnitude. Multiple mechanisms can cause such variability, such as the 'propeller effect' in which accretion is halted at the magnetospheric radius, and super-orbital periods caused by precession in the system. Monitoring ULX-rich galaxies with Chandra can begin to provide us with a population-level understanding of long-term ULX variability and its rarity.

    Lightning Talk in Ultraluminous X-ray Sources on Tuesday, Oct 20th 09:00–10:30 EDT
    Spectral Variability: A Road to Discovery with the Chandra Source Catalog Rafael Martínez-Galarza (Smithsonian Astrophysical Observatory) Co-author(s): Megan Freeman (University of Southampton), Giuseppina Fabbiano (Smithsonian Astrophysical Observatory), Andreas Zezas (University of Crete)

    Spectral variability is among the many properties recorded in the Chandra Source Catalog 2.0 (CSC2), and also one of the most relevant astrophysical properties for X-ray sources. TDEs, ULXs, X-ray binaries, all show specific spectral variability features that facilitate their identification. We show results of using an anomaly detection algorithm to identify potentially interesting transient sources in CSC 2.0 that are also spectrally variable. We report on the identification of several luminous transients detected in CSC2 using an anomaly detection algorithm, and on the characterization of their spectral properties. These transients have been hiding for several years as serendipitous sources in Chandra data without being previously spotted or fully characterized. Mining CSC2 has made their discovery and characterization possible. Among the transients reported are soft luminous transients that are compatible with being accreting white dwarfs, X-ray binaries with evolved companions, and flaring young stars. We provide a summary of our early findings.

    Contributed Talk in Ultraluminous X-ray Sources on Tuesday, Oct 20th 09:00–10:30 EDT
    Gamma-ray Bursts in the Era of Multimessenger Astrophysics Eleonora Troja (University of Maryland & NASA-GSFC)


    Invited Talk in Gamma-ray Bursts on Tuesday, Oct 20th 11:00–12:30 EDT
    The X-ray afterglow and host galaxy of the unusual GRB 180418A Alicia Rouco Escorial (CIERA (Northwestern University)) Co-author(s): Wen-fai Fong (CIERA (Northwestern University)), Amy Lien (NASA Goddard Space Flight Center (University of Maryland)), Peter Veres (Center for Space Plasma and Aeronomic Research (University of Alabama in Huntsville)), Kerry Paterson (Center for Astrophysics|Harvard & Smithsonian), Anya Nugent (Science and Technology Institute (Universities Space Research Association)), Dylaan Cornish (and Steward Observatory (University of Arizona))

    In this talk, I present the late-time afterglow monitoring and host galaxy discovery of the intermediate-duration GRB180418A, using X-ray facilities and large ground-based telescopes. I present deep Chandra observations till 39 days after the burst, which constrain the GRB outflow to have a wide opening angle >10 deg. I also present a comparison of the afterglow behavior of GRB 180418A to the short and long GRB populations.

    Contributed Talk in Gamma-ray Bursts on Tuesday, Oct 20th 11:00–12:30 EDT
    The merger environments of short gamma-ray bursts Brendan O'Connor (George Washington University, University of Maryland - College Park, NASA Goddard Space Flight Center) Co-author(s): Paz Beniamini (Caltech), Chryssa Kouveliotou (George Washington University)

    The association of a host galaxy with a short gamma-ray burst (SGRB) depends on an accurate localization of the SGRB. 20-30% of well-localized SGRBs lack a coincident host to deep optical and NIR limits. These SGRBs have been identified as observationally hostless due to their lack of strong host associations. Considering early time \textit{Swift} observations of short GRB afterglows we derive lower limits on their circumburst densities. We calculate the gas density at the virial redshift of an average SGRB host galaxy, and by adopting this threshold identify that <16% of our sample could have merged within such densities. We find that out of the five observationally hostless bursts in our sample, none are consistent with having occurred outside that radius. This implies one of two scenarios. Either the binary neutron stars leading to those SGRBs merged at a large enough offset from their birth galaxy that the probability of galaxy misidentification is large (but such that they still merged well within their host's galactic halo) or else these binaries merged in faint host galaxies at moderate to high redshifts that were missed by follow-up observations.

    Contributed Talk in Gamma-ray Bursts on Tuesday, Oct 20th 11:00–12:30 EDT
    Search for short GRBs within 200 Mpc with Neil Gehrels Swift observatory Simone Dichiara (University of Maryland / NASA-Goddard)

    The discovery of the first binary neutron star merger directly associated with a short gamma-ray burst (GW170817/GRB170817A) reveal the presence of a local population of off-axis events. The onset of the X-ray ray emission from these events is expected after several days from the GRB discovery and is significantly fainter than the on-axis afterglow as revealed by Chandra observation of GRB170817A. In our work we investigate whether similar nearby (<200 Mpc) events were observed by NASA's Neil Gehrels Swift observatory finding 4 possible local events. We use this subsample to constrain the rate of local SGRBs and derive information about the outflow collimation and its structure.

    Lightning Talk in Gamma-ray Bursts on Tuesday, Oct 20th 11:00–12:30 EDT
    GW170817: from merger to jet to outflow Davide Lazzati (Oregon State University)

    The non-thermal emission from GW170817 came from a structured relativistic outflow from a binary neutron star merger. Understanding how the structure is imprinted on the outflow holds the keys to understanding the nature of the remnant and the jet launching process and composition. I will combine numerical simulations and theoretical considerations to gain insight into such an important process.

    Contributed Talk in Gamma-ray Bursts on Tuesday, Oct 20th 11:00–12:30 EDT
    Thursday, October 22nd
    A unique view of variability in exotic X-ray binaries from Chandra Arash Bahramian (International Centre for Radio Astronomy Research – Curtin University)

    Chandra’s fantastic combination of high spatial resolution and sensitivity has paved the path to careful studies of accretion in some of the most fascinating subclasses of X-ray binaries, especially in crowded regions. From the elusive black hole (candidate) X-ray binaries in Galactic and extragalactic globular clusters to the rare transitional millisecond pulsars, such studies have provided invaluable insights about accretion mechanisms in these systems, and subsequently about their population and evolution. Specifically, characterizing variability on short (minutes to hours) and long (days to months/years) timescales in these systems, and possible links with emission in other bands (e.g., radio), have been key diagnostic tools and furthered our understanding of accretion mechanisms. In this talk, I will review some of these findings from the last few years, made possible in part through the eyes of Chandra. I will discuss their impact on our understanding of the evolution of these classes of X-ray binaries.

    Invited Talk in X-ray Binaries I on Thursday, Oct 22nd 08:00–09:30 EDT
    GS 2000+25: The Least Luminous Black Hole X-Ray Binary Jennifer Rodriguez (Michigan State University) Co-author(s): Ryan Urquhart (Michigan State University), Richard M. Plotkin (University of Nevada), Teresa Panurach (Michigan State University), Laura Chomiuk (Michigan State University), Jay Strader (Michigan State University), James C. A. Miller-Jones (Curtin University), Elena Gallo (University of Michigan), and Gregory R. Sivakoff (University of Alberta)

    We report new, strictly simultaneous radio and X-ray observations of the nearby stellar-mass black hole X-ray binary GS 2000+25 in its quiescent state. In deep Chandra observations we detect the system at a faint X-ray luminosity of Lx = 1.1x10^30(d/2 kpc)^2 erg/s (1–10 keV). This is the lowest X-ray luminosity yet observed for a quiescent black hole X-ray binary, corresponding to an Eddington ratio Lx/LEdd ~ 10^−9. In 15 hours of observations with the Karl G. Jansky Very Large Array, no radio continuum emission is detected to a 3σ limit of <2.8 μJy at 6 GHz. Including GS 2000+25, four quiescent stellar-mass black holes with Lx < 1032 erg s^−1 have deep simultaneous radio and X-ray observations and known distances. These sources all have radio to X-ray luminosity ratios generally consistent with, but slightly lower than, the low-state radio/X-ray correlation for stellar-mass black holes with Lx > 1032 erg s^−1. Observations of these sources tax the limits of our current X-ray and radio facilities, and new routes to black hole discovery are needed to study the lowest-luminosity black holes.

    Lightning Talk in X-ray Binaries I on Thursday, Oct 22nd 08:00–09:30 EDT
    X-ray Binary New Early Warning System (XB-NEWS) Payaswini Saikia (New York University Abu Dhabi) Co-author(s): Dave Russell (New York University Abu Dhabi)

    Low-mass X-ray binaries (LMXBs) can lay dormant in quiescence, remaining undetected for decades, accreting at very low rates. Despite quiescent LMXBs being notoriously difficult to detect at X-ray energies, they can be routinely detected using relatively small optical telescopes. We have been monitoring ~40-50 LMXBs for 15 years using the Faulkes Telescopes/Las Cumbres Observatory (LCO). We are now detecting the early stages of these outbursts with these optical telescopes, before they become bright enough for X-ray detection. Our new real-time optical monitoring pipeline, the "X-ray Binary New Early Warning System (XB-NEWS)" aims to detect and announce new XRB outbursts within a day of first optical detection. This allows us to trigger X-ray and multi-wavelength campaigns during the very early stages of outbursts, to constrain the outburst triggering mechanism. We have an active very fast TOO trigger with Chandra to achieve very early detections coming out of quiescence. Using the results from the XB-NEWS pipeline, we present long term optical monitoring of some LMXBs, and show how Chandra is the key to solving the outburst triggering mechanism.

    Contributed Talk in X-ray Binaries I on Thursday, Oct 22nd 08:00–09:30 EDT
    Extragalactic X-ray Binary Populations Andreas Zezas (University of Crete & CfA)


    Invited Talk in X-ray Binaries I on Thursday, Oct 22nd 08:00–09:30 EDT
    A Fine Balance: Chandra, Accretion, and Outflows in X-ray Binaries Joey Neilsen (Villanova University)

    If X-ray binaries are an ideal laboratory for studying the physics of black hole accretion, then Chandra is a microscope, offering a close-up view of these transients and the intricate life cycles of stellar mass black holes. With a unique combination of high-resolution spectroscopy and high-resolution imaging, Chandra uncovers the microphysics of black hole feedback, enabling us to draw connections between ionized outflows, relativistic jets, and the variable underlying accretion flow. In this talk, I will highlight some of Chandra's insights into the structure, evolution, and inflow-outflow dynamics of disks around black holes. Along the way, I'll discuss some lessons learned from the last 21 years and some hopes and challenges for the future.

    Invited Talk in X-ray Binaries II on Thursday, Oct 22nd 10:00–11:30 EDT
    High resolution X-ray spectroscopy of the Supergiant HMXB 4U1700-37 during the compact object eclipse Maria Martínez-Chicharro (Instituto Universitario de Física Aplicada a las Ciencias y las Tecnologías (IUFACyT) Universidad de Alicante E-03690 Alicante Spain) Co-author(s): V. Grinberg (Institut für Astronomie und Astrophysik Universität Tübingen), J.M. Torrejón (Instituto Universitario de Física Aplicada a las Ciencias y las Tecnologías (IUFACyT) Universidad de Alicante), N. Schulz (MIT: Kavli Institute for Astrophysics and Space Research), L. Oskinova (Massachusetts Institute of Technology), M. Nowak (Postdam Institut ür Physik und Astronomie Universität Potsdam), F. Fürst (Department of Physics Washington University), N. Hell (ESA/ESAC: European Space Astronomy Centre (ESA/ESAC) Science Operations Department Villanueva de la Cañada), R. Hainich (LLNL:Lawrence Livermore National Laboratory)

    We present an analysis of the first observation of the iconic High Mass X-ray Binary 4U 1700-37 with Chandra High Energy Transmission Gratings during X-ray eclipse. This allow us to study in depth the back illuminated stellar wind of the O6Ia star HD153919 =V884 Sco, the earliest donor in any Galactic HMXB, with unprecedented detail. We analyse the behaviour of the emission line spectrum as a function of the continuum emission and present physical properties of the irradiated stellar wind.

    Lightning Talk in X-ray Binaries II on Thursday, Oct 22nd 10:00–11:30 EDT
    Understanding X-ray shots in Cygnus X-1 Yash Bhargava (Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, India)) Co-author(s): Ranjeev Misra (IUCAA), Jayashree Roy (IUCAA), Nandini Hazra (Gran Sasso Science Institute), Md. Shah Alam (IUCAA), Dipankar Bhattacharya (IUCAA), A. R. Rao (Tata Insititute of Fundamental Research)

    Short timescale variability in the lightcurves of X-ray binaries provides an interesting insight into the accretion dynamics. We analyse the "shots" observed in Cygnus X-1 in 0.1-80 keV energy band using simultaneous observation with AstroSat and NICER. We detect simultaneous shots in the soft X-ray band with NICER and in the hard X-rays band with AstroSat-LAXPC. We observe the shot profile for the first time in soft X-rays (0.1-3 keV) and determine the features of the profile in different spectral bands. The relative shot profile peaks in 1.5-3 keV energy range. Using the shot-phase resolved spectroscopy, we break the degeneracy regarding the origin of the sudden surge of the thermal photons. We find that during a shot, the accretion rate remains constant and the inner edge of the accretion disk moves inwards as the shot rises and outwards as the shot decays.This event produces a surge of photons which then get upscattered by the comptonising cloud, thus producing the asymmetric shot profile. We discuss the possible mechanisms causing the swing in the inner radius

    Contributed Talk in X-ray Binaries II on Thursday, Oct 22nd 10:00–11:30 EDT
    Using the Chandra HETG for Time Resolved X-Ray Spectroscopy of X-Ray Binaries Michael McCollough (CXC/SAO/CfA) Co-author(s): T. Kallman (NASA/GSFC), K. Koljonen (FINCA), D. Liedahl (LLNL), J. Miller (U. Mich.), F. Paerels (Columbia), G. Pooley (MRAO), M. Sako (U. Penn), N. Schulz (RAS), S. Trushkin (U. Mich.)

    For relatively bright X-ray binaries the Chandra HETG allows one to do time resolved X-ray spectroscopy on timescales of hours. It is possible to examine how various spectral features vary as a function of activity and orbital phase. As a case study we will review some of the result from a recent study of Cygnus X-3 (Kallman, et al. 2019).

    Lightning Talk in X-ray Binaries II on Thursday, Oct 22nd 10:00–11:30 EDT
    Blackhole X-ray binaries in state transition (a short review) Armin Vahdat Motlagh (University of Tubingen)

    I will be summarizing the spectral and timing evolution of black hole low-mass X-ray binaries during state transition focusing on the outburst decay stage. I will compare the luminosity distribution of these sources in different states and discuss the impact of observables on the distribution.

    Contributed Talk in X-ray Binaries II on Thursday, Oct 22nd 10:00–11:30 EDT
    Multi-wavelength fast timing in X-ray binaries Alexandra Tetarenko (East Asian Observatory) Co-author(s): P. Casella (INAF), J.C.A Miller-Jones (ICRAR - Curtin University), G.R. Sivakoff (University of Alberta), T.J. Maccarone (Texas Tech University), P. Gandhi (University of Southampton)

    Time-domain observations now offer a promising new way to study accretion and jet physics in X-ray binaries. Through detecting and characterizing rapid flux variability in these systems across a wide range of wavelengths/energy bands (probing emission from different regions of the accretion flow and jet), we can measure properties that are difficult, if not impossible, to measure by traditional spectral and imaging methods (e.g., size scales, geometry, jet speeds, the sequence of events leading to jet launching). While variability studies in the X-ray bands are a staple in the X-ray binary community, there are many challenges that accompany such studies at longer wavelengths. However, with recent advances to observing techniques/instrumentation, the availability of new computational tools, and today's improved coordination capabilities, we are no longer limited by these challenges. In this talk, I will discuss new results from fast timing observations of MAXI J1820+070 from the radio through X-ray bands, highlighting how we can directly connect variability properties to internal jet physics. Additionally, I will discuss the role that Chandra can play in this science.

    Invited Talk in X-ray Binaries III on Thursday, Oct 22nd 16:00–17:30 EDT
    Chandra Monitoring and Follow-Up of NGC 300 ULX-1 Breanna Binder (Cal Poly Pomona) Co-author(s): Georgios Vasilopoulos (Yale)

    NGC 300 ULX-1 is a pulsar-high mass X-ray binary that has undergone extreme flux variations (by nearly four order of magnitude) since its discovery ten years ago. The outbursting, ultraluminous X-ray behavior has been followed by prolonged decreases in flux - observations with Chandra, Swift, XMM-Newton, and NuSTAR suggest that this variation in flux may be the result of an accretion disk that is precessing due to the Lense-Thirring effect. We present preliminary results of recent efforts with Chandra and Swift to monitor the flux of NGC 300 ULX-1, and discuss Chandra's important contributions to ultraluminous X-ray source monitoring campaigns.

    Lightning Talk in X-ray Binaries III on Thursday, Oct 22nd 16:00–17:30 EDT
    Observations of the disk/jet coupling of X-ray binaries during their descent to quiescence: The case for rapid follow-up Aarran Shaw (University of Nevada Reno) Co-author(s): R. M. Plotkin (University of Nevada Reno), J. C. A. Miller-Jones (Curtin University), J. Homan (SRON), E. Gallo (University of Michigan), D. M. Russell (New York University Abu Dhabi), J. A. Tomsick (University of California Berkeley), P. Kaaret (University of Iowa), S. Corbel (Université Paris-Saclay), M. Espinasse (Université Paris-Saclay), J. Bright (University of Oxford)

    Black hole X-ray binaries in the quiescent state display softer X-ray spectra compared to higher-luminosity black hole X-ray binaries in the hard state. However, the cause of this softening, and its implications for the underlying accretion flow, are still uncertain. Here, we present quasi-simultaneous X-ray and radio spectral monitoring of the black hole X-ray binary MAXI J1820+070 during the decay of its 2018 outburst and of a subsequent re-flare in 2019, providing an opportunity to monitor a black hole X-ray binary as it actively transitions into quiescence. We use our dense coverage of MAXI J1820+070 over four decades in X-ray luminosity to show how rapid multi-wavelength follow-up of fading X-ray binaries can help us understand accretion at low luminosities.

    Contributed Talk in X-ray Binaries III on Thursday, Oct 22nd 16:00–17:30 EDT
    The spin of GRS 1915+105, revisited Bri Mills (University of Virginia) Co-author(s): Shane W. Davis (University of Virginia), Matthew J. Middleton (University of Southampton)

    The spin of the black hole in the X-ray binary GRS 1915+105 has long been debated, largely due to the variability of GRS 1915+105 and the need for constraining the mass, distance, and inclination of the black hole. We present a re-analysis of both Middleton et al. 2006 and McClintock et al. 2006, accounting for new constraints on the mass, distance, and inclination and report our findings for the spin of GRS 1915+105.

    Lightning Talk in X-ray Binaries III on Thursday, Oct 22nd 16:00–17:30 EDT
    Measure the gas to dust ratio towards bright sources in the Galactic Bulge Jun Yang (MIT Kavli Institute for Astrophysics and Space Research) Co-author(s): Norbert Schulz (MIT Kavli Institute for Astrophysics and Space Research), Claude Canizares (MIT Kavli Institute for Astrophysics and Space Research), and Lia Corrales (University of Michigan)

    Knowing the dust content in interstellar matter is necessary to understand composition and evolution of the interstellar medium (ISM). The metal composition of the ISM enables us to study the cooling and heating processes that dominate the star formation rates in our Galaxy. The Chandra High Energy Transmission Grating (HETG) Spectrometer provides a unique opportunity to measure element dust compositions through X-ray edge absorption structure. We measure gas to dust optical depth ratios towards several bright Low-Mass X-ray Binaries (LMXBs) in the Galactic Bulge with the highest precision so far. We also explore edge variability due to ionized Si as was proposed in a previous study. Well calibrated and pile-up free optical depths are measured for a large range broadband hydrogen equivalent absorptions (log N_H [cm^-2] = 21.6 - 22.8). From the optical depths we deduce gas to dust ratios for various silicates in the ISM. The final goal is to model neutral Si gas, Si dust and contributions of ionized Si for different lines of sight towards the Galactic Bulge.

    Contributed Talk in X-ray Binaries III on Thursday, Oct 22nd 16:00–17:30 EDT
    Monday, October 26th
    Accretion and outflows in neutron star low-mass X-ray binaries Nathalie Degenaar (University of Amsterdam, Anton Pannekoek Institute for Astronomy)

    Accretion is a very important physical process that occurs on many different scales throughout our universe. Binary systems that are composed of a neutron star and a low-mass companion, are exciting laboratories to study accretion and associated outflows: Apart from devouring gas, these collapsed stellar remnants also spit matter and energy back into space via collimated radio jets and dense disk winds. These outflows can have a significant impact on the accretion process, the evolution of the binary, and the environment. In this talk, I will highlight Chandra’s past, present, and future contribution to time-domain studies of accretion and outflows in neutron star low-mass X-ray binaries.

    Invited Talk in Neutron Stars and Magnetars I on Monday, Oct 26th 09:30–11:00 EDT
    Jet–Accretion Coupling in Luminous Accreting Neutron Stars in Galactic Globular Clusters Teresa Panurach (Michigan State University) Co-author(s): Jay Strader (Michigan State University), Arash Bahramian (International Centre for Radio Astronomy Research Curtin University), Laura Chomiuk (Michigan State University), James Miller-Jones (International Centre for Radio Astronomy Research Curtin University)

    It is now established that hard-state accreting neutron stars in low-mass X-ray binaries show outflows — and sometimes jets — in the general manner of accreting black holes. However, the quantitative link between the accretion flow (traced by X-rays) and the outflow/jet (traced by radio emission) is much less well-understood for neutron stars than for black holes. Here we use the deep MAVERIC radio continuum survey of 50 Galactic globular clusters to do a systematic study of the radio and X-ray properties of all the luminous (L_X > 10^34 erg/s) persistent neutron star X-ray binaries in our survey, as well as two other transients also captured in our data. We find that these neutron star X-ray binaries show a much larger range in radio luminosity than previously observed, and some have outflows as luminous as those of black holes. These results show that neutron stars do not evince a single relation between inflow and outflow and that the accretion dynamics are more complex than for black holes.

    Lightning Talk in Neutron Stars and Magnetars I on Monday, Oct 26th 09:30–11:00 EDT
    Chandra Eclipse Timing Determination of the Third Body Period of 4U 2129+47 Michael Nowak (Washington University in St. Louis) Co-author(s): Deepto Chakrabarty (MIT Kavli Institute), Joern Wilms (Dr. Karl Remeis-Sternwarte University of Erlangen-Nuremberg)

    We present a Bayesian analysis, using the fact that Chandra is a "single photon detector", showing eclipse timing of the quiescent neutron star binary 4U 2129+47. Variations in this eclipse time reveal the presence of a third body in this likely hierarchical triple, and allow us to determine the most likely triple orbital parameters. Furthermore, we discuss long term cooling of the NS, as revealed by both Chandra and XMM-Newton observations conducted over a time span of a decade.

    Lightning Talk in Neutron Stars and Magnetars I on Monday, Oct 26th 09:30–11:00 EDT
    Role of temporal features in machine-learning (ML) classification of X-ray sources based on Chandra Source Catalog (CSCv2) Hui Yang (George Washington University) Co-author(s): Oleg Kargaltsev (George Washington University (GWU)), Jeremy Hare (NASA GSFC and NASA Postdoctoral Program Fellow), Igor Volkov (GWU), Blagoy Rangelov (Texas State University), Rafael Martínez-Galarza (Center for Astrophysics | Harvard & Smithsonian)

    ML algorithms provide an efficient way of identifying the astrophysical nature of many thousands of unclassified X-rays sources in large catalogs, such as CSCv2. The addition of multi-wavelength (MW) and temporal features provides a wealth of information about the sources that need to be analyzed. Variability, which can be quantified in many ways (significance, magnitude, timescale, etc), is one of the most important features that sheds light on the physical processes occurring in different kinds of astrophysical objects. In this talk, I will outline our approach to supervised ML classification of X-ray sources based on CSCv2 and MW catalogs, focusing on the variability and temporal features used in our ML pipeline. The limitations and possible extensions of the variability features in CSCv2 will be briefly discussed. I will also talk about the correlations between the long-term (inter-observation on year long time scales) and the short term (intra-observation on time scales of days or less) variability for different astrophysical classes of sources in CSCv2. Preliminary results regarding the classifications of some variable sources in Galactic fields will be presented.

    Lightning Talk in Neutron Stars and Magnetars I on Monday, Oct 26th 09:30–11:00 EDT
    Strongly magnetised neutron stars in the Chandra era Nanda Rea (Institute of Space Sciences (CSIC-IEEC, Barcelona))

    I will summarise all what we have learned in the past two decades about strongly magnetised neutron stars, and the unexpected Chandra discoveries in the field. From their steady X-ray emission, their bright bursts, their possible cyclotron lines, their powerful outburst events, to the physics involved in their extreme emission and their connection with the rest of the pulsar population.

    Invited Talk in Neutron Stars and Magnetars I on Monday, Oct 26th 09:30–11:00 EDT
    Isolated Neutron Stars Vicky Kaspi (McGill University)


    Invited Talk in Neutron Stars and Magnetars II on Monday, Oct 26th 11:30–13:00 EDT
    The Chandra ACIS Timing Survey Project (CATS@BAR) GianLuca Israel (INAF, Astronomical Observatory of Roma) Co-author(s): Paolo Esposito (IUSS Pavia), Guillermo Rodriguez (INAF - IASF Palermo), Lara Sidoli (INAF - IASF Milano)

    The "Chandra ACIS Timing Survey at Brera And Roma observatories" project (CATS@BAR), aimed at searching for new pulsators in the X-rays, is entirely based on 20 years of Chandra ACIS TE archival data. This timing survey, with 14000 analyzed ACIS pointing and almost 500000 time series extracted, more than 200000 of which has been searched for coherent signals, is one of the largest ever carried out in the 0.5-10keV band. So far we discovered about 50 new X-ray pulsators of different nature, such as cataclysmic variables, accreting neutron stars, and black hole candidates both in the Milky Way and in nearby galaxies. To test the signal goodness, we started a parallel project aimed at identifying and studying their optical counterparts and by requesting follow-up X-ray observations with Chandra and XMM. As a by-product of this project, we studied in details the spurious signals which are present in the ACIS time series. Additionally, this is a living project and the detection algorithm will continue to be routinely applied to the new Chandra data as they become public. Based on the results obtained so far, we expect to discover about three new pulsators every year.

    Contributed Talk in Neutron Stars and Magnetars II on Monday, Oct 26th 11:30–13:00 EDT
    Crust cooling emission from high-magnetic field neutron stars in Be/X-ray transients Alicia Rouco Escorial (CIERA (Northwestern University)) Co-author(s): Rudy Wijnands (University of Amsterdam), Nathalie Degenaar (University of Oxford), Jakob van den Eijnden (Universidad autónoma de México), Dany Page (Universitat Autònoma de Barcelona)

    In this talk, I summarize the current status in the field of the crust cooling emission for strongly magnetized neutron stars in Be/X-ray transients under an observational point of view. Additionally, I highlight the role of Chandra in the long-term monitoring of these systems that allow to study the potential effects of different crustal magnetic-field configuration on the crust cooling behavior.

    Lightning Talk in Neutron Stars and Magnetars II on Monday, Oct 26th 11:30–13:00 EDT
    A Fast Radio Burst Associated with a Galactic Magnetar Christopher Bochenek (Caltech) Co-author(s): Vikram Ravi (Caltech), Konstantin Belov (JPL), Gregg Hallinan (Caltech), Jonathon Kocz (UC Berkeley), Shri Kulkarni (Caltech), Dan McKenna (Caltech)

    Since their discovery in 2007, much effort has been devoted to uncovering the sources of the extragalactic, millisecond-duration fast radio bursts (FRBs). The short durations and energetics of FRBs favored magnetar progenitors. In this talk, we present the discovery of a millisecond-duration radio burst from the Galactic magnetar SGR J1935+2154, with a fluence of 1.5±0.3 Mega-Jansky milliseconds. The isotropic-equivalent energy released in this event, termed ST 200428A (=FRB 200428) is 4000 times greater than in any Galactic radio burst previously observed on similar timescales. ST 200428A is just 40 times less energetic than the weakest extragalactic FRB observed to date, and is arguably drawn from the same population as the observed FRB sample. This event is the first FRB with an X-ray counterpart, which was typical in energy and duration for a magnetar burst, but has a significantly harder spectrum than a typical magnetar burst. The discovery of ST 200428A implies that active magnetars like SGR 1935+2154 can produce FRBs at extragalactic distances.

    Contributed Talk in Neutron Stars and Magnetars II on Monday, Oct 26th 11:30–13:00 EDT
    Tuesday, October 27th
    Monitoring AGN variability over two decades with Chandra Deep Surveys Maurizio Paolillo (Università di Napoli Federico II) Co-author(s): I.Papadakis (University of Crete), W.N.Brandt (Pennsylvania State University), F.Bauer (Pontificia Universidad Católica de Chile), O.Shemmer (University of North Texas), Y.Xue (Nanjing University), B.Luo (University of Science and Technology of China)

    Over the last two decades, Chandra Deep Surveys allowed to monitor AGN variability on a large range of timescales, redshifts and luminosities. I will discuss the progresses made on the use of X-ray variability of AGNs in order to constrain the physics and the main properties of accreting supermassive BH through cosmic time, as well as a tool for cosmological studies.

    Invited Talk in Active Galactic Nuclei I on Tuesday, Oct 27th 09:00–10:30 EDT
    Chandra Observations of the Broad Absorption Line AGN Narrow-Line Seyfert 1 galaxy WPVS 007 Dirk Grupe (Morehead State University)

    I will report on Chandra observations of the unusual Narrow-Line Seyfert 1 Galaxy WPVS 007, and AGN that exhibits Broad Absorption lines which are only seen in Quasars. Models explaining the strong UV variability observed by HST while the AGN remains in an extreme X-ray low state will be discussed.

    Contributed Talk in Active Galactic Nuclei I on Tuesday, Oct 27th 09:00–10:30 EDT
    Flaring in NGC 4869 and the effect of Kelvin Helmholtz instability Dharam V. LAL (National Centre for Radio Astrophysics TIFR)

    Chandra and uGMRT have imaged NGC4869 and its X-ray environment. We detect a steep-spectrum sheath layer enveloping a flat-spectrum spine, hinting at transverse velocity structure with fast-moving spine surrounded by a slow-moving sheath. Also seen is a ridge of radio emission, i.e, flaring of a collimated jet as it crosses a surface brightness edge, which is due to Kelvin-Helmholtz instabilities.

    Contributed Talk in Active Galactic Nuclei I on Tuesday, Oct 27th 09:00–10:30 EDT
    Complementary Chandra and XMM-Newton measurements of rapid AGN variability Erin Kara (MIT)

    Active Galactic Nuclei show X-ray variability on timescales down to minutes, indicating a compact emission region close to the central supermassive black hole. In the past decade Chandra and XMM-Newton have made pivotal and complementary observations to probe these compact environments through variability studies. In this review talk, I will discuss how Chandra's exquisite spatial resolution enables studies of microlensing of distant quasars to probe the inner accretion flow geometry, and how XMM-Newton's complementary large effective area has enabled X-ray reverberation mapping of these same compact regions. Chandra and XMM-Newton’s independent measurements show that the X-ray corona in luminous AGN is much more compact than the accretion disc, which is not highly truncated and extends down to the scale of the innermost stable circular orbit.

    Invited Talk in Active Galactic Nuclei II on Tuesday, Oct 27th 14:00–15:30 EDT
    A time-domain gamma-ray blazar sequence Manel Errando (Washington University in St Louis) Co-author(s): Benjamin Groebe (Washington University in St Louis)

    We use Fermi-LAT data to establish a blazar sequence based solely on the flux variability properties of blazars at timescales of days to years. We find that variability patterns are correlated with blazar spectral properties, and that BL Lac-type blazars and flat spectrum radio quasars show distinguishable flaring features. Our results align with predictions from leptonic emission scenarios with the differences in flux variability properties being explained by varying rates of radiative cooling. BL Lac-type blazars display higher levels of gamma-ray flux variability as their luminosity increases and their broadband spectral energy distribution shifts to redder frequencies. The variability observed in flat spectrum radio quasars also displays a range of behaviors, but a correlation between physical properties of the AGN and the measures of variability was not found.

    Lightning Talk in Active Galactic Nuclei II on Tuesday, Oct 27th 14:00–15:30 EDT
    Chandra Observations of Candidate Subparsec Binary Supermassive Black Holes Lynnie Saade (University of California Los Angeles) Co-author(s): Daniel Stern (Jet Propulsion Laboratory), Murray Brightman (California Institute of Technology), Zoltán Haiman (Columbia University), S. G. Djorgovski (California Institute of Technology), Daniel D’Orazio (Harvard University), K. E. S. Ford (American Museum of Natural History), Matthew J. Graham (California Institute of Technology), Hyunsung D. Jun (Korea Institute for Advanced Study), Ralph P. Kraft (Harvard-Smithsonian Center for Astrophysics), Barry McKernan (American Museum of Natural History), Alexei Vikhlinin (Harvard-Smithsonian Center for Astrophysics), Dominic J. Walton (University of Cambridge)

    I will present analysis of Chandra X-ray observations of seven quasars that were identified as candidate subparsec binary supermassive black hole (SMBH) systems in the Catalina Real-Time Transient Survey based on apparent periodicity in their optical light curves. Simulations predict that close-separation accreting SMBH binaries will have different X-ray spectra than single accreting SMBHs, including harder or softer X-ray spectra, ripple-like profiles in the Fe K-α line, and distinct peaks in the spectrum due to the separation of the accretion disk into a circumbinary disk and mini disks around each SMBH. I found these seven quasar spectra were all well fit by simple absorbed power-law models, with the rest-frame 2–10 keV photon indices, Γ, and the X-ray-to-optical power slopes, α(OX), indistinguishable from those of the larger quasar population. This could mean that these seven quasars are not truly SMBH binaries, and/or that differences between single and binary SMBH spectra lie outside the Chandra band, and/or that models of binary SMBHs need to be adjusted.

    Lightning Talk in Active Galactic Nuclei II on Tuesday, Oct 27th 14:00–15:30 EDT
    A new statistical scheme to model the time evolution of characteristic variability time scales of accreting objects Darlin Soto (Pontificia Universidad Católica de Chile) Co-author(s): Giovanni Motta (Texas A&M University), Malgosia Sobolewska (Harvard-Smithsonian Center for Astrophysics)

    We present the class of Locally Stationary ARMA models with parameters that are smooth functions of time and have potential to statistically describe the X-ray PSD evolution of AGN and X-ray binaries. Chandra’s 20 years archive, in synergy with other X-ray observatories, provides means to test the LSARMA description and study physical processes governing the X-ray variability of accreting objects.

    Contributed Talk in Active Galactic Nuclei II on Tuesday, Oct 27th 14:00–15:30 EDT
    X-ray Spectral-Timing Modeling of Accreting Black Holes Javier Adolfo García (Caltech)

    I will present a new unified model for time-dependent relativistic X-ray reflection in accreting compact objects. We self-consistently merge the best spectral and timing reflection models to make accurate predictions for the flux-energy, lag-frequency, lag-energy spectra simultaneously. I will show the application of this model to X-ray data from accreting black holes.

    Contributed Talk in Active Galactic Nuclei II on Tuesday, Oct 27th 14:00–15:30 EDT
    Friday, October 30th
    An X-ray View of Engine-Powered Transients Brian Metzger (Columbia University, Flatiron Institute CCA)

    As optical time-domain surveys uncover new classes of astrophysical transients, it has become clear that classical mechanisms for powering their light curves (e.g. nuclear fusion, radioactive decay, residual heat of explosion) are insufficient to explain many of their luminosities and timescales. These classes potentially include: superluminous supernovae, neutron star mergers, white dwarf-neutron star mergers, classical novae, binary star mergers, "fast blue optical transients" (FBOT), and tidal disruption events. In many cases, the light from these events may be powered by a long-lived "central engine" enshrouded in the ejecta, whether that engine is a bonafide compact object (magnetar or black hole) or internal shocks generated as the ejecta collides with an external medium. I will describe a unifying picture for engine-powered optical transients, focusing on the crucial role X-ray observations play in directly probing the "engine" (particularly in cases where X-rays can escape the ejecta to be observed). I will highlight a few illustrative examples: long-lived magnetars in binary neutron star mergers, superluminous supernovae, and the FBOT AT2018cow.

    Invited Talk in Multi-messenger Astronomy I on Friday, Oct 30th 09:00–10:30 EDT
    The X-ray view of IceCube neutrino events Fe Krauss (Penn State University) Co-author(s): Azadeh Keivani (Columbia University), Derek B. Fox (Penn State University), Kohta Murase (Penn State University)

    The origin of very-high energy cosmic rays remains a mystery, decades after the initial discovery. Neutrinos provide a unique tool in the search for the sources of cosmic rays. The blazar TXS 0506+056 was one of the first AGN connected to a neutrino event, IceCube-170922A. X-ray observations have played a crucial role in identifying the origin of astrophysical neutrinos, in particular related to the flux constraints expected from secondary cascades. Follow-up observations of IceCube alerts at all wavelengths have not revealed obvious source counterparts, with blazars not being able to explain all of the IceCube neutrinos. We present results from the IceCube-190331A neutrino event and have identified a possible type as the dominant IceCube neutrino emitter. We show that radio-quiet, gamma-ray quiet AGN are in agreement with observed neutrinos, particularly for the case of the neutrino IceCube-190331A.

    Contributed Talk in Multi-messenger Astronomy I on Friday, Oct 30th 09:00–10:30 EDT
    The search for LISA Detectable Gravitational Wave Sources using the Zwicky Transient Facility Kevin Burdge (California Institute of Technology) Co-author(s): Michael Coughlin (University of Minnesota Minneapolis), David Kaplan (University of Wisconsin-Milwaukee), Shri Kulkarni (California Institute of Technology), Thomas Marsh (University of Warwick), Tom Prince (California Institute of Technology), Jan van Roestel (California Institute of Technology)

    In this talk, I will describe ongoing work using the Zwicky Transient Facility (ZTF) to identify ultracompact binaries (P<60 min) which are candidate gravitational wave sources detectable by the Laser Interferometer Space Antenna (LISA). So far, this work has resulted in the discovery of 18 newly identified binary systems with orbital periods under an hour, including seven eclipsing systems, two of which have orbital periods under 10 minutes (making them the shortest period eclipsing binary systems known). Three of the systems will be very strong (SNR>50) LISA gravitational wave sources, and many of the others should be detectable at lower SNR.

    Contributed Talk in Multi-messenger Astronomy I on Friday, Oct 30th 09:00–10:30 EDT
    Multi-messenger astronomy with high-energy neutrinos Anna Franckowiak (DESY, Ruhr University Bochum)

    The recent discovery of high-energy astrophysical neutrinos has opened a new window to the Universe. In September 2017, the detection of a high-energy neutrino in coincidence with a flaring gamma-ray blazar revealed the first compelling high-energy neutrino source candidate. At the same time, gamma-ray blazars are disfavored as the dominant neutrino source class. Other plausible source candidates are tidal disruption events, low-luminosity gamma-ray bursts and supernovae. Combining neutrino data with electromagnetic measurements in a multi-messenger approach will increase the sensitivity to identify neutrino sources and help to solve long-standing problems in astrophysics such as the origin of cosmic rays. I will review the recent progress in neutrino multi-messenger astronomy.

    Invited Talk in Multi-messenger Astronomy I on Friday, Oct 30th 09:00–10:30 EDT
    X-raying compact binary mergers Eleonora Troja (University of Maryland & NASA-GSFC)

    In 2017 our understanding of compact binary mergers was transformed by the spectacular discovery of GW170817, the first neutron star merger observed through gravitational waves and light. Like all revolutionary discoveries, GW170817 posed as many questions as it answered. What is the fate of the merger remnant? Do all NS mergers launch successful relativistic jets? Is radioactive decay the only power-source of kilonovae? How does a neutron star - black hole merger look like? Stemming from the experience of GW170817, I'll discuss the crucial role of X-ray observations of gravitational wave sources.

    Invited Panelist in Multi-messenger Astronomy II (Panel & Discussion) on Friday, Oct 30th 11:00–12:30 EDT
    Preparing to Prioritize (and Pounce) in the LIGO-Virgo O4 Era and Beyond Daryl Haggard (McGill University, McGill Space Institute)

    As advanced LIGO-Virgo approach “design sensitivity” these gravitational wave experiments anticipate one or more high-significance events per day. Even if only a small fraction of these are likely to boast an electromagnetic counterpart, rapid target selection, characterization, and vetting will be essential for prioritizing candidates for other elite ground- and space-based facilities. Are scientists prepared to design and launch campaigns that maximize discovery from these and other time domain events? How prepared are Chandra and other observatories to meet the demand for these time domain observations? I’ll share a few ideas and likely ask more questions than I answer.

    Invited Panelist in Multi-messenger Astronomy II (Panel & Discussion) on Friday, Oct 30th 11:00–12:30 EDT
    Mapping the Diverse Outcomes of BNS Mergers Brian Metzger (Columbia University, Flatiron Institute CCA)

    I will briefly highlight the predicted diversity in the outcomes of binary neutron star mergers and their associated electromagnetic counterparts (and, time permitting, of neutron star-black hole mergers). An empirical map between gravitational wave and electromagnetic observables could become a powerful probe for constraining the equation of state of neutron stars.

    Invited Panelist in Multi-messenger Astronomy II (Panel & Discussion) on Friday, Oct 30th 11:00–12:30 EDT
    The bright future of GW170817 Raffaella Margutti (Northwestern University/CIERA)

    I will present a critical review of the observational constraints on the neutron star merger GW170817

    Invited Panelist in Multi-messenger Astronomy II (Panel & Discussion) on Friday, Oct 30th 11:00–12:30 EDT