Workshop Abstracts

Black-hole X-ray binaries allow us to observe accretion processes to take place on human timescales: over days to months, a single outburst redistributes mass and energy among the disk, corona, wind, and jet. In this talk I will describe the phenomenology of an idealized outburst from trigger to decay, emphasizing at each stage Chandra’s unique vantage point to measure the most important physical properties. Through the bright phase, high-resolution grating spectra have turned highly ionized disk winds into velocity-resolved, structured plasmas that respond to the inner flow within minutes and that may, in the most extreme systems, carry away more mass than the black hole accretes. The same photons, crossing the Galaxy on their way to us, make these binaries backlights for the atomic, ionic, and solid-state content of the interstellar medium. As the source fades, grating resolution helps us to determine the broadening of iron fluorescent emission, which is used for reflection-based spin measurements. Years later, sub-arcsecond imaging finds the ejecta still moving, decelerating, and accelerating multi-TeV particles, showing that a radio flare at launch can badly undercount an ejection. I will conclude with the next decade of Chandra studies, the synergy with XRISM and the current fleet, and the longer roadmap toward NewAthena.

Cygnus X-3 (Cyg X-3) is a microquasar composed of a compact object (likely a Black Hole) and a Wolf-Rayet star in a 4.8 hour orbit. Cyg X-3 is known to produce major radio flares associated with jets that can reach fluxes of over 20 Jy @ 15 GHz. Cyg X-3 has also been recently shown by LHAASO to produce PeV emission. In this presentation we will present a time domain analysis of the Chandra results (both in spectra and images). This will include how the spectra change as a function of state and orbital phase. This will also be compared with time-domain datasets on other wavelengths (gamma-ray, hard X-ray, submillimeter, and radio). We will include what has been found from supporting/followup observations done with SMA, XRISM, and IXPE. We will present a time domain analysis of Chandra and XRISM data on how the spectra and flux vary as a function of state and orbital phase. We will also discuss how the nature of Cyg X-3's Little Friend was discovered using phase resolved images. We will present the initial results of a VLBA observing campaign of a major Cyg X-3 flare and discuss the results of a VERITAS campaign during major flaring of Cyg X-3 and how they may relate to PeV emission from the source.

X-ray transients have been studied for sixty years, and represent the brightest class of X-ray sources seen from Earth. The cause of these X-ray brightenings is the sudden increase of accretion of matter onto a black hole or neutron star. The Disc Instability Model predicts a delay between the hydrogen ionization front sweeping through the accretion disc, and the onset of accretion onto the black hole. However, due to the unpredictability of the outbursts, and the sensitivity limitations of X-ray all-sky monitors, the early stages of outbursts are usually missed altogether. Optical and X-ray monitoring of the early rise of the black hole X-ray binary, Swift J1753.5-0127 in 2023 are presented, including a very early detection from Chandra. A delay is measured between a thermal instability developing in the accretion disc, causing heating fronts to begin propagating through the disc (seen by an optical brightening), and the onset of accretion onto the black hole (a delayed X-ray brightening by 4.4 +/- 0.2 days). Crucially, a Very Fast trigger from Chandra showed elevated X-ray flux above the quiescent level, but before the fast X-ray rise, just 3 days after first optical detection. We show that the ionization of hydrogen from a ring at an intermediate radius in the disc initiates the optical outburst. We witness the propagation of the heating wave, as a steady increase in the flux and surface area of the disc. The rise profile and heating wave spreading rate imply the viscosity parameter alpha ~ 0.08. The results have implications for the switching on of AGN, and tidal disruption events.

Supernova remnants (SNRs) offer the means to study supernovae long after the original explosions. These objects, often characterized by X-ray emission from shock-heated material, exhibit complex morphologies with structure down to the sub-arcsecond level for some Galactic SNRs. In this talk, I will discuss how Chandra studies have been used to greatly improve astronomers’ understanding of SNRs through spatially-resolved imaging and spectroscopic analysis. Chandra observations have enabled astronomers to quantify SNR asymmetries, create ejecta maps, study the evolution of forward shocks, measure the outward motion of ejecta knots, and perform detailed studies of SNR-embedded compact objects and pulsar wind nebulae. This information has in turn been used to further our understanding of supernova explosion mechanisms and the interactions between SNRs and their environment.

Supernova remnants provide key constraints on explosion physics through the spatial and dynamical distribution of their ejecta. We present a combined analysis of Tycho supernova remnant using archival Chandra data and new observations from XRISM/Resolve to probe its three-dimensional structure.
With Chandra, we reconstruct a velocity vector field in the plane of the sky with an excellent spatial resolution using a Poisson Optical Flow method, and map average line-of-sight velocity through blind source separation (GMCA) and Doppler shift, providing a first detailed view of the 3D ejecta dynamics across the remnant. Building on this, XRISM’s high spectral resolution enables us to directly measure velocities and fluxes of individual elements (Si, S, Ar, Ca, Fe) and to disentangle emission from the front and back sides of the shell.
 We reveal significant asymmetries in both velocity and morphology, and constrain the stratification of outer Si/S-rich layers and inner Fe-rich ejecta. This combined approach demonstrates the power of linking high-resolution imaging and spectroscopy to probe the 3D structure of Type Ia remnants, and provides new constraints on explosion models and progenitor environments.

Supernova remnants (SNRs) are primary candidates for Galactic cosmic-ray acceleration, with their X-ray synchrotron emission serving as a vital diagnostic for understanding high-energy particle acceleration. Leveraging Chandra’s unparalleled angular resolution and long-term monitoring capabilities, we present a study of the synchrotron flux time evolution in Kepler’s SNR (SN 1604). By comparing the deepest Chandra/ACIS observations performed in 2006 and 2014, we analyzed the non-thermal flux evolution across various filamentary structures at the rim of the shell. Our analysis confirms the existence of two distinct acceleration regimes: in the northern region, interaction with a dense and turbulent circumstellar medium (CSM) enhances acceleration efficiency, which proceeds close to the Bohm limit. A key finding of this two-epochs analysis is the detection of fading synchrotron emission in a specific filament (Region 5). In this area, the low shock velocity caused by the dense CSM increases the acceleration timescale to the point where synchrotron radiative losses become dominant. This rapid decay allowed for a direct estimate of the local magnetic field strength (B ~ 130-190 μG), providing a rare real-time view of how the local environment modulates the efficiency of cosmic accelerators. Our findings provide a coherent understanding of the different regimes of electron acceleration observed in Kepler’s SNR through various diagnostics, highlighting the role of the Chandra X-ray Observatory in understanding the particle acceleration processes.

We report our measurements of radial velocities and proper motions of clumpy X-ray ejecta knots in the "Ear" regions of Kepler's supernova remnant (SNR), based on our 600 ks Chandra ACIS-HETG observations and ~1 Ms of the archival Chandra ACIS data spanning a time baseline of ~22 years. Our measurements indicate high space velocities (5000-7000 km/s) for ejecta knots projected within both Ears beyond the main shell of the SNR, which are significantly faster than those projected within the main shell (~2000-4000 km/s). These results support the presence of bipolar outflows along the axis of the Ears in Kepler's SNR, probably created either by a bipolar SN explosion or by some particular low-density circumstellar structure.

M dwarf stars are the most abundant stars in the galaxy and have a high frequency of Earth-sized planets, making them favored targets of upcoming missions to detect and characterize exoplanets. However, these stars are known to exhibit high levels of activity and flaring over their entire lifetime, raising questions about the habitability of planets around these stars. If an active star flares so frequently that an atmosphere does not have sufficient time to recover, it could lead to long-lasting depletion or enhancement of key biosignatures such as oxygen and ozone (O2 and O3). Ultimately, this could impact our ability to accurately assess habitability when we obtain snapshot observations of a planetary atmosphere. I will present recent results characterizing stellar flares across the electromagnetic spectrum — from radio through X-ray wavelengths — and discuss advances in our understanding of how these flares shape the atmospheres of surrounding planets. This work is especially time critical to inform the interpretation of JWST observations of exoplanet atmospheric spectra and ultimately target selection for HWO.

I will present a study of highly absorbed high-mass X-ray binaries using high-resolution X-ray spectroscopy from Chandra HETG and XRISM. My analysis focuses on variability in the iron line complex on short timescales, from a few kiloseconds to sampling orbital phases. By tracking changes in line flux, width, and centroid energy, I investigate the dynamics, ionization structure, and clumpiness of stellar winds from the massive companion stars. These variations provide direct insight into wind inhomogeneities, accretion processes, and the geometry of the reprocessing material. My results demonstrate that time-resolved high-resolution spectroscopy is a powerful tool for constraining the physical conditions and structure of stellar winds in HMXBs.

Interactions between stars and planets shape the evolution of young planetary atmospheres, especially when stars are young and active and planets orbit nearby. Early M stars are compelling targets because their habitable zones are close enough for frequent transits yet distant enough that atmospheres may survive. We model the evolving activity of early M-type stars using empirical data. Although flares are common in the first few hundred million years, we find that total X-ray output is dominated by quiescent emission rather than flaring. Using this stellar model, we simulate hydrodynamic escape for Jupiter-like, (sub)Neptune-like, and (super)Earth-like planets at different orbital distances. Jupiter-like planets remain largely unaffected, while Earth-like planets lose their atmospheres. Some sub-Neptunes may evolve into super-Earths, and some super-Earths may become more Earth-like, retaining portions of secondary atmospheres and potentially achieving habitability. We apply these results to planets orbiting M2–3 stars near the habitable zone. We find only a narrow mass range ( 2–4 M_Earth) can retain an Earth-like atmosphere.

Magnetically regulated hot (1-10 MK) coronae of late-type stars radiate directly in kilovolt soft X-rays; indirectly in 100,000 K ultraviolet emissions; and vary on time scales of kiloseconds (flares) to decades (dynamo cycles). The high-energy lives of coronal stars are a long-standing challenge for theorists, but also crucial to exoplanet enthusiasts mulling over space weather threats to habitability. Ten cool dwarfs -- visual binaries Alpha Cen (G2V+K1V), Xi Boo (G9V+K5V), 70 Oph (K0V+K4V); single K-dwarf Epsilon Eri (K2V); plus dMe flare stars AU Mic, AD Leo, Proxima Cen -- are well enough documented in X-rays (Chandra, XMM, Swift) and UV (HST joint programs) to explore their high-energy behavior on relevant time scales. What is known so far: The multi-MK XUV (0.05-10 keV: 1-250 Å) dominates over the T>30,000 K FUV (250-1700 Å) in all but the lowest activity G/K dwarfs. Bright chromospheric (10,000 K) hydrogen Lyα (1215 Å) is competitive with XUV at low activity, but lags behind at higher activity. X-ray cycles can have solar-class decadal durations and large amplitudes (Max/Min~ 0.7 dex) in lower activity G/K dwarfs such as Alp Cen AB, but flat-line examples similar to the Sun's 70-year spot-less Maunder Minimum in the 17th Century also are known; high-energy cycles decline in contrast and possibly shorten in period with increasing X-ray luminosity, as for Eps Eri; and are drowned out by flaring in dMe stars like Prox Cen. Counterpart cycles are seen in the FUV, but more muted than at higher energies. At the same time, transient flares are rare at lower activity, but rise in strength and occurrence rate at higher LX, until they completely rule the light curves of dMe stars (albeit much less so for active G/K dwarfs). While FUV usually pales against XUV in raw energetics, HST spectra reveal surprising redshifts of Si IV 1393 Å and C IV 1548 Å during large outbursts; contrary to expected, but rarely seen, blueshifts that otherwise would signify outward explosive mass ejections (putative core driver of exoplanet atmospheric escape). Instead, the FUV hot lines apparently dominantly form in impulsive chromospheric shocks caused by inward directed particle beams accelerated by the overlying flare kernel, or possibly cooling downflows in post-flare magnetic loop systems, analogous to ``coronal rain'' on the Sun.

Fast X-ray Transients (FXTs) are minute-to-hours long extra-galactic flashes of X-rays, now discovered at a rate of about 2 per week by Einstein Probe. FXTs have been proposed to arise from double neutron star mergers, tidal disruption events involving an intermediate-mass black hole and a white dwarf, and from off-axis or sub-luminous gamma-ray bursts. Contemporaneous multi-wavelength detections, possible only in the current Einstein Probe era, show that FXTs indeed originate from more than 1 progenitor. I will show the most recent findings including Chandra and JWST multi-wavelength follow-up observations, discussing examples of each of the three progenitors mentioned above.

Abstract not available.

I will discuss theoretical models for the evolution of tidal disruption event accretion flows -- transient disks formed when an unfortunate star is ripped apart by the tidal forces of a super-massive black hole. These disks are now known to dominate the emission observed across multiple wavelengths at different stages in a TDE systems evolution. I will discuss how Chandra has helped shape three exciting avenues of research: (i) accretion disk state transitions in super-massive black hole systems; (ii) quasi-periodic eruptions and their TDE connection; and (iii) off-nuclear ''wandering'' super massive black holes revealed by tidal disruption events. I will argue that TDEs offer clean probes of accretion disk physics, black hole demographics and novel transient phenomena, and discuss what fundamental questions in astrophysics may be answered by continued follow up of these systems with sensitive X-ray instruments.

Although large TDE samples now span a wide range of optical/UV and X-ray behaviors, the physical origin of extreme variability and its link to reprocessing remains unclear. I will present a remarkable TDE in a previously inactive galaxy that shows a years-long nuclear outburst punctuated by a giant, hard X-ray flare lasting about a month. The X-ray emission is hard-dominated throughout and ends in a rapid, order-of-magnitude decline. Strikingly, the UV emission exhibits a simultaneous sharp drop, directly coupling the high-energy engine to the reprocessed component. The long-term X-ray evolution is further modulated by four large-amplitude oscillations whose recurrence times increase approximately geometrically; the giant flare occurs near the peak of the third oscillation and is accompanied by pronounced UV dips. I will discuss implications for episodic inner accretion and time-dependent reprocessing in TDEs.

Transients associated with neutron star systems come in a variety of flavors, such as episodically bursting X-ray binaries, flaring magnetars, ultraluminous X-ray sources, and high-mass gamma-ray binaries, and exhibit a dizzying array of observational behaviors at X-ray energies. In this talk, I will provide illustrative examples of how Chandra has made and continues to make unique contributions to advancing our understanding of the neutron star transient menagerie.

The Rapid Burster MXB 1730-335 is a peculiar system discovered in 1976. It is a unique LMXB since it shows both type-I and type-II X-ray bursts. For about 50 years, despite a few attempts to search it, the optical/infrared counterpart remained undetected, mainly because of the challenging location in the innermost region of the largely extincted and crowded complex stellar system Liller 1. In this talk I will report the possible identification of the optical counterpart. The identification was performed by taking advantage of a set of images acquired with the Hubble Space Telescope/Advanced Camera for Surveys in the optical band, and with the Gemini South Telescope in the near-infrared. The analysis of these images revealed a star with a position that might be compatible with the X-ray and radio-band coordinates of the Rapid Burster, and it varies significantly in the optical. According to its location in the color-magnitude diagram, the candidate companion appears to belong to the young (only ∼1−2 Gyr old) supersolar metallicity ([M/H]=+0.3) subpopulation recently discovered in Liller 1. Finally, I will also discuss the need for further observations to confirm the association with the X-ray source and to better clarify the physical properties of the system.

Cas A is a nearby supernova remnant (SNR) with an X-ray point source at the center of the SNR believed to be a young neutron star (NS). Light echo observations suggest it is a Type IIb SN, and some models suggest that these types of SNe are the result of a binary system. In an attempt to shed more light on Type IIb SN in general, and Cas A in particular, I will present in this talk a novel method to search for a companion of the NS remnant. This method consists of using the measured X-ray spectrum of the NS observed by the Chandra X-ray Observatory over 3 months with a total of ~ 85000 X-ray photons, to search for the signature of a companion as a result of the Doppler beaming effect on the NS X-ray spectrum. Using injection simulations and an analytical treatment, I will show that our method is sensitive to companions with orbital velocities ≳ 2000 km/s, and that we currently do not find the beaming signal for which the false alarm probability for all trials is < 1%.

Interstellar dust plays a vital role in galactic evolution: it cools interstellar clouds, enabling star formation; provides surfaces for complex chemical reactions; and serves as a fundamental building block for new planets. Yet, the dust grain properties remain an open question in astrophysics. High-resolution X-ray spectroscopy offers a powerful means to probe interstellar dust. The X-ray band is particularly sensitive to absorption from K- and L-shell electrons of abundant interstellar elements such as C, N, O, Ne, Si, Mg, and Fe. Spectral features of dust appear near the photoelectric absorption edges and reveal the dust grain chemical composition, physical size and lattice structure. I will present our efforts to understand the dust grain properties in both the diffuse and dense interstellar medium using high-resolution X-ray spectroscopy and laboratory data. I will highlight recent results from the diffuse interstellar medium (Psaradaki et al. 2020, 2023, 2025 submitted), based on Chandra and XMM-Newton data from dedicated observing and laboratory campaigns. I will then discuss new results on dust grain chemistry in dense interstellar environments from recent XRISM observations of the bright X-ray binary GX 340+0 (Psaradaki et al., in preparation). Finally, I will summarize our current understanding of dust grain composition across different Galactic environments using X-ray observations - from Chandra to XRISM - highlighting the unique capabilities of Chandra for this science.

NASA’s 4th TDAMM workshop occurred in Huntsville, AL on Oct. 27-30, 2025, and was focused on the discussion of rare, high-importance transient events - cases with trigger rates much less than 1/year for a particular class, where facility response time is key and there is broad community agreement that flagship facility-level observations are worthwhile. These events may be intrinsically rare detections (e.g. kilonovae) or more common events in a particularly fortuitous location (e.g. Galactic supernovae). Events from the classes of GRBs, X-ray binaries, neutrino MMA events, compact binary mergers, TDEs, magnetars, novae, and supernovae were discussed. An open white paper is being written that will describe these rare trigger events and develop community observing plans to enable the rapid and coordinated follow-up of these time-sensitive explosive transients by space-based and ground-based observatories, and community feedback on the white paper is welcome. This talk will provide a broad overview of the workshop’s outcomes, the observing plans being discussed in the white paper, and potential next steps for implementing the community observing plans.

The Astro2020 Decadal Survey recommended an investment in time domain and multimessenger (TDAMM) astrophysics as a top priority for the coming decade. In response, NASA commissioned the Astrophysics Cross-Observatory Science Support (ACROSS) initiative to investigate ways of increasing the operational efficiency of NASA’s fleet of observatories, enabling more agile follow-up of TDAMM events. In this talk I will describe the software infrastructure that has been developed by ACROSS to meet this goal. I will detail the real-time science situational awareness data, such as standardized observation schedules and visibility calculators, made available through ACROSS’s publicly accessible API and web portal. I will also discuss scientific analysis tools, including a user-friendly client to interface with the ACROSS system, as well as in-development Target of Opportunity (ToO) functionality modularized across NASA missions to enable joint observations of TDAMM events. Finally, I will conclude by discussing how Chandra fits into the ACROSS system and how ACROSS can support a potential Chandra community ToO follow-up program.

We present a novel classification system, SOMClassifier, that employs Self-Organizing Maps (SOMs) and the spectral and time-variability properties of 36,904 X-ray detections in the Chandra Source Catalog version 2.1 to investigate their astrophysical nature, a critical step for time-domain investigations. By training the SOM on labeled sources, we create a two-dimensional map where similar objects cluster together and that is used by the SOMClassifier to assign high statistical significance labels to 6,225 previously unclassified detections. The SOM model and the classification workflows are now publicly available via a web application, that provides an essential resource for rapidly identifying newly discovered transient or unknown X-ray sources.

Due to degradation of Chandra's thermal insulation, mission planning has largely become about maintaining a delicate balance between the temperatures of various on-board subsystems, while satisfying scientific, engineering, and other observing constraints. Despite these significant challenges, Chandra continues to operate with an extremely high level of success, with key performance metrics remaining on par with historical values. This success has been made possible by several (mostly proactive) updates to processes, policies, and software that alleviate the effects of changing spacecraft conditions. I will discuss the mission-planning process and the issues that drive it, how targets of opportunity are incorporated into this process, how Chandra has historically supported TDAMM science, and possible operational approaches for maximizing that support through the remainder of the 2020s and beyond.

Fast Blue Optical Transients (FBOTs) are characterized by luminous, rapidly evolving, blue emission that cannot be explained by standard supernova models. The luminous sub-population of FBOTs (LFBOTs), with AT2018cow as its prototype, represents a rare and unique class that exhibits remarkable observational characteristics. Over the past decade, multiwavelength observations have transformed our understanding of these events, providing key insights into their energetics and environments. In particular, X-ray observations have revealed luminous and highly variable emission pointing to a central engine rather than shock interaction alone powering these explosions. In this talk, I will review major advances in the field of LFBOTs with an emphasis on the role of Chandra. I will conclude by outlining the key open questions and future directions in the field, emphasizing the importance of continued multiwavelength observations for uncovering the nature of these exotic transients.

Late-time observations of type II supernovae (SNe II) are a window into the uncertain mass-loss history of their red supergiant (RSG) star progenitors as well as a direct probe of the physics of shock interaction as SN ejecta collide with distant circumstellar material (CSM). To date, a growing number of SNe II are being detected years to decades after their radioactive decay power has become subdominant, indicating that they are powered almost exclusively by shock power generated from the interaction of the SN ejecta with the RSG wind. Here we present multi-wavelength (X-ray - MIR) observations of CSM-interacting SNe II, including the some of the closest SNe II of the decade 2023ixf and 2024ggi, which allow us to robustly constrain the thermalization efficiency of shock power as well as quantify the unknown mass-loss rates of RSGs in their final decades, centuries and even millennia before explosion. We highlight to essential role of Chandra and XMM-Newton observations in constraining the shock physics of old SNe. We will present non-LTE radiative transfer simulations that are able to reproduce the complete UV-MIR spectra of such events through the unique combination of X-ray shock power and dust emission. Finally, we will highlight the new “X-ray Supernova Analysis Pipeline (XSNAP)” a user-friendly, Python-based tool for easy reduction and analysis of SN X-ray spectra from all available X-ray telescopes.

The evolution of a young supernova (SN) to eventually become a supernova remnant is of importance in understanding the dispersal of mass and energy into the interstellar medium. However it is difficult to study, given that supernovae are generally followed only for years to decades, whereas known supernova remnants are mostly several hundred years old. In order to understand how the shock velocity and temperature evolve over time, it is necessary to follow young SNe for as long as possible, especially at X-ray wavelengths. The sensitivity of the Chandra telescope helps significantly in this regard, as it is possible to monitor young SNe to quite low flux levels. In this talk we will discuss 3 young SNe that have been monitored closely at X-ray wavelengths, primarily with Chandra, for over a decade. SN 2011dh, a Type IIb SN, has 14 years of X-ray data. SN 1993J, a Type IIb, and one of the closest SNe in the northern hemisphere when discovered, was regularly monitored for over 30 years. SN 1986J, a Type IIn SN that probably exploded in 1983, has an astonishing 40+ years of X-ray observations. And we have an ongoing campaign to observe it with the Chandra telescope. We show the X-ray light curves and spectra, discuss the evolution of various dynamical and kinematical quantities that can be derived from the X-ray emission, and what they reveal about the SN evolution. In some cases, the mass-loss rate of the medium in which the SN evolves can be evaluated. We will also compare the X-ray lightcurves to those at other wavelengths, primarily in the radio. This talk will highlight the role of Chandra's excellent spatial resolution in these observations. SN 1986J has several bright X-ray sources within 30 arc-seconds. SN 1993J has an X-ray binary about 1 arc-minute away, and the bright galactic nucleus 3 arc-minutes away. However Chandra's exquisite spatial resolution allows us to clearly delineate the emission of the source from that of surrounding sources, enabling us to obtain an accurate X-ray light curve.

Based on archival Chandra ACIS data in two epochs separated by 10-year time baseline, we present a preliminary result from our study of 3-D ejecta kinematics of the oxygen-rich Galactic supernova remnant G292+1.8. We employed a two-dimensional image cross-correlation method to measure the proper motions of ejecta knots across the entire remnant. Our measurements yield velocities that vary from ~1000 km/s to over 3000 km/s, assuming the distance of ~6 kpc to the remnant, generally exhibiting radial expansion from the remnant’s optical expansion center. Combining our proper motion measurements with radial velocities for common ejecta knots in the literature, we estimate the space velocities for a subset of our sample ejecta knots. Our estimated space velocities for ejecta knots range from ~1900 to ~2300 km/s. Based on this space velocity distribution of ejecta knots, we discuss 3-D kinematic structure of clumpy metal-rich ejecta in G292.0+1.8.

X-ray spectra of novae manifest the most exciting physical processes, and provide us with the perfect laboratory for studying them. The talk will present new results of high-resolution nova spectra that have been a mystery for two decades. Our novel analysis methods reveal several incidents of unique nucleosynthesis that result in clean nuggets (~10^-7 solar mass) of intermediate-mass elements (Si, S, Ar) ejected from the white dwarf into the interstellar medium. The transient ejections last for a few hours only; the ejecta are shock-heated, and identified for the first time through their mixing with cold ambient gas. The mixing produces narrow radiative recombination continua (RRCs) and high-order lines resulting from distinct charge exchange (CX) collisions with neutral species. In fact, the X-ray spectra of nova feature the most unambiguous evidence of CX outside the solar system. We interpret these results as indicative of turbulence, which is particularly intense in novae of heavy white dwarfs approaching the Chandrasekhar mass.

The emergence of high-cadence time-domain surveys like TESS and LSST has highlighted a critical need for high-resolution X-ray diagnostics to decode the physics behind optical variability. In this review, we synthesize over two decades of Chandra observations of magnetic cataclysmic variables (mCVs), symbiotic stars, and novae to demonstrate how sub-arcsecond imaging and grating spectroscopy (HETG/LETG) remain the "gold standard" for understanding accretion and outflows. We will revisit foundational results from the intermediate polar EX Hya, where Chandra HETG spectra were instrumental in testing cooling flow models and resolving long-standing controversies regarding white dwarf masses. We then examine the broader impact of Chandra on the study of classical and recurrent novae, spanning from the spatial resolution of the RS Oph bipolar ejecta to the detection of supersoft X-ray from novae during their post-outburst phase. Finally, we connect these classic studies to recent "dynamic" discoveries, such as the extreme spin-down and "discless propeller" state of V1405 Cas. We discuss how the legacy of Chandra provides the necessary physical framework to interpret new transients, emphasizing its ongoing role in providing the spectral ground truth required to model the complex interaction between white dwarf magnetospheres and their environments in the TDAMM era.

Cosmic microwave background telescopes have brought the millimeter-band into the range of wavelengths for which wide field transient surveys can be conducted. Until very recently, this work had focused largely on high Galactic latitude regions of the sky. The South Pole Telescope has been conducting Galactic Plane surveys for about one month each year since 2023. The first two transients seen in these data are both wide binaries with accreting white dwarfs, and both had already been seen by Chandra and Swift, which was essential for their identification as the correct counterparts to the millimeter transients. These flares likely represent magnetic reconnection events in the accretion disks of the binaries, a type of transient which may start to become more commonly seen from a variety of classes of accreting compact objects in upcoming time domain surveys across a range of wavelengths.

V1723 Sco (Nova Sco 2024) was monitored in X-rays for several months. The only Chandra high spectral resolution observation with the HETG caught it in the "shock phase", reveling peculiar abundances that are typical of side branches of CNO burning on oxygen-neon novae, though to occur on oxygen-neo white dwarfs. I will describe the Chandra spectrum and the results of the long term monitoring that has involved Swift, NICER and XMM-Newton and compare the Chandra HETG spectrum with previous, very different but spectacularly interesting spectra of novae obtained with both the LETG and the HETG.

We present recent NICER monitoring of the high-mass X-ray binary IGR J16320-4751, comprising approximately 40 observations over multiple orbits. These data were used to measure absorption and constrain its variation over the orbital phase. In addition, we also present the first Chandra grating observation of this source. We performed detailed spectroscopic measurements of the complex Fe line emission, including Fe K$\alpha$, Fe K$\beta$, and the ionized Fe XXV and Fe XXVI lines, as well as the K$\beta$ absorption edge. We place upper limits on the Compton shoulder of the Fe K$\alpha$ line, which will constrain the geometry and composition (e.g., gas-to-dust ratio) of the clumpy absorbers surrounding the compact object in the source. In addition to direct spectral fitting, we will use the XSTAR package to simulate the physical conditions in and around IGR J16320-4751, to derive more quantitative constraints on the ionization of the absorber.

Gamma-ray bursts (GRBs) are the most luminous explosions in the Universe. While long-duration GRBs signal the deaths of massive stars, short-duration GRBs originate from the mergers of two compact objects. It has been twenty-seven years since the first detection of a GRB with Chandra, and this flexible and unique facility still continues to advance our knowledge of the explosion physics and progenitor systems that power GRBs. In this talk, I present some of the seminal observations of GRBs as seen through Chandra’s eyes. These observations have revealed evidence for jet collimation, uncovered mysterious behavior such as X-ray flares and excess emission, and localized their positions to sub-arcsecond precision within their host galaxies, all of paramount importance for understanding their stellar progenitors. I will also highlight how how Chandra’s natural synergy with other facilities have lead to breakthrough discoveries.

Fast radio bursts (FRBs) are extremely luminous, short-duration radio transients produced by unknown astrophysical sources outside the Milky Way. While the detection of an X-ray burst accompanying an FRB-like radio burst from the Galactic magnetar SGR 1935+2154 strongly supports a magnetar origin for at least some FRBs, no confirmed X-ray counterpart has yet been detected from any extragalactic FRB source. Nearby FRBs therefore provide a unique opportunity to search for multiwavelength emission and constrain FRB progenitor models. I will present results from a sensitive, simultaneous X-ray and radio campaign targeting FRB 20200120E, the closest known extragalactic repeating FRB source, located in a ~10 Gyr-old globular cluster within the M81 galactic system. Deep observations with the Chandra X-ray Observatory were critical for placing the most stringent constraints to date on persistent X-ray emission from an extragalactic FRB source, ruling out an association between FRB 20200120E and ultraluminous X-ray sources. Prompt X-ray limits from coordinated observations with NICER and XMM-Newton further disfavor several classes of high-energy burst emission from young magnetars, including giant flares and SGR 1935+2154-like intermediate flares. Our results favor source classes consistent with the globular cluster environment, such as a giant radio pulse-emitting pulsar or a magnetar formed through a delayed evolutionary pathway, and demonstrate the power of sensitive X-ray observatories for uncovering the high-energy nature and origins of nearby FRBs.

Although they were not designed as transient detectors, I have been using imaging X-ray telescopes such as Swift/XRT and NuSTAR to hunt for serendipitous X-ray transients in the fields of view of pointed observations. This search has been fruitful, uncovering a tidal disruption event (Brightman et al. 2021), several transient ultraluminous X-ray sources (Brightman et al. 2023) and a sample of fast X-ray transients detected by NuSTAR (Brightman et al. 2026). I will discuss these results and how Chandra’s sub-arcsecond positional accuracy is key in the identification and characterization of these events.

The origin of the extended X-ray emission from the microquasar SS 433 presents a critical test of its classification as an ultraluminous X-ray source (ULX) analogue. The extended emission has previously been attributed to reheating by internal shocks within its jets; however, an alternative hypothesis proposes that this emission is reflected light from an obscured, super-Eddington central engine. To discriminate between these two models, we conducted a variability test using the Chandra and Swift observatories to search for a time-delayed correlation between the core X-ray flux and the extended features. Although the extended emission was weaker than in earlier epochs, we resolved compact brightenings at 0.8″ to 1.5″ from the core, many of which are consistent with the expected jet trajectories when modest velocity variations are allowed. Their locations support a reflection interpretation, but require a broader or more conical illuminating beam than the narrow pencil-beam geometry originally assumed.

Accreting supermassive black holes are intrinsically variable, changing in brightness across the electromagnetic spectrum and over a wide range of timescales. In recent years, a remarkable subset of active galactic nuclei has been found to exhibit extreme spectral variability. These changing-state events are thought to be driven by rapid and dramatic changes in the black hole accretion rate, challenging the standard expectations for accretion timescales in systems hosting black holes with masses of millions to billions of solar masses. Such transitions provide a unique opportunity to probe the structure and evolution of accretion flows around massive black holes. In my talk, I will present recent observational results on nearby changing-state AGN, highlighting how their X-ray emission evolves as a function of the mass-normalised accretion rate and what these changes reveal about the structure of the accretion flow.

The first supermassive black holes (SMBHs) are seen reaching masses in excess of one billion solar masses less than one billion years after the Big Bang. Even with massive seeds, for black holes to attain such masses so rapidly they must have extreme accretion, averaging a sustained Eddington rate for hundreds of millions of years. Yet exactly how well these AGN can sustain their accretion is not clear. In this work, I will present X-ray observations of J1429+5447, the most luminous X-ray source known at z > 6. In independent observations with NuSTAR and Chandra 110 days apart (~2 weeks rest frame), the source flared in brightness a factor of ~3x in the shared 3--7 keV band. Since that time, we have obtained two additional epochs of Chandra observations. I will discuss these results, contextualize two additional high-redshift quasars with X-ray-detected flux variation, and describe ongoing efforts to measure X-ray variability at Cosmic Dawn.

Timing studies have demonstrated that AGNs are variable. But, except for episodical flaring are state change, we still do not have clear observational examples of their secular variability cycle. Accretion disk modelling suggests time scales of order thousands of years, way longer than our current human observing cycle. Sub-arcsecond observations of the spatial features of the interstellar medium (ISM) of AGN host galaxies gives us a means to explore AGN variability on longer time scales. Chandra high resolution imaging of nearby AGNs have constrained variability times ranging from ~100 to ~104-5 yrs. Together with high resolution optical excitation mapping with Hubble they can map the secular interaction of the AGN with its host. This talk will present the results of recent work in this field and illustrate how sub-arcsecond imaging associated with high-resolution spectra is essential for a future X-ray flagship.

The origin of high-energy extragalactic neutrinos is still an open question. It is unknown where and how cosmic rays, which are the progenitors of high energy neutrinos, are accelerated to such high energies to produce neutrinos with energies in the TeV range. Recently, there has been promising evidence from the IceCube Neutrino Observatory that TeV neutrinos are produced in AGN corona, with > 4σ detection of TeV neutrino emission from the central region of NGC 1068. The emission was constrained to within 15 Schwarzschild radii of the central supermassive black hole, and a likely candidate for the emission region is the AGN corona. X-ray observations provide the best probes of the physical processes in AGN corona and may provide insight into the corona as a potential origin of neutrino emission, but the continuous monitoring of AGN needed to construct a detailed timeline of the corona is too expensive with our most sensitive X-ray instruments. We discuss the potential of using data from the Swift Burst Alert Telescope with supplemental data from a fleet of pointed X-ray telescopes, including Chandra, to discover potential correlations between the X-ray emission and neutrino emission in nearby AGN in this interesting case of multimessenger astrophysics.

We present the first systematic study of short-timescale X-ray variability in radio-quiet active galactic nuclei (AGN), utilizing archival Chandra observations of approximately 3000 broad-line AGN selected from the SDSS and DESI. We identify 14 AGN exhibiting rapid (on timescales of tens of kiloseconds) X-ray flux variations by factors of 2 or more that are statistically significant, indicative of fast coronal variability. By converting minimum variability timescales to light-crossing times, we place upper limits on the sizes of the variable coronal regions, finding typical scales of <10−4 pc. The coronal variable region size upper limits of an AGN in our sample are found to be much smaller than the typical coronal sizes inferred from microlensing, suggesting that its corona is composed of localized, transient structures rather than smooth, homogeneous plasmas. This is the first evidence of a clumpy, turbulent corona of AGN, thanks to the extremely low background of Chandra. Such efficient magnetic energy dissipation in compact volumes is consistent with expectations for magnetically dominated coronae and is supported by recent general relativistic magnetohydrodynamic simulations.

X-ray Quasi-Periodic Eruptions (QPEs) are the latest and perhaps the most peculiar addition to the zoo of extragalactic nuclear transients. They are high-amplitude bursts of X-ray radiation recurring every few hours to few days, with a lifetime of years, originating from the central black holes in the nuclei of low-mass galaxies. So far, only a dozen such events have been found, although with rising interest in the broader community given their observational and theoretical connection with tidal disruption events and, possibly, low-frequency gravitational wave sources. I will outline the observational properties of QPE sources and the latest insights from our tests of the current theoretical models. I will conclude with an outlook on the current and future landscape of X-ray facilities and their capabilities for carrying out transformative QPE science, with a focus on Chandra’s possible role.

The 4 million solar mass supermassive black hole at the Galactic center, Sgr A*, produces hourly-timescale “flaring” emission across the electromagnetic spectrum. At X-ray energies, flares typically occur approximately once per day at a peak brightness 4 to 600 times brighter than the background Bremstrahlung emission. Simultaneous X-ray observations with other multiwavelength facilities have shown that all X-ray flares have IR counterparts, suggesting a physical connection between these different regimes. Yet, not all IR flares have an X-ray counterpart, hinting that two different mechanisms are responsible for their production. Chandra, with its high angular resolution and superior sensitivity, has been the primary X-ray instrument over the last 25 years of Sgr A* monitoring, having observed the Galactic Center for a total of approximately 7 Ms. Many of these observations were taken in concert with multiwavelength facilities including JWST/MIRI and SMA, enabling tests of single-zone emission models by combining the submillimeter linear polarization measurements with multiwavelength data. We will focus on initial results from joint multiwavelength campaigns in 2024 and 2025 with these observatories.

Sagittarius A* (Sgr A*), the supermassive black hole (SMBH) at the center of the Milky Way, exhibits rapid (minutes to hours-long) flaring behavior in the sub-millimeter, infrared, and X-ray. In this study we present the Chandra 25-year Sgr A* X-ray flare catalog: a systematic analysis of 6.8 Ms of Sgr A* monitoring spanning Chandra's mission lifetime. This is the most complete Chandra Sgr A* X-ray flare catalog to date, consisting of 100 flares with 2-10 keV unabsorbed luminosities ranging from ~4x10^33 - 6x10^35 erg s^-1. 18 flares are reported for the first time, including the second brightest Sgr A* flare ever observed by Chandra. The expanded dataset supports previous indications of a correlation between X-ray flare hardness and luminosity, showing a change in the X-ray spectral index from 3 to 2 with increasing flare brightness. The flare sample exhibits correlations between flare duration, fluence, and maximum count rate. These results likely reflect variations in the underlying particle distribution that produce weak and strong flares. The new catalog serves as a rich archive for ongoing observational and numerical investigations into the physical mechanisms driving SMBH accretion and variability.

The XRISM observatory is providing meaningful new insights into the nature of ultra fast outflows observed in X-rays. These winds with velocities of 0.1c and higher have been known for more than a decade. Nonetheless, XRISM’s combination of unprecedented high spectral resolution above 6 keV, along with its high collecting efficiency at these energies has revealed the outflows to be both clumpy and transient on day timescales. The talk will describe the observations, the uniqueness of the data, and how they change our understanding of the launching mechanism and impact of these powerful winds. Specifically, it will present new evidence tying the outflows to the AGN corona.

We report on a high variable X-ray transient that is being followed up with the Chandra telescope. When first detected, it barely scraped into ultra-luminous X-ray (ULX) source territory. Four years later, a Chandra observation revealed that the flux was up by around a factor of 3, clearly delineating it as a ULX. Less than two months later the flux had decreased by about 10%. Two months after that the count rate dropped by about a factor of two, and has dropped slightly another 1.5 months later. We fit both a power-law and a diskbb model to the source, and discuss its properties.The ULX shows clear variations not only in its flux but also the X-ray properties with time. As a population, ULXs are known to show a wide range of flux changes, especially when changing accretion states. In this talk we will speculate upon the identity of the compact object, and discuss the nature of this interesting source. These observations would not have been possible without Chandra's superb spatial resolution. The ULX is surrounded by several bright X-ray sources within about 40 arc-seconds, with the closest being about 20 arc-seconds away. Only the Chandra telescope can effectively resolve the X-ray emission from the source.

We present X-ray spectral analysis of a low-mass X-ray binary system, MS 1603.6+2600, to constrain the plasma properties very close to the compact object. MS 1603.6+2600 (UW Coronae Borealis), we investigated its accretion disk corona (ADC), where X-rays from the binary illuminate the disk, heating gas that settles into a corona just above/below the disk. Chandra/HETG observations revealed flares in the X-ray spectra; we detected a 6.7 keV line during the flare, likely originating in the corona. This provides the first direct evidence of a corona in this system. Together, our results offer new insights into the structure and dynamics of plasma near compact objects in low-mass X-ray binaries, demonstrating the power of time-resolved spectroscopy to constrain the physical environment near the compact objects in the system.

The Galactic center contains the highest concentration of X-ray sources in the Milky Way.  Nearly ten thousand X-ray point sources were previously detected by Chandra in the central ~2°0.8° of the Galaxy.  We present a new, expanded X-ray source catalog of the central 6°4° of the Milky Way, based on Chandra/ACIS-I observations with exposures ranging between 4–6 ks. With a contribution of thousands of previously undetected sources, our survey will substantially increase the known X-ray population in this region. To construct the catalog, we combine wavelet-based source detection with a maximum likelihood estimation (MLE) algorithm adapted from the CSC pipeline to classify candidate sources. This pipeline improves source reliability, particularly for faint sources in the high-background Galactic center region. This catalog will provide a robust foundation for future studies of Galactic center X-ray source populations – including multi-wavelength classification methods – and will contribute to a more complete understanding of compact object populations and their distributions in the Galactic center and bulge.

Ultra-luminous X-ray sources (ULXs) are generally defined as non-nuclear point sources that have a high luminosity > 1e39 erg/s (assuming isotropic emission), which corresponds to the Eddington luminosity of a 10 solar mass black hole. The nature of most ULX systems is unknown, but they are thought to harbor a compact object. It is generally believed that the population of ULXs as a whole comprises mainly of super-Eddington accreting compact objects, with several being confirmed as neutron stars since 2014. In this poster we investigate the X-ray luminosity of 2 ULXs in the galaxy NGC 891, labelled ULX1 and ULX2. ULX1 has been persistently visible in X-ray observations starting from the ROSAT observation in 1991, and is still persistent in our ongoing series of bimonthly Chandra observations. ULX2 appeared in 2011 XMM and Chandra observations, and is detectable in the Mar 2026 observation. It appears to be fading over time. We report on the lightcurves and spectra of the two ULXs, and describe the variation in luminosity on multi-year timescales as well as over bimonthly intervals. Given the distance between the two ULXs of around 24 arc-seconds, as well as the presence of other sources nearby, Chandra's exceptional spatial resolution is needed to isolate the emission from the source and accurately measure the flux.

We present detailed magnetohydrodynamic simulations of the merging galaxy cluster MACS J0018.5+1626 as part of the ICM-SHOX project, which combines Chandra X-ray, SZ, optical, and lensing observables to constrain merger geometry. Our modeling indicates a binary merger near pericenter passage, viewed nearly along the merger axis. Three-dimensional MHD simulations coupled to tracer particles and a Fokker–Planck solver consistently produce two merger-driven shocks with Mach numbers M_s ~ 2–3, matching the cluster's multi-wavelength signatures. We assess the conditions under which diffusive shock acceleration can reproduce the observed LOFAR radio emission and key correlations between the radio and Chandra X-ray surface brightness.

The X-ray afterglow plateau phase of Gamma-Ray Bursts (GRBs) holds key hints about the ongoing energy injection from a long-lived central source, like magnetar spin-down or fallback accretion. Even though the well-known empirical 1D Dainotti anti-correlation between plateau luminosity (L_a) and rest-frame plateau duration (T_a_rest) is pretty much accepted, the fact that it has noticeable scatter feels like something is missing in the actual event energy budget. Here we look into this multivariate setting using an agnostic symbolic regression search on 251 long GRBs observed with Swift. The method finds a very strong 3D prompt–plateau “fundamental plane” that ties the prompt emission energetics straight to late-time X-ray afterglow structures. The best-fit plane from multivariate regression is log L_a = 4.718 + 0.776 log L_peak - 0.805 log T_a_rest producing a tight relation ( R = 0.91 ) . For the energy-centered sanity checks, the plateau energy proxy E_plateau is proportional L_a * T_a_rest turns out to be deeply entangled with the prompt peak luminosity L_peak, so a simple constant-ratio picture seems unlikely. To separate the physics signal from observational artifacts, we apply an Efron-Petrosian style de-evolution, and we also do distance-controlled residualization. After that, the intrinsic connection still looks solid at R^2 ≈ 0.62, meaning there is likely a real physical coupling between the prompt radiation channel and the later, continuous afterglow injection stage. Overall, these results suggest the classical Dainotti relation is more like a lower-dimensional slice of a wider coupled energy–timescale structure, one that is governed by a single unified central engine reservoir, in some sense.

Bispectral analysis of timing structures offers a novel approach to interpreting QPOs in time-variable systems. The bispectrum is a three point correlation function evaluated between two frequencies and the sum of the frequency pair. In this work, we present bispectra of three Chandra observations of the AGN MCG-6-30-15 in which we identify "hypotenuse" structures in the mHz frequency range. This has analogy to timing investigations of the black hole X-ray binary GRS 1915+105, in which bispectra showed features including the hypotenuse, revealing nonlinear coupling between QPOs and noise, which can not be discerned from power spectra alone.

The historical Supernova (SN) 1181 is a peculiar SN in our Galaxy, whose remnant has been extensively observed from radio to X-rays. The SN today hosts a bright white dwarf (WD) remnant that is launching an extremely fast (15000 km/s) wind, which may have been created from a past WD merger. Past Chandra observations in 2021 have revealed a compact yet extended central X-ray source, whose origin remains poorly understood. I will present our ongoing theoretical work towards understanding this enigmatic central source, and future prospects from the planned observations in Chandra Cycle 27 (PI H. Suzuki) that would help distinguish the proposed scenarios.