Posters

1

Vladimir Karas (Astronomical Institute, Prague)

Acceleration of electrically charged particles along an escape corridor from an accretion disk: the onset of chaos near a magnetised black hole

We examine a mechanism of destabilisation of equatorial orbits of electrically charged particles and dust grains. Near a magnetised black hole, initially bound matter can be accelerated along trajectories emerging from an accretion disk that eventually may escape in the vertical direction. A fraction of these trajectories exhibit chaotic behaviour; even for large-scale, ordered magnetic fields it appears that the chaotic dynamics controls the outflow. We employ Recurrence Plots to characterize the onset of chaos in the medium. The role of black hole spin and the magnetic field strength are discussed, and the maximal escape velocity is computed (based on a recent paper, Kopacek & Karas 2018, ApJ, 853, id. 53, 2018; arXiv:1801.01576).

2

Zhuo Chen (University of Rochester)

General Equation of State Astrophysical Flow

The need to incorporate complex equations of state (EOS) in hydrodynamics is becoming increasingly important in computational astrophysics. The state-of-the-art of many numerically intensive research problems that include radiation transfer in such diverse subfields as accretion disks, binary mergers, star formation, supernovae, are approaching the state where more accurate EOS relating fluid state variables (e.g. density, pressure and temperature) are needed. Although the assumption of an ideal gas EOS with a fixed constant ratio of heat capacities/adiabatic index (γ) is prevalent, such constancy is usually not fully justified, especially where ionization or phase transitions occur (e.g. hydrogen ionization transition). When C_V and γ vary drastically, the assumed constancy will be inapplicable. In this research, we present methods to solve the Riemann problem exactly for quite general equations of state (EOS) to facilitate realistic modeling and understanding of astrophysical flows. The existence and uniqueness of the Riemann Solution can be guaranteed if the EOS is convex. We confirm that: (1) the solution of the exact general EOS Riemann solver and the solution of the original exact Riemann solver match; and (2) the solution of the Harten-Lax-van Leer-Contact (HLLC) general EOS Riemann solver approaches the exact solution.

3

Evan Nunez (California State Polytechnic University, Pomona)

Characterizing Intermediate-Mass Pre-Main-Sequence Stars via X-Ray Emission

The X-ray emission from intermediate-mass, pre-main-sequence stars (IMPS) can provide useful constraints on the ages of very young (<5 Myr) massive star forming regions. IMPS have masses between 2 and 8 M_☉ and are getting power from the gravitational contraction of the star. Main-sequence late-B and A-type stars are not expected to be strong X-ray emitters, because they lack the both strong winds of more massive stars and the magneto-coronal activity of lower-mass stars. There is, however, mounting evidence that IMPS are powerful intrinsic x-ray emitters during their convection-dominated early evolution, before the development and rapid growth of a radiation zone. We present our prime candidates for intrinsic, coronal X-ray emission from IMPS identified in the Chandra Carina Complex Project. The Carina massive star-forming complex is of special interest due to the wide variation of star formation stages within the region. Candidate IMPS were identified using infrared spectral energy distribution (SED) models. X-ray properties, including thermal plasma temperatures and absorption-corrected fluxes, were derived from XSPEC fits performed using absorption (N_H) constrained by the extinction values returned by the infrared SED fits. We find that IMPS have systematically higher X-ray luminosities compared to their lower-mass cousins, the TTauri stars. This work is supported by the National Science Foundation under grant CAREER-1454334 and by NASA through Chandra Award 18200040.

4

Michael McCollough (SAO/CXC/CfA)

A phase/time dependent HETG Chandra study of the Wolf-Rayet X-ray binary Cygnus X-3

Cygnus X-3 is a well known high mass X-ray binary (HXRB) which is also a Wolf-Rayet (WR) X-ray binary. It is believed that this type of system will be a progenitor of a double compact object binary. This will, in turn, lead to a merger that produces gravitational waves. In this study, we examine the interaction of the winds from the massive WR star with the compact object using Chandra HETG data. The Chandra HETG data for Cygnus X-3 show many strong lines (emission and absorption), radiative recombination continua (RRCs), and P-Cygni profiles on many of the emission lines. The 3rd order HETG spectra provide a more detailed look at the Fe K alpha spectral region, in particular, the He-like Fe line. During Cygnus X-3's hypersoft/quenched state we a find variable absorption feature which is "blueward" of Fe K alpha. Some of the other features show a phase/time dependencies which will be examined in this presentation.

5

Norbert S. Schulz (Massachusetts Institute of Technology)

The Young Be-Star X-Ray Binary Cir X-1

Accretion conditions and morphologies of X-ray transients containing neutron stars are still poorly understood. Circinus X-1 is a specifically enigmatic case where we observe X-ray flux changes covering four orders of magnitude. We observed Circinus X-1 many times since the launch of the Chandra X-ray Observatory using the high energy transmission grating spectrometer and each time the source gave us a vastly different look. Most recently we caught the source at its very lowest X-ray flux at a flux of 1.8×10^-11 erg cm^-2 s^-1. Its spectrum, a single 1.7 keV blackbody spectrum, showed a low emission radius of 0.5 km which implies a high magnetic field between 0.4 and 2.5×10¹¹ G depending on neutron star radius. Photoionized line emissions suggest a large emission volume and low plasma densities. The observed bluehifts of ~400 km s^-1 and emission volume is consistent with the ionized but distorted wind of a B5Ia supergiant companion confirming a previous identification. We argue that the companion of Cir X-1 is fast rotating Be-star and its stellar disk provides much of the observed excess column densities. We paint a scenario in which a precessing oblate Be-star rotator may explain the vast X-ray flux variations in the past.

6

Thanawuth Thanathibodee (University of Michigan)

The End of Accretion: the Case of a Very Slowly Accreting T Tauri Star CVSO 1335

Magnetospheric accretion is the accepted view of accretion in low-mass, pre-main sequence stars (T Tauri Stars; TTS). Under this paradigm, the stellar magnetic field truncates the inner region of the surrounding protoplanetary disk at a few stellar radii and material flows onto the star along the field lines. The accretion flows and the resulting accretion shocks are probed via atomic emission line profiles and the excess over photospheric fluxes, particularly the Balmer jump, respectively. Although this main concept has been accepted, it is still unclear how accretion proceeds at a very low accretion rate, or how accretion stops. Here we report multi-epoch spectroscopic observations of CVSO 1335, a K5 TTS with a very low accretion rate. The star belongs to the Orion OB1b subassociation with the age of 5 Myr, a critical age for the evolution of accretion in which more than 80 percent of the stars in a given population have already lost their inner disk. The spectra do not show any clear excess over chromospheric emission beyond the Balmer jump, indicating that the star is a very low accretor. However, the Hα profiles, observed simultaneously, show a high level of variability, as well as conspicuous, multi-component redshifted absorptions, which provide a direct evidence of the star being an accretor. We estimate the accretion rate using accretion shock models. By modeling Hα profiles using magnetospheric accretion models, we self-consistently estimate the geometry of the stellar magnetosphere and accretion flows. Our results give insight into the properties of T Tauri accretion near its final stages.

7

Saeqa Vrtilek (SAO)

Identifying the nature of point sources in external galaxies

Chandra's exquisite spatial resolution allows it to resolve individual X-ray point sources in nearby external galaxies. We find that point sources associated with external galaxies agree well with distributions of XRBs within our own Galaxy in color-color-intensity diagrams. By comparing these point sources with the location of known source types we will identify the distribution of classes of binaries in external galaxies as well as identify background AGN, super-soft sources, and ultra-luminous sources in external galaxies. We use a probabilistic (Bayesian) model providing a supervised learning approach: unknown classifications are predicted given known classifications. We take a set of objects whose types are well established by other means (e.g., as X-ray pulsars, black holes, non-pulsing neutron stars within our own Galaxy) as a training set and calculate probabilities that an unknown system from an external galaxy is of a given type. Our study provides important information on populations of X-ray sources in different galaxy types, with implications for both the evolution of galaxies and the evolution of XRBs, as well as significant clues about how the different classes of XRBs are related to each other.

8

Dipankar Maitra (Wheaton College)

Astrophysics with a 12-inch telescope: Observations of V404 Cygni During its 2015 Outburst Suggest a Highly Energetic Jet-base

We present some results that show how interesting science can be carried out using small telescopes if you are at the right place at the right time. Specifically, we present V-, R, and I-band optical photometry of the black hole X-ray binary system V404 Cygni obtained using Wheaton College's 12-inch telescope. The correlated multiwavelength flux variability, and the spectral index between V- and I-band, suggest that the optical emission originated very close to the base of the jet. Our data, in conjunction with simultaneous data obtained by INTEGRAL and Swift, strongly suggest that the jet-base was extremely compact and energetic during this phase of the outburst.

9

Sibasish Laha (University California San Diego)

Probing the active galactic nuclei (AGN) Torus structure using X-ray spectral variability

It is still a big puzzle as to how the host galaxy gas (at kpc scales) feeds the accretion disk of a supermassive black hole (SMBH), at sub-pc scales, and thereby fuels the central engines of active galactic nuclei (AGN). Circumnuclear structures such as the cold, dusty "torus" are believed to be an active step in SMBH accretion, and understanding the structure of this medium is essential for understanding disk/SMBH fueling. However, the torus structure is still highly debated, with various theoretical models encompassing continuous or clumpy distributions. A major probe of the torus' properties is the variable absorption detected in the 0.3-10 keV X-ray spectra, which serves to probe the torus' morphology and location. Here, we present the latest results from an extensive X-ray spectral variability study of a sample of 20 Compton-thin Seyfert 2 galaxies using XMM, Chandra, Suzaku and NuSTAR. The column density variations (or lack thereof) enable us to probe various torus morphology scenarios and to also consider possible host galaxy contributions to total absorption. Along the same vein of probing the nature of AGN accretion, we also present results from two additional sample studies: 1. a sample of low luminosity quasars (LLQSOs) in the local Universe, where we find that the accretion properties of these local quasars are similar to those of higher redshift ones, and their lower luminosity scales with having lower black hole masses, and 2. an X-ray study of a sample of local IR-bright galaxies detected with molecular outflows (MOX sample), where we find that both the central AGN as well as starbursts contribute to driving large scale molecular outflows and thereby clearing matter from the central regions of the AGN. These sources have been detected to be extremely X-ray weak, a property also seen in other outflowed AGN such as broad absorption line (BAL) quasars.

10

David Principe (MIT)

Investigating Star-disk Interactions with High Resolution Spectroscopy

Young stars, which host planet-forming circumstellar disks, display evidence of magnetic activity in the form of coronal and accretion generated X-ray emission. High resolution X-ray spectroscopy of young star-disk systems with instruments like Chandra-HETG can distinguish between these X-ray generating mechanisms and, when coupled with optical spectroscopy, can elucidate physical conditions of the accretion shock region. We present near-simultaneous Chandra high-resolution X-ray and SSO optical H-alpha spectroscopy observations of the two actively-accreting star-disk systems T Cha and RY Lup. Both systems have highly inclined viewing geometries and we investigate star-disk interactions in the form of accretion, flaring, and variability arising from inner disk warps rotating into and out of our line of sight.

11

Vallia Antoniou (Smithsonian Astrophysical Observatory)

Different generations of HMXBs: clues about their formation efficiency from Magellanic Clouds studies

Nearby star-forming galaxies offer a unique environment to study the populations of young (<100 Myr) X-ray binaries (XRBs), which consist of a compact object - typically a neutron star or a black hole - powered by accretion from a companion star. These systems are tracers of past populations of massive stars that heavily affect their immediate environment and parent galaxies. The SMC is the ideal environment for population studies of young XRBs by providing us with what the Milky Way cannot: A complete sample of X-ray sources within a galaxy. Using a Chandra X-ray Visionary program, we investigate the young neutron-star binary population in this low-metallicity, nearby, star-forming galaxy by reaching quiescent X-ray luminosity levels (~few times 10³² erg/s). In this talk, I will present the first measurement of the formation efficiency of high-mass X-ray binaries (HMXBs) as a function of the age of their parent stellar populations. We use three indicators of the formation efficiency of young accreting binaries in the low SMC metallicity: the number ratio of the HMXBs, N(HMXBs), to the number of OB stars, to the star-formation rate (SFR), and to the stellar mass produced during the specific star-formation burst they are associated with, all as a function of the age of their parent stellar populations. In all cases, we find that the HMXB formation efficiency increases as a function of time up to ~40-60 Myr, and then gradually decreases. The peak formation efficiency N(HMXB)/SFR is in good agreement with previous estimates of the average formation efficiency in the broad ~20-60 Myr age range. I will also present the deepest luminosity function ever recorded for a galaxy, and discuss the X-ray properties of the largest sample of extragalactic accreting pulsars as well. Moreover, I will present the formation efficiency of HMXBs in the LMC based on the N(HMXBs)/SFR ratio, and discuss how these findings are compared to the lower metallicity environment of the SMC.

12

Matthew Coleman (Institute for Advanced Study)

Convection Affects Magnetic Turbulence in White Dwarf Accretion Disks

I examine the accretion disks which power outbursts in two types of white dwarf binary systems: dwarf novae (DNe) and AM CVns. Accretion disks in these systems are thermally unstable, causing some of the observed variations. The source of "normal outbursts" in these systems ultimately originates from ionization transitions (H for DNe and He for AM CVns). These ionization transitions cause significant temperature dependence in opacities and equation of states, culminating in the occurrence of convection within these accretion disks. Local stratified shearing-box simulations were used to show that this convection has a significant impact on the turbulence and dynamos generated by the magnetorotational instability (MRI). Most notably, convection enhances the stress to pressure ratio, often denoted by alpha. These results were then incorporated into the disk instability model to generate the first theoretical lightcurves for dwarf novae outbursts which incorporate MRI physics.

13

Sergei Dyda (University of Nevada Las Vegas (UNLV))

3D Line Driven Winds: Clumpy Outflows

We study non-axisymmetric features of 3D line driven winds in the Sobolev approximation and discuss implications for O stars, CVs and AGN. We find that non-axisymmetric density features, so called clumps, form primarily at the base of the wind on super-Sobolev length scales. The density of clumps differs by a factor of ~3 from the azimuthal average, the magnitude of their velocity dispersion is comparable to the flow velocity and they produce ~20% variations in the column density. Clumps may be observable because differences in density produce enhancements in emission and absorption profiles or through their velocity dispersion which enhances line broadening.

14

Kristen Dage (Michigan State University)

Ultraluminous X-ray Sources in Extragalactic Globular Clusters

The question whether globular clusters host black holes has been of longstanding interest. This interest has grown dramatically with the LIGO detection of merging black holes, as black hole mergers in globular clusters is one of the leading explanations for these LIGO sources. Determining whether black holes are common in globular clusters has been an observational challenge. One of the most successful ways to identify candidate black holes in globular clusters is to identify globular cluster X-ray sources with very high luminosities that are much greater than the Eddington limit for neutron stars (known as ULXs). A number of ULXs have been found within extragalactic globular clusters (GCs), and are candidate accreting black holes. We fit a number of these sources over a large span of Chandra data. We find that the globular cluster ULXs seem to follow one of two distinct trends: one group show a strong correlation between the accretion disk temperature and X-ray luminosity, while another group show no change in disk temperature with significant variations in X-ray luminosity. We discuss how these observational result impacts our understanding of the nature of these sources.

15

Jeremy Drake (SAO)

The peculiar X-ray/UV accretion rate schism in the puzzling dM+DA binary QS Vir

The period distribution of close binaries, cataclysmic variables, novae and single-degenerate SN1a progenitor candidates is largely controlled by magnetically-driven mass and angular momentum loss (AML) from the M dwarf secondary. The mass loss rates for these spun-up stars remain essentially unknown and impossible to observe directly, with likely values in the range 10^-12-10^-15 M_☉ yr^-1. We began studying the detached close dM+DA binary QS Vir to crack this problem, using observations of the M dwarf wind accreting onto the WD as probe of the M dwarf mass loss rate. But we found X-ray and UV diagnostics give accretion rates differing by 2-3 orders of magnitude! We present HST and XMM-Newton data that illustrate the problem which is yet to be resolved. The most likely explanation is additional mixing processes in cool WD atmospheres that decrease effective diffusion timescales for metals.

16

Rosaria Bonito (INAF - Osservatorio Astronomico di Palermo)

Observations, Simulations, and Laboratory Experiments: an Interdisciplinary Approach to Study Accretion Impacts in Young Stars with Disks

Young stars with disks are complex systems consisting of several components: star, disk, accretion columns, outflows, all contributing to the emission in different bands. The properties of these systems and the corresponding emission can be also important in the context of formation of stars and exo-planetary systems. A more complete description of the accretion phenomena characterizing young accreting stars is the interdisciplinary approach which combines multi-wavelength observations, magnetohydrodynamical models, and laboratory experiments (e.g. Revet, Chen, Bonito et al., Science Advances 2017). We will show the comparison between our magnetohydrodynamical models prediction and high energy observations (in the UV and X-ray bands) of TW Hya, a promising object to perform Doppler shift measurements with currently available instruments like Chandra. We will also discuss how future missions, as Athena and Large Synoptic Survey Telescope in different bands, will allow us to investigate in more details the accretion/ejection processes in young stars and their variability.

17

Elena Mason (INAF-OATS)

Insights from High Resolution (Multi-Band) Spectroscopy

Using high resolution (HR) multi-wavelength spectroscopy we investigated the ejecta kinematics for a few recent bright classical novae and derived a number of their physical parameters, including the geometry. The observable ejecta are consistent with polar-caps/biconical geometry and do not behave as a wind but are in ballistic expansion (V~R). Using HR multi-wavelength spectroscopy we also constrained the dust location, size, and evolution in dust forming novae. The dust is not destroyed by the high energy radiation from the unveiling hot white dwarf but slowly dilutes and mixes within the circumstellar environment.

18

Eric M. Schlegel (The University of Texas at San Antonio)

The Cataclysmic Variable AC Cnc: Accretion Variations and Stunted Bursts Reveals by Kepler2

We describe the K2 observation, using Kepler at 1-minute cadence, of the nova-like cataclysmic variable AC Cnc. The K2 data set lasted 74 days, covered 246 eclipses, and yielded 106,703 photometric measurements. This CV exhibits stunted outbursts and we describe them in relation to other stunted-bursting CVs. The light curve shows no evidence of disk brightening before either burst, suggesting that mass transfer is not the answer.