Poster Abstracts

Outflows of ionized gas are nearly ubiquitous components of active galactic nuclei (AGN). Their prevalence in the X-ray spectra and detected blueshifts make them promising candidates for being the missing link between the central engine and the large-scale effects observed in the host galaxy environment. However, their energetics and launching mechanisms are poorly understood, as the gas density and distance to the ionizing source are currently highly uncertain. Here, we show that the density and location of these outflows can be constrained from the frequency-dependent variability, namely from the Fourier spectral-timing signatures of the outflows in currently available X-ray data. Furthermore, by exploiting extensive simulations of the time-dependent outflow behavior, we present constraints on the elusive but critically important properties for outflows in a sample of highly variable AGN.

We present Chandra ACIS-S imaging spectroscopy results of the extended (300 -1600 pc) hard X-ray emission of NGC 5728, the host galaxy of a Compton thick active galactic nucleus (CT AGN). We find spectrally and spatially-resolved features in the Fe Kα complex (5.0-7.5 keV), redward and blueward of the neutral Fe line in the extended ionized bicone. A simple phenomenological fit of a power law plus Gaussians gives a significance of 5.4σ and 3.7σ for the red and blue features, respectively. Spectral fits employing a set of physically motivated models confirm a significance ≥3σ for the red wing. The significance of the blue wing may be diminished by the presence of rest frame highly ionized Fe XXV (at 6.7 keV) and Fe XXVI (at 6.9 keV) lines (1.4σ-3.7σ range in significance). A detailed investigation of the Chandra ACIS-S point spread function (PSF) and comparison with the observed morphology demonstrates that these red and blue features are radially extended (~5′′, ~1 kpc) along the optical bicone axis. If the wings emission is due solely to redshifted and blueshifted high-velocity neutral Fe Kα, then the implied line-of-sight velocities are symmetrically ~0.1c, and their fluxes are consistent with being equal. This outflow has deprojected velocities ~100x larger than the outflows detected in optical spectroscopic studies, potentially dominating the kinetic feedback power.

X-ray continuum emission of active galactic nuclei (AGNs) may be reflected by circumnuclear dusty tori, producing prominent fluorescence iron lines at X-ray frequencies. I will present the broadband emission modelling of three radio-loud AGNs belonging to the class of compact symmetric objects (CSOs), with detected narrow Fe Kα lines. CSOs have newly born radio jets, forming compact radio lobes with projected linear sizes of the order of a few to hundreds of parsecs. We modeled the radio-to-γ-ray spectra of compact lobes in J1407+2827, J1511+0518, and J2022+6137, which are among the nearest and the youngest CSOs known to date, and are characterized by a high intrinsic X-ray absorption. Based on the modeling, we argue that the inverse-Compton emission of compact radio lobes may account for the intrinsic X-ray continuum in all these sources. Furthermore, we propose that the observed iron lines may be produced by a reflection of the lobes' continuum from the surrounding cold dust.

In the nearby Universe the thermal equilibrium of galaxy groups and clusters is maintained by heating from the jets of their central radio galaxies. At higher redshifts these AGN are expected to be quasars, emitting most of their energy as radiation, with limited impact on the hot intra-cluster medium (ICM). To understand why jet-mode feedback now dominates, we need to understand the conditions which trigger quasar mode. IRAS 09104+4109 hosts the lowest-redshift (z=0.44) example of an obscured (type II) quasar in a cool-core cluster, with a star-formation and jet activity history that suggest the AGN recently realigned its axis and shifted into quasar mode. Cooling from the ICM is evident; the dominant elliptical is surrounded by ionized gas filaments and has one of the most extended molecular gas distributions known in any cluster, with 4.5x10^10 Msol of cold gas along the lines of the old, fading radio jets. We will present results from ~550 ks of Chandra ACIS-S observations of this unique system, focussing on the conditions around the AGN to answer the question: why does this cluster host a central QSO, and what triggered its transition from jet to quasar mode?

One of the key questions in extragalactic astrophysics is understanding how actively accreting supermassive black holes interact with their host galaxies. A crucial aspect is studying energetic jetted outflows that transport energy from the galactic center over distances from milliparsecs to megaparsecs. While jets are primarily observed at radio wavelengths in nearby quasars, several factors favor X-ray jets at high redshifts. The X-ray emission via inverse Compton scattering of the cosmic microwave background (IC/CMB) is enhanced at higher redshifts due to the (1+z)^4 increase in CMB energy density. Observations of X-ray jets in high-redshift quasars are rare, making each detection significant for understanding jet populations across cosmic time. This study focuses on two z~3 radio-loud quasars with kiloparsec-scale extended X-ray emission but no continuous radio jet. I will present results from deeper Chandra X-ray observations and new high-resolution, high-sensitivity Very Large Array 6 GHz radio data. Using the IC/CMB model, I will discuss constraints on relativistic and jet parameters and introduce a novel method to estimate the jet's angle to our line of sight. Our analysis indicates that the synchrotron break is real and shows that low-energy electrons can still produce X-ray emission via IC/CMB.

NGC 3227 is a nearby, Seyfert 1.5 galaxy with a highly variable X-ray spectrum. We present the results of an analysis of the Chandra-HETG spectra of NGC 3227 from two observations completed in 2016 and 2017. The first (~30 ks) observation reveals evidence of unusual double-peaked Fe Ka line emission, with centroid energies separated by ~60 eV. In the second (~120 ks) observation, an automated line search found several emission lines that indicate the presence of a warm outflow of material at ~600 km/s. Fe XXV and Fe XXVI line features appear to vary from absorption in the first observation to emission in the second.

Recent discoveries of quasars powered by supermassive black holes weighing billions of solar masses existing ~500 million years after the Big Bang represent a major puzzle. How did these black holes grow so much so fast? The formation and growth paths of these black holes are largely unexplored and remain in the domain of speculation. In this talk, I will discuss, from the observational point of view, how early Universe environmental conditions could have acted as an evolutionary mechanism for the first black holes. Various observational, computational, and theoretical works investigated the relationship between galaxy interactions and black hole activity. However, none of these works probed populations of galaxies that are prevalent in the early Universe. Observations of the ultraviolet luminosity function imply that beyond z=8, an unseen population of low-mass galaxies overwhelmingly dominates the galaxy population and, as a result, dominates the merger rate per unit volume. Even though these high-z low-mass galaxies are beyond our reach, using local dwarf galaxies as their contemporary analogs to establish the relationship between dwarf-dwarf interactions and black hole accretion can provide vital clues for understanding how the first black holes evolved and acquired mass. However, only a small number of local dwarf-dwarf galaxy pairs and dwarf galaxy groups have been discovered, and only one confirmed AGN in a dwarf-dwarf merger is known in the whole Universe. We focused on detecting the signatures of accretion using the Chandra X-ray Telescope from the subset of these dwarf interacting systems, residing in the GOODS-S field, where the deepest X-ray data exists (t_exp=7 Msec) and low detection limits allow probing low-luminosity AGN. We discovered six new confirmed AGN, increasing the known sample of dwarf-dwarf-merger-related AGN from one to seven. The inferred AGN incidence in the paired/grouped dwarf sample is 10-15%. We then constructed the control sample, consisting of isolated dwarf galaxies that reside in the same Chandra footprint where the same detection limits can be reached, and that follow the same distribution in the redshift-stellar mass space as paired/grouped dwarfs. In other words, dwarfs from the control sample have the same intrinsic properties and are exposed to the same observational conditions as grouped/paired dwarfs, with the environment being the only difference that could cause a discrepancy in AGN incidence. Among 178 isolated control dwarfs, we find only three AGN, implying a 1.6% incidence. This number is in line, with various works from the literature that tried to estimate the frequency of dwarf AGN. This study, the first of its kind at the lowest mass scales, unambiguously demonstrates that the presence of a nearby dwarf neighbor is efficient in triggering black hole accretion. Since the prevailing wisdom suggests that concurrent and consecutive dwarf-dwarf mergers are a common sight in the young Universe, these findings are opening new avenues for future studies of mechanisms that dictate the emergence of the first supermassive black holes and demonstrating the instrumental role that the Chandra telescope plays.

High-$\lambda_\mathrm{Edd}$ dust-obscured galaxies (DOGs) may represent a key phase closest to the peak of both the black-hole and galaxy growth in the coevolution framework for supermassive black holes (SMBHs) and galaxies. In this work, we present the a 68 ks XMM-Newton observation of the high-$\lambda_\mathrm{Edd}$ DOG J1324+4501 at $z \sim 0.8$, which was initially observed by Chandra. We analyze the XMM-Newton spectra jointly with archival Chandra spectra. In performing a detailed \mbox{X-ray} spectral analysis, we find that the source is intrinsically \mbox{X-ray} bright ($L_\mathrm{X} > 10^{44.7}$ erg s$^{-1}$) and heavily obscured ($N_\mathrm{H} \sim 10^{23.4}$ cm$^{-2}$). We further utilize UV-to-IR archival photometry to measure and fit the source's spectral energy distribution (SED) to estimate its host-galaxy properties. We additionally present a supplementary comparison sample of 21 \mbox{X-ray} luminous DOGs from the XMM-SERVS survey with sufficient ($> 200$) $0.5-10$ keV counts to perform a similarly detailed X-ray spectral analysis. Of the X-ray luminous DOGs in our sample, we find that J1324+4501 is the most remarkable, possessing one of the highest \mbox{X-ray} luminosities, column densities, and star formation rates. We demonstrate that J1324+4501 is in an extreme evolutionary stage where SMBH accretion and galaxy growth are at their peaks.

Bipolar jets of relativistic plasma, emitting across the electromagnetic spectrum from radio to X-rays, are a hallmark of active galaxies. While the radio emission from these jets is well understood as synchrotron radiation, recent observations with HST, ALMA, and Chandra have unveiled a mysterious high-energy component at optical and X-ray wavelengths. This component challenges our understanding of particle acceleration and jet physics, with implications for the energetics and impact of these jets on their environments. In this presentation, we focus on the investigation of three extragalactic jets exhibiting a hard optical spectral index, indicative of a second emission component distinct from the radio synchrotron tail. By analyzing multi-wavelength data, we aim to determine the nature of this high-energy component, testing between synchrotron and inverse Compton (IC) emission scenarios. Distinguishing the emission mechanism is crucial, as it directly influences our understanding of jet power, particle acceleration, and the broader impact of these jets on galaxy evolution. Through this study, we will address one of the enduring mysteries in the field of jetted AGN, contributing to the ongoing legacy of discoveries enabled by the Chandra X-ray Observatory over the past 25 years. The results will offer new insights into the physical processes driving these powerful cosmic phenomena and inform future models of jet behavior and their interaction with the surrounding environment.

Chandra high-resolution ACIS-S observations of many CT AGNs have provided clear evidence of three ways in which the AGNs interact with the ISM of the host galaxy: (1) radiation, affecting both the gaseous ISM and dense molecular clouds; (2) embedded jets, causing localized shocks and lateral outflows; and (3) fast nuclear outflows. We will illustrate these phenomena that lead to a revised “AGN standard model”.

Approximately half of all active supermassive black holes (SMBHs; M > 10^6 Msun) are expected to grow behind Compton-thick columns of obscuring gas. However, there are currently no known examples of a Compton-thick accreting intermediate mass black hole (IMBH; M < 10^5 Msun), despite being a viable seed mechanism for SMBHs. In this talk I will report on the deep 177 ks Chandra X-ray observations of IC 750; a nearby AGN powered by accretion onto an IMBH with mass ~70,000 Msun. The exquisite angular resolution of Chandra is able to spatially disentangle a number of contaminating sources in the host galaxy, revealing substantial spatially-resolved iron Kalpha emission coincident with the nucleus. We test three unique physically-motivated X-ray spectral obscuration models, and find that the circum-nuclear environment surrounding the AGN in IC 750 is Compton-thick along the line-of-sight to >99% probability. By considering a number of multi-wavelength considerations, we show that the AGN is consistent with an intrinsic accretion rate of ~10-100% of the Eddington limit which is in broad agreement with the known SMBH AGN population within the nearby Universe. Given that Compton-thick AGN are known to constitute a substantial portion of growing SMBHs, our results suggest that heavy obscuration may prove to be a vital consideration in the pursuit of IMBHs.

X-rays from Active Galactic Nuclei (AGN) offer an exceptional probe into the surrounding s of supermassive black holes. Due to their high piercing power, X-rays provide a wealth of information even for Compton-Thick AGNs, where the central engine is heavily obscured by large column densities (~1.5E24 cm-2). X-ray data from Chandra, thanks to its unparalleled imaging capabilities, offer a unique view into the central regions of obscured AGN. In particular, Chandra has revealed the presence of extended X-ray regions with cone/counter cone-like structures, approximately perpendicular/parallel to the plane of the accretion disk. Using NGC 1386 as a case study, I will present a standardized procedure aimed at homogeneously determining the physical properties of this extended X-ray emission, which we will apply to all Compton-Thick AGNs observed with Chandra. This will allow us, for the first time, to build a catalog of the X-ray spectroscopic properties of this diffuse X-ray emission, and provide insights into the interaction between the central supermassive black hole and its host galaxy.

Supermassive black holes weighing of order 1 billion Solar masses are surprisingly ubiquitous in the first billion years of the Universe (z > 5.6). Even with heavy seeds, generating such robust populations requires significant, sustained growth preceding and through the Epoch of Reionization. Recent results have hinted that jets may be a common feature of the AGN powered by these SMBHs, and jets have been posed as a means of enabling faster growth in the early Universe. In this poster, I will discuss the results of a Chandra LP targeting the radio-loudest quasars in the early Universe, including the detection of the most distant (z=7) blazar, extended X-ray emission unseen at radio frequencies, and short-time scale variability.

We present a focused X-ray and multiwavelength study of the ultraluminous weak-line quasar (WLQ) SDSS J1521+5202, one of the few X-ray weak WLQs that is amenable to basic X-ray spectral and variability investigations. J1521+5202 shows striking X-ray variability during 2006–2023, by up to a factor of ≈ 23 in 0.5–2 keV flux, and our new 2023 Chandra observation caught it in its brightest X-ray flux state to date. Concurrent infrared/optical observations show only mild variability. The 2023 Chandra spectrum can be acceptably described by a power law with intrinsic X-ray absorption, and it reveals a nominal intrinsic level of X-ray emission. In contrast, an earlier Chandra spectrum from 2013 shows apparent spectral complexity that is not well fit by a variety of models, including ionized- absorption or standard Compton-reflection models. Overall, the observations are consistent with the thick-disk plus outflow model previously advanced for WLQs, where a nominal level of underlying X-ray emission plus variable absorption lead to the remarkable observed X-ray variability. In the case of J1521+5202 it appears likely that the outflow, and not the thick disk itself, lies along our line-of-sight and causes the X-ray absorption.

Massive stars are hot, and luminous, and drive strong stellar winds. At the end of their evolution, they undergo gravitational collapse, typically accompanied by supernova explosions, resulting in the formation of black holes or neutron stars. These compact objects can be part of binary systems, accreting material from companion stars and emitting X-rays. The most luminous of these systems are known as ultra-luminous X-ray sources (ULXs). Only Chandra can disentangle the point sources and diffuse X-rays in compact starburst galaxies. As such, we present the first results of our new deep Chandra observations of the template low-metallicity starburst ESO 338-4, which contains a large population of massive stars and star clusters. These observations are supplemented by XMM-Newton, HST, and ESO-VLT data to complete the multiwavelength picture of ESO 338-4. We present the discovery of the ULX population in this galaxy. Our new multiwavelength images uncover the role ULXs and massive star feedback play in low-metallicity galaxies.

Recently, diffuse, hard (>2 keV) X-ray, kpc-scale nebulae have been reported in several nearby star-forming galaxies, such as M82, NGC 891, and NGC 3079. However, the origin of these nebulae is still unknown. This hard emission may be interpreted as superheated gas, which may drive galactic outflows from regions of intense star formation. If this is the case, the hard diffuse X-ray emission may help understand how the superheated gas connects to the surrounding kpc-scale winds seen from radio through the X-rays. We are conducting the Chandra archival survey to quantify the frequency and properties of diffuse, hard emission in starbursts and star-forming galaxies. Our sample consists of 20 galaxies, ranging from 3 - 12 Mpc in distance and with SFR ranging from 0.2 - 43 Msun/yr. We present our preliminary results.

Spectroscopic observations of high-redshift galaxies and their nearby analogs have uncovered puzzling nebular emission line features, indicative of excitation by a source of very high-energy photons. The low metallicities and bursty star formation histories of such galaxies are conducive to the efficient formation of ultra-luminous X-ray sources (ULXs), which has led to a surge of interest in whether ULXs are an important excitation mechanism in high redshift galaxies and their analogs. Yet the age-dependence for forming such sources also impacts their ionizing potential. We will present preliminary results from Chandra on the X-ray properties of a carefully selected SDSS sample of extreme emission line galaxies. We will discuss these results in the context of predictions from state-of-the-art photoionization simulations incorporating ULXs, which predict that ionizing photon production from ULXs operates most efficiently relative to stars on timescales > 10 Myr post-starburst at low metallicities. Utilizing the multi-wavelength spectro-photometric data for these galaxies we constrain the star formation histories to present consistency checks with the photoionization models. This analysis provides insight into the major sources of high-energy ionizing photons and the host galaxy environments where they operate, which is key for interpreting spectroscopic observations of high redshift galaxies just now becoming more accessible with JWST.

We present results from a study combining spatially resolved star formation histories (SFHs) and Bayesian X-ray spectral fitting to study the relationship between star formation and X-ray emission from hot gas on ~kpc scales in NGC 4254 (M99). We derive maps of the SFH from UV-IR data, including AstroSat UV and VLT MUSE IFU data from PHANGS. From archival Chandra data, we produce high-quality point-source-subtracted maps of the diffuse X-ray emission, from which we extract and fit the spectra of subgalactic regions, deriving luminosities and plasma temperatures. We derive kT-SFR and LX-SFR scaling relationships from our maps and spectral fits, and use them to constrain theoretical models of the energy and mass input into the ISM from galactic winds. Future application of our methods to a larger, more diverse sample of galaxies will yield insights into how subgalactic scaling relations combine to produce the observed, tight, galaxy-integrated LX-SFR scaling relation.

In Chandra's earlier days, photons as soft as 0.15 keV could be detected with the ACIS-S detector, yet few studies considered photons below 0.3 keV. Our archival study of well-observed nearby galaxies has revealed a population of X-ray sources within these galaxies that only emit at energies below 0.3 keV, which we dub 'hypersoft' X-ray sources. These X-ray sources are largely missing from the Chandra Source Catalog, as well as literature source lists compiled by individual investigators. The most luminous sources have 0.15-0.3 keV luminosities exceeding 1e38 erg/s, and considering reasonable bolometric corrections, have bolometric luminosities in the 1e39-1e40 erg/s range. Thus, these hypersoft X-ray sources may represent the highest luminosity individual objects in most non-AGN galaxies. We discuss possible origins of hypersoft X-ray sources and prospects for nailing down their true bolometric luminosities.

The Milky Way Halo as observed in X-rays is known to be a multi-phased gaseous medium with an ubiquitous and anisotropic warm-hot (~2x10^6 K) gas near the Galaxy’s virial temperature and a hotter-than-virial-temperature phase that is less well understood. During our Suzaku survey of the hot (super-virial) phase, we found the temperature to be fairly uniform across the sky, while the emission measures were higher towards the Galactic bubbles and mostly varied little between neighboring sightlines. Anomalously, two nearby sightlines (<2.0º) exhibit a nearly 8-fold discrepancy in the hot phase emission measure though the warm-hot emission measure between the two remained comparable. I examined the point source population of both fields with new Chandra observations as a culprit for the variance. Our results suggest an alternate explanation is necessary and future work exploring the morphology of the Milky Way hot phase is warranted, particularly at sightline separations <2.0º.

Feedback by active galactic nuclei (AGN) is one of the most common explanations for quenching of star formation in galaxies. Recently quenched galaxies, selected based on post-starburst characteristics, provide an ideal laboratory for searching for signatures of AGN activity responsible for or occurring simultaneous with quenching. We studied X-ray observations of a sample of recently quenched galaxies, primarily observed serendipitously by Chandra. Many of these galaxies do not have sufficient counts for spectral modeling. We developed and applied a forward-modeling technique to use the number of photons and their hardness ratio to constrain either (1) the intrinsic 2-10 keV luminosity and obscuration column of a power-law or (2) the 0.5-8 keV luminosity and APEC plasma temperature consistent with the observation of each galaxy. While we find that a large fraction of these galaxies has nuclear X-ray emission, few are sufficiently luminous to exert significant radiative feedback capable of driving out gas reservoirs, leading to quenching. I will discuss our forward modeling methodology and the results of its application to quenching galaxies.

X-ray observations are potentially powerful for probing the galactic ecosystem, especially its hot and energetic components. However, existing X-ray studies of nearby star-forming galaxies are largely limited by the inadequate counting statistics of the observations and the lack of a suitable spectral modeling or analysis tool to account even approximately for the expected distribution geometry of X-ray emission and absorption. We report here the results of an X-ray spectral study of M51 based on the 1.3-Ms Chandra observations, the deepest available for such a galaxy. We designed a new hierarchical Bayesian approach to jointly analyze these spectra and test spectral models of varying sophistication, ranging from 1-T diffuse hot plasma to those that allow for distributed hot plasma and X-ray absorbing cool gas properties. With this new approach, spatially resolved spectral analysis is possible. From that, we recommend a simple model that gives a satisfactory fit to the spectra, consisting of a galactic corona with a logarithmic temperature distribution and a galactic disk with mixed X-ray emission (contributed mainly by unresolved stellar sources) and X-ray absorbing cool gas. In this model, only half of the corona emission is subject to this internal absorption. The best-fit absorbing gas column density is about a factor of ~2 larger than that inferred from the mean effective optical extinction of stellar light. The temperature distribution can be characterized by a mean lognormal temperature of $\sim 0.1$~keV and a dispersion of $\sim 1$~keV; as may be expected, the dispersion is enhanced in spiral arm regions. This characterization, if close to the reality, then suggests that the radiative cooling of the corona could largely balance the energy input from the stellar energy feedback of the galaxy, the "missing energy" from stellar feedback actually is dominated by emission below 0.5 keV, which is difficult to detect. These results demonstrate the power of X-ray mapping both the corona and the cool gas in disk galaxies.

Classical novae are known to demonstrate a supersoft X-ray source (SSS) state following outbursts. This state is associated with residual thermonuclear burning on the white dwarf (WD) surface. We performed a spectral analysis of the supersoft X-ray phase of the classical nova AT 2018bej, which was observed in X-rays by the eROSITA and XMM-Newton telescopes. To describe the spectrum we calculated high-gravity hot LTE model atmospheres of hot WDs with different chemical compositions, assuming equal hydrogen and helium number fractions, and five different values of carbon and nitrogen abundances. The code developed by Suleimanov et al. (2024) was used for this aim. The 0.3-0.6 keV analysis yields a WD temperature Teff~600kK, gravity logg~8.3—8.4, and a WD radius R~8000–8700km, which gives luminosity L~6-6.5*1e37 erg/s. The derived WD mass is estimated to be ~1.1*M_sun. We traced a minor evolution of the source on a half-year timescale accompanied by a decrease in carbon abundance, decrease in temperature and increase in radius, and concluded that LTE model atmospheres can be used to analyse the available X-ray spectra of classical novae during their SSS state.

X-rays from young, pre-main sequence (pre-MS) stars play an important role in the chemical evolution of circumstellar disks and the eventual formation of planets. Young, low-mass stars are ubiquitous X-ray sources due to their rapid rotation and intense magnetic activity capable of heating coronae to millions of degrees Kelvin. Over the past 25 years, Chandra and XMM-Newton have identified thousands of these stars demonstrating that hot coronae are typical of pre-MS stellar evolution. However, it’s not yet clear when coronae around young stars first develop after the gravitational collapse of a molecular cloud clump begins the star formation process. Recent radio campaigns using high-sensitivity and high spatial resolution observations with ALMA and the VLA have identified a sample of several hundred nearby ‘Class 0’ protostars, the earliest stage of pre-MS evolution representing the first ~10,000 years of a star’s life. Utilizing Chandra’s excellent spatial resolution, we present a preliminary analysis of several Class 0 protostar candidates that have been detected with X-rays in archival Chandra observations of stellar clusters. These sources may represent the youngest sample of X-ray emitting protostars to date indicating that coronae may exist even at the earliest stages of the star formation process.

The sudden release of stored magnetic energy in the coronae of late-type stars via magnetic reconnection, known as flaring, is manifest across decades of the electromagnetic spectrum from radio to X-rays. X-ray flare emission is produced by the heating of coronal plasma, either in situ or via chromospheric evaporation. "White light" optical and ultraviolet emission is produced in the chromosphere and photosphere by heating from the impact of particles accelerated near the site of magnetic reconnection in the corona. In solar flares, the white light energy can exceed the X-ray energy by up to two orders of magnitude. For stellar flares, the partition of energy between different wavelengths and their associated heating mechanisms is very poorly known. This is a problem for understanding the energy budgets of active stars and their flare-dominated coronae. Here, we analyse Chandra X-ray light curves for Proxima Centuri to determine the flare frequency vs energy distribution. We compare the results to analogous white light studies and asses how far magnetic reconnection flare models honed on solar observations can apply to the very different atmospheric and magnetic properties of active M dwarfs.

Wolf-Rayet (WR) stars are the latest stage in the evolution of very massive stars, before they eventually explode as supernovae (SN) or possibly gamma-ray bursts. They exhibit dense and powerful stellar winds that, along with their ultimate death as core-collapse SN, dominate the feedback to the local interstellar medium in star-forming galaxies. Studying in more detail the properties of the short-lived WR phase, will advance our understanding on star-formation processes and will test stellar evolutionary predictions. The ideal laboratory to investigate the WR phase is the massive young star cluster Westerlund 1. It is the closest supermassive star cluster to the Sun, and it contains an impressive large sample of coeval massive stars including the largest population (24) of WR stars in our Galaxy.  In this meeting, I will present the  results of the EWOCS  (Extended Westerlund 1 and 2 Open Clusters Survey) project on the WR stars in Westerlund 1 based on a 1Msec Chandra/ACIS-I Large Project. Through this study, made possible only by the high resolution of Chandra, we can unveil the X-ray spectral, and timing properties of the entire WR population, shedding light on their X-ray production mechanism. I will discuss these results in the context of different spectral subtypes of WR stars, as well as their binarity. This is particularly relevant as the majority of these stars show clear signs of very hot plasma produced in the colliding-wind region of a binary system, contributing to a broader understanding of their formation pathway.

Determination of the frequency and properties of low-mass stars that are members of multiple star systems is difficult in general. However, the X-ray wavelength range is the one spectral region where a cool main sequence companion can outshine a supergiant, particularly in systems with relatively young primaries such as Cepheids. Twenty Cepheids have been observed in X-Rays by Chandra or XMM-Newton. Of this sample, 4 have been found to have F, G, or K companions, and one (S Mus) has X-rays from a hot early B companion. This provides an estimate that 20\% of the Cepheids have low mass companions. Combing this with the fraction of more massive companions identified in the ultraviolet, results in a lower limit to the binary fraction for Cepheids of 57 +/- 12 \% for mass ratios greater than 0.1 and separations greater than 1 au. The fraction of low mass companions is significantly lower than would be produced by random pairing with a field initial mass function.

We present a new method to distinguish between different states (e.g., high and low, quiescent and flaring) in astronomical sources with count data. The method models the underlying physical process as latent variables following a continuous-space Markov chain that determines the expected Poisson counts in observed light curves in multiple passbands. For the underlying state process, we consider several autoregressive processes, yielding continuous-space hidden Markov models of varying complexity. Under these models, we can infer the state that the object is in at any given time. The continuous state predictions from these models are then dichotomized with the help of a finite mixture model to produce state classifications. We apply these techniques to X-ray data from the active dMe flare star EVLac, splitting the data into quiescent and flaring states. We find that a first-order vector autoregressive process efficiently separates flaring from quiescence: flaring occurs over 30-40% of the observation durations, a well-defined persistent quiescent state can be identified, and the flaring state is characterized by higher temperatures and emission measures. Vinay Kashyap will present a more detailed analysis of EVLac using the methods we present here.

The defining, yet still mysterious, property of solar magnetic activity is its 11-year ebb and flow. The sunspot cycle is a hundred times longer than either the solar rotation period or the convective turnover time, the two main ingredients thought to control the elusive internal dynamo. The solar cycle is exaggerated in coronal (T~1-10 MK) soft X-rays, with amplitudes of several to hundreds, depending on energy band. Famous chromospheric Ca II HK (10,000 K), on the other hand, varies a mere tens of percents. The high-contrast solar X-ray modulation impacts our local Space Weather; stellar counterparts likewise for their exoplanets. Little is known about stellar X-ray variability and cycles, however: there is no high-energy equivalent to the heroic Mt. Wilson HK Project, which captured chromospheric emissions of hundreds of late-type stars seasonally over several decades. Absent a dedicated X-ray time-domain program, the alternative is to troll the archives of the three major long-lived modern observatories: Chandra, XMM-Newton, and Swift. About twenty late-type stars have useful-enough records to explore long-term X-ray variability. Among these are the Chandra "Seven Dwarfs:" Alpha Cen (G2+K1), Xi Boo (G7+K5), 70 Oph (K0+K4), and Alpha CMi (Procyon: F5). Within the "Big Three" observatories sample, large-amplitude X-ray cycles are found exclusively in the lower activity tiers (in terms of X-ray-to-bolometric ratios), whereas the hyperactive objects (like rapidly spinning "cannibal" K-dwarf AB Dor, RS CVn binary AR Lac, and close-by dM flare star Proxima Cen) show high, but stable long-term X-ray trends, although punctuated on the short term by frequent flaring. Oddly, some stars display ultra-short cycle periods, only 1-2 years; whereas others are longer than the Sun's: nearly 20 years for solar-twin Alpha Cen A. There are "flat-line" coronal cases, as well, epitomized by mid-F Procyon. The diversity of activity cycles on ostensibly similar stars is puzzling, a major headache for already stressed dynamo theories.

We analyze Chandra ACIS-S/HETG light curves of EV Lac using a Hidden Markov Model with continuous states, and classify the states such that intervals where flaring is dominant can be isolated. We find that the quiescent emission is persistent and stable across epochs separated by nearly a decade. The flaring emission occurs intermittently, is highly variable in both emission measure and temperature, shows indications of abundance changes, and is at significantly higher temperatures than the plasma giving rise to the quiescent emission. Flaring dominates approximately 30-40 percent of the observation duration. The clear dichotomy between flaring and quiescent emission suggests that different heating processes are responsible for the different components. [arXiv:2405.06540]

Dynamically relaxed galaxy clusters are the most robust cosmic laboratories to study the evolution of structure in the Universe as they most closely align with our astrophysical assumptions of spherical symmetry and hydrostatic equilibrium. Here, we present the joint (Chandra+XMM-Newton) characterization of ACT-CL J0123.5-0428, the newly identified highest redshift relaxed cool core system. At z = 1.51, the spatial resolution of Chandra is critical to resolve gas on the 5-10kpc scales necessary to classify its morphology and probe the nature of the cluster core. In addition to the morphological measurements enabled by Chandra, we combine the Chandra and XMM-Newton data to present the spatially resolved thermodynamic and chemical properties of ACT-CL J0123 out to r500.

In this poster, we investigate the impact of f(Q) gravity on key cosmological parameters, extending beyond the framework of General Relativity (GR) by incorporating non-metricity. We analyze the modified Friedmann equations derived from this theory to understand the evolution of the Hubble parameter, energy density, pressure, and the equation of state parameter. Our findings suggest that f(Q) gravity shows promise as an alternative to GR, particularly in explaining the accelerated expansion of the universe. By solving these modified Friedmann equations, we present a comprehensive set of differential equations that describe the cosmological evolution within the FLRW model under f(Q) gravity. These results pave the way for further exploration of modified gravity theories and their potential to address unresolved challenges in cosmology.

Abell 2029 (A2029) is a massive, nearby (z=0.0767), relaxed, cool-core galaxy cluster that hosts the longest, continuous sloshing spiral ever observed. A2029 also contains a wide-angle tail (WAT) radio source near its core, making it an outstanding laboratory for studying both the dynamics of the sloshing intracluster medium (ICM) and the intricate interplay between ICM dynamics and the cluster active galactic nucleus (AGN). I will present the results from new, deep (~400 ks) Chandra X-ray Observations of A2029 that, when combined with previous observations, represent the most detailed view of the cluster to date. I will present a deep dive into the thermodynamic structure of the ICM, particularly in regions of the sloshing spiral. Additionally, I will discuss the interaction between the ICM and the central WAT source, providing updated measurements of the relative velocities between the ICM and AGN, as well as revised estimates of the AGN energy injection rate and the central ICM mass cooling rate. This combined analysis of AGN feedback and sloshing dynamics will further our understanding of heating mechanisms in cool-core clusters and shed light on the interactions between ICM and AGN activity.

We present an analysis of a sample of T > 6 keV galaxy clusters observed with the Chandra X-ray Observatory which display large cluster-wide gas sloshing spirals despite the absence of strong cool cores at their centers. Within each cluster, cold fronts are detected coincident with the spiral-shaped surface brightness excesses, consistent with the spirals having formed through the disruption of a previously intact strong cool core. Recent simulations have shown that gas sloshing spirals which extend out to hundreds of kiloparsecs from the cluster core require Gyrs to reach such distances, suggesting a formation initiated by an off-axis merger that had transpired long before the current epoch of observation. Nonetheless, merger shocks are detected in each cluster, some associated with infalling gaseous subclusters, indicating ongoing dynamical activity which is likely further contributing to core disruption.

We are conducting a survey of distant clusters of galaxies (0.5 < z < 3.0) using bent, double-lobed radio sources driven by AGN as tracers. These types of radio sources are easily detected in the ~10,000 square degree VLA FIRST survey to large distances and are frequently found in clusters since interaction with a surrounding intracluster medium (ICM) can create the ram pressure necessary to bend the radio lobes. Unlike many other cluster surveys that are biased towards the most massive clusters, our clusters span a wide mass range. Our COBRA (Clusters Occupied by Bent Radio AGN) sample includes 646 radio sources, all of which have follow-up Spitzer infrared observations. In addition, we have deep optical imaging for a significant subset of the sample. Here, we present Chandra observations of the highest redshfit COBRA source that we have observed in the X-ray -- a bent-lobed quasar at z=1.8. We find extended emission in the X-ray consistent with cluster ICM at this redshift. A jet from the quasar is also detected in the Chandra observation. Combining our Chandra observations with HST spectroscopic observations of this source, we find that it is likely a high-redshift cluster, making this object the highest-redshift cluster discovered using a bent radio source as a cluster tracer.

As the largest gravitationally bound structures in the Universe, galaxy clusters serve as a unique and valuable laboratory for probing cosmological models and understanding the astrophysics of AGN feedback. Clusters that are dynamically relaxed, or have not undergone any recent merging events, are especially useful targets of study due to their morphological and dynamical simplicity. However, at redshifts >1, very few such clusters have been identified so far. I will present results from new observations of cluster SPT-CL J2215-3537 (z=1.16), the second most distant relaxed, cool core cluster identified to date. We place constraints on the cluster’s total mass profile and investigate the gas temperature, density and metallicity profiles of the cluster, resolving its cool core on the scale of arcseconds and providing context for the massive starburst seen in its central galaxy. In conjunction with thoroughly studied relaxed clusters at lower redshifts, SPT-CL J2215-3537 and new relaxed clusters at z>1 provide a powerful benchmark for our understanding of the formation of cool cores and the evolution of massive clusters of galaxies.

The Cluster Evolutionary Revolutionary Ensemble At Low-z (CEREAL) sample and its low-mass extension consist of more than 150 galaxy clusters in the local universe (z < 0.25) observed with Chandra, including ~100 new and ~50 archival observations. The clusters are selected from the Planck 2nd Sunyaev-Zel’dovich survey and comprise a statistically complete sample in mass-redshift space, painting the most representative picture to date of clusters at low redshift. The ability to uniformly observe low-z clusters across a range of masses allows us to extract properties that trend with mass as we seek to understand how clusters grow over cosmic time. In addition, the CEREAL sample will serve as an anchor for ongoing studies of cluster evolution in the era of high-z observations.

Quasar microlensing provides a novel approach to probing discrete masses within galaxies and galaxy clusters by studying their induced microlensing signatures and the energy shifts of X-ray emission lines from high-redshift background quasars. We employ this technique to place effective constraints on the distribution of planet-mass objects within two lensing systems, Q J0158-4325 (zl​=0.317) and SDSS J1004+4112 (zl​=0.68), using Chandra observations of these gravitationally lensed quasars. The observed variations in the peak energy of the emission lines can be explained as microlensing of the FeKα emission region by planet-mass microlenses. To confirm this, we perform microlensing simulations to estimate the probability of a caustic transiting the source region and compare this with the observed line shift rates. Our analysis yields constraints on the substellar population, with masses ranging from Moon-sized (∼10−8M⊙​) to Jupiter-sized (∼10−3M⊙​) bodies, within these galaxy- or cluster-scale structures, with total mass fractions of approximately 3×10−4 and 1×10−4 with respect to halo mass for Q J0158-4325 and SDSS J1004+4112, respectively. This study offers the first constraints on substellar mass distributions within intracluster light, suggesting the universality of unbound planet-mass objects in galaxies, potentially comprising free-floating planets or primordial black holes. Our results provide the most stringent limit on the mass fraction of primordial black holes in this mass range.

Several studies have shown that entropy profiles of the intracluster medium (ICM) generally do not match self-similar predictions near the virial radius, with the measured values typically being less than predicted. There are several proposed mechanisms to explain this discrepancy, including weakening accretion shocks, electron-ion non-equilibrium, and unresolved cool gas clumps that bias X-ray measurements. These effects are expected to correlate with the local orientation of cosmic filaments, as they interface with the outskirts of the ICM. I will present results from deep X-ray observation of early-stage, binary cluster mergers, where the merger axis is expected to be aligned with a local large scale filament. We find that the ICM entropy profiles are consistent with self-similar predictions, even in these dynamically active, merging systems, likely due to the relatively low subcluster masses (3-4 keV). We also find tantalizing evidence for diffuse emission with properties that are consistent with the dense end of the warm-hot intergalactic medium (WHIM) only along the merger axes (but not at the same radii away from the merger axes), consistent with the expectation that the merger axes in these systems are aligned with local cosmic filaments.

Together, XMM-Newton and Chandra, both launched in 1999, have revolutionised our view of the X-ray sky. With around 400 peer-reviewed publications per year, XMM-Newton is one of ESA's most successful scientific missions ever. The talk will give an overview of the status of the mission and present the key performance indicators that characterise the scientific impact of the mission. The talk will then give an overview of the latest research results highlighting the most important developments in current X-ray astronomy. Finally, the talk will outlines possible challenges for research and observation in the next decade.

We present the results of 15 years of characterizing comets using the Chandra X-ray Observatory Advanced CCD Imaging Spectrometer (ACIS). Targets have ranged from opportunistically bright, disrupting comets C/2001 LS4 (Lisse et al. 2001) and 17P/Holmes (Christian et al. 2010) to fainter, but well known, periodically recurring comets like 2P/Encke (Lisse et al. 2005), 9P/Tempel 1 (Lisse et al. 2007), and 73P/Schwassman-Wachmann 3 (Wolk et al. 2009).

The discovery of high energy x-ray emission in 1996 from C/1996 B2 (Hyakutake) created a surprising new class of x-ray emitting objects not involving ultra-hot stellar coronae or compact massive objects. The original discovery (Lisse et al. 1996) and subsequent detection of x-rays from 30+ comets suggested that the very soft (E < 1 keV) emission is due to an interaction between the solar wind and the comet's atmosphere, and that x-ray emission is a fundamental property of all comets. As first determined in 2001 by Chandra observations of comet C/2001 LS4, the dominant x-ray emission flux in the 0.4 – 1.0 keV energy range is consistent with charge exchange emission (CXE) between hydrogenic and heliogenic highly charged minor C,N,O, Fe, Ne ions in the solar wind and outflowing cometary neutrals, explaining how such cold, low mass objects can generate x-rays - via the Sun’s coronal heating and ionization of the solar wind. CXE models allow us to produce estimates of a cometʼs x-ray luminosity and emission spectrum given a comet’s neutral gas production rate, the solar wind ionization state, and the solar wind flux density at the comet. Our models agree with the observational spectra and generate composition ratios for heavy, highly charged SW ions interacting with the cometary atmosphere that are consistent with in situ spacecraft SW measurements.

These results demonstrate the utility of cometary CXE emission as a remote diagnostics tool for heliospheric SW composition and flux density, as well as a local, well understood example of astrophysical hot plasma-cold neutral gas interaction. Questions still remain from the Chandra observations as to the importance of solar coronal x-ray scattering from cometary dust comae (Snios et al. 2015), and the role of dust in potentially inhibiting CXE emission from extremely dusty comets like C/1995 O1 Hale-Bopp and 17P/Holmes (Christian et al. 2010).

We discuss the current state of our analysis of the X-ray fluence experienced by 38 planets. Assuming hydrodynamic equilibrium we estimated the mass loss rates of these planets and found 85% of them exceed the mass escape rate of the Earth. At least 3 planets have an estimated mass escape rate of over 1 million times that of the Earth. We specifically examine Two systems, Wolf 359, and HD 189733 for which we have over 10 epochs of observations that determine the different environments these planets have experienced over the past 2 decades. In addition, we focus on GJ 887 b TRAPPIST d, e, f, and g all of which have an Earth similarity index of > 0.5. We also examine the impact of individual flares and discuss the implications for like around M stars.

In about one-fifth of all Type I X-ray bursts, the ignition of accreted H/He produces radiation forces so powerful the Neutron Star (NS) photosphere is lifted off of the stellar surface entirely, driving a wind of ejected material that may contain heavy elements. To date, Chandra has delivered some of the highest spectral resolution observations of these events, called Photospheric Radius Expansion (PRE) bursts, including six such bursts detected with Chandra from 4U 1728-34 (Galloway et al. 2010). Observations like these can enable identification of absorption features from heavy elements in the wind and constrain the NS radius via measurement of the Eddington flux, but these inferences are limited by systematic spectral modeling uncertainties and the steady-state approximation that current wind models assume. We therefore present time-dependent numerical simulations of NS PRE bursts which couple the 1D MESA models of Yu & Weinberg (2018) to Athena++ (Stone et al. 2020), a radiation hydrodynamics code that excels at realistic radiative transfer. The 1D MESA models provide both initial conditions and a time-dependent inner boundary condition for the Athena++ simulation to follow as the burst evolves. By synthesizing the best abilities of these two powerful codes, this work aims to deliver the most accurate wind solutions for PRE bursts to date.

X-ray observations of Cas A, RCW 86 and HESS J1731-347 have revealed filamentary non-thermal rims tracing the forward shocks of these supernova remnant (SNRs). These structures have been identified as synchrotron radiation from shock-accelerated electrons with TeV energies, interacting with the compressed, and probably amplified, local magnetic field. Magnetic field amplification (MFA) is broadly believed to result from, and contribute to, cosmic ray acceleration at the shocks of SNRs. Using the data from the repeated Chandra observations of these remnants, we have estimated the expansion of the non-thermal rims at the forward shocks of the SNRs, as well as the width of these filaments. Since the size of the synchrotron filaments places constraints on the magnetic field strength, this study allows us to establish a connection between the shock velocity and the characteristics of the particle acceleration

We present evidence for atomic absorption in the photospheric spectrum of RX J0822-4300, the neutron star in Puppis A. The atmosphere is mainly composed of Oxygen and Neon. We see the predicted Zeeman splits and shifts and spectroscopically confirm the previously measured surface magnetic field strength. The gravitational redshift is z = 0.29. We also detect the pressure broadening of the absorption lines; this will allow a spectroscopic measurement of mass and radius of the star.

We analyze new X-ray data obtained with Chandra from 2021/2022 to produce continued measurements of the proper motion of the shockwave from Tycho’s supernova remnant (SNR). As the remnant of a historical supernova (observed in 1572 CE), Tycho’s SNR presents the rare opportunity to study the dynamical evolution of such an object in real time. Our new measurements build on the work of Tanaka et al. (2021) to confirm non-uniform deceleration in parts of the shockwave of Tycho’s SNR. Working in the 2.0-8.0 keV band to mitigate the effects of contamination buildup on the Chandra ACIS detectors over the years, we compared data from 6 different observations (2000, 2003, 2007, 2009, 2015, and 2021/2022) to measure the velocity of the shockwave and the deceleration. The results of Tanaka et al. (2021) show a substantial velocity decrease over a 12 year span (2003-2015) in several regions of the forward shock. In this work, we repeat the analysis of Tanaka et al., adding in the 2021/2022 observations and using our own independently drawn regions around the periphery of the remnant. We also find a substantial decrease in velocity in regions similar to those used in Tanaka et al. when incorporating the new data. This broadly confirms the result that the shockwave in Tycho has experienced rapid deceleration along the west and southwest rims. We also explored the width of the thin rim filaments, which are known to vary with energy, to see if there is a systematic trend of varying with time over the two decade baseline. We find no such trend. Finally, we are examining the spectral variations of the nonthermal synchrotron emission in the rim filaments over the more than 20 year baseline.

For the first time, we have separated supernova remnant Sgr A East from the Sgr A* accretion flow. We applied machine learning technique, Generalized Morphological Component Analysis (GMCA), to 1.5 Ms of cleaned and stacked Chandra ACIS-I data and are able to separate the supernova remnant from the accretion flow in multiple energy bands, identifying different physical components. We are able to successfully separate Sgr A East and analyze its X-ray morphology, and we present preliminary results from our analysis.

We present results from a deep (900 ks) observation of the Large Magellanic Cloud supernova remnant N132D with the Chandra X-ray Observatory. N132D is an O-rich supernova remnant (SNR) of a 15-25 Mdot progenitor and is the most X-ray luminous SNR in the Local Group (L_x~1.0e38 ergs/s [0.3-10.0 keV]). The Chandra images reveal the spatial distribution of the elements Ne, Mg, Si, S, and Fe in unprecedented detail. The spatial distributions of Ne, Mg, Si, and S are in general similar on large spatial scales but differ significantly on small spatial scales. The distribution of Fe is dramatically different from the lower Z elements with a centrally-concentrated morphology spread across the southern half of the remnant. We use these band images to identify features that have significantly different abundances than the average abundances in the remnant. We present spectral results on some interesting regions such as the so-called ``runaway knot'' which is associated with high velocity optical ejecta, a central region that appears to be interacting with a molecular cloud, and a bright feature in the ``void'' region in the southwestern part of the remnant. The spectral parameters are used to determine if the emission in the regions is dominated by ejecta or swept-up interstellar material.

Pulsar Wind Nebulae (PWNe) are some of nature's most interesting laboratories for studying the high-energy particles produced by pulsars. The synchrotron X-ray emission from PWNe carries information about the underlying energy spectra of accelerated particles which determine the observed spectral indices and luminosities of PWNe.  Our analysis of around 80 PWNe spectra shows that their photon indices exhibit a large spread (between 1.0 and 2.5) even for the most compact PWN structures in the immediate vicinity of the pulsar where radiative cooling should play no role. In addition, the structures themselves show significant diversity (e.g. strongly varying relative strengths of the jet and torus components). Finally, the X-ray radiative efficiencies of PWNe vary by nearly 4 orders of magnitude. A likely explanation for the diversity of compact structures formed in the vicinity of the termination shock  (and possibly also for their spectral differences) is the different magnetic obliquities and viewing angles. The same angles are found to determine the shape and brightness of pulsar lightcurves in gamma-rays and radio. In addition to performing a comprehensive and systematic census of X-ray properties of an increased (compared to previous studies) sample of PWNe, we explore the relations between these properties and pulsar geometries inferred from their lightcurve modeling.  Chandra's unrivaled angular resolution and low detector background make it the only instrument capable of studying the arcsecond-scale structure of X-ray emission from PWNe.

Supernovae are among the most energetic events in the universe, playing a key role in shaping the energy balance, structure, and chemical composition of galaxies. Despite their significant importance in astrophysics, the details of how these explosions occur—from the final stages of progenitor evolution to identifying which massive stars produce specific supernova subtypes—are not yet fully understood. Supernova remnants (SNRs), the nearby remains of supernova explosions, allow us to spatially resolve and study the ejected material and the surrounding circumstellar environment in detail. This talk will explore how the synergy between X-ray and infrared observations enhances our understanding of the properties of SNRs, their progenitor systems, and the explosions that formed them. I will focus on recent studies of SNRs that host pulsar wind nebulae and the latest findings from the Chandra and JWST study of Cassiopeia A.

Grating spectra of super-soft X-ray sources obtained by Chandra and XMM-Newton open the possibility for a detailed investigation of these sources. Previous studies of these spectra were based on blackbody fits or they used model spectra of non-LTE atmospheres. Here we present new sets of LTE model atmospheres of hot white dwarfs (WDs) computed for much more extended numbers of input parameters, namely effective temperature, surface gravity, and chemical composition, compared to existing sets of non-LTE atmospheres. The model atmosphere grids were computed for solar chemical composition, and for chemical compositions corresponding to the Large and Small Magellanic Clouds.The model spectra were used for fitting Chandra and XMM-Newton grating spectra of the classical super-soft sources CAL 83 and RX J0513.9-6951 (RX J0513) in the Large Magellanic Cloud. The obtained parameters of CAL 83 (T_eff about 560 kK, log g about 8.6 - 8.7, WD mass in the range 1.1-1.4 solar masses) are very close to the parameters obtained using non-LTE model atmospheres. RX J0513 demonstrates an evolution in accordance with a model track in the T_eff - log g plane, corresponding to thermonuclear burning on the surface of a WD with 1.1-1.2 solar masses. The effective temperature changes within the range 550 - 620 kK, and log g within 8.2-8.75. For most observations, RX J0513 is situated in the stable burning strip, whereas CAL 83 was below this strip during the observations.

We present first results from the \chandra Large Project observation of the Large Magellanic Cloud (LMC) supernova remnant N132D. N132D was observed on 28 separate observations during 2019 and 2020 totaling more than 878~ks. We compare the new data to archival data acquired in 2006 to measure the expansion of the forward shock in the bright southern rim to be $0\farcs111\pm0\farcs023$ over the $\sim14.15$~yr baseline which corresponds to a velocity of $1853\pm386\kms$. We measure a shock velocity of $3630\pm263\kms$ for a feature in an apparent blowout region in the north-east. After accounting for the PSF azimuthal difference, the corrected expansion velocity of the southern ring is $1623\pm402\kms$, and the velocity of north-east feature is $3843\pm263\kms$. The average temperature inferred from X-ray spectral fits to regions in the southern rim is $0.99$~keV. It is consistent with the electron temperature implied by the shock velocity we measured, assuming full non-equilibration between electrons and ions. We fitted 1-D evolutionary models for the SNR shock in the Southern rim and Northeast region, using the measured forward shock radius and velocity for propagation into a constant density and power-law profile circumstellar medium. We found good agreement with the age derived from optical measurements of approximately 2500 years, for explosion energies of $1.5-3.0 \times 10^{51} ergs$ and ejecta masses of $2-5 \msol$ in a constant density medium.

We present our recent studies of X-rays and shocked molecular clouds traced by radio-line emission (CO) in supernova remnants (SNRs). Multiwavelength studies using Chandra and ALMA allow us to reveal the efficient acceleration mechanism of cosmic rays and the shock-ionization processes in Galactic/Magellanic SNRs at a good resolution of ~1-6 arcseconds for the first time. The shock-cloud interaction enhances turbulent magnetic up to mG, which causes synchrotron X-ray limb-brightening on the surface of the shocked cloud, as well as short-time flux variation of the X-rays. The reflected shocks will efficiently produce the hard X-rays from high-temperature plasma. The spatially resolved X-ray spectroscopy following the CO cloud distribution is critical to understanding the spatial variation of thermal plasma conditions. Moreover, an expanding shell of neutral gas in Type Ia SNRs provides alternative evidence for the single-degenerate origin of the Type supernovae. In this poster, we also discuss future perspectives for combining studies of Chandra and ALMA.

Supernova remnants (SNRs) are fundamental to galaxy evolution, playing a key role through their energetic shocks and the enrichment of metals. Particularly, the interactions between shocks and molecular clouds are crucial not only for cosmic-ray acceleration but also for regulating the star formation rate (e.g., Kortgen et al. 2016). This study explores the universality of the physical processes driven by shock-cloud interactions. The SNR N132D, located in the Large Magellanic Cloud (LMC), is an ideal target for investigating a range of shock-cloud interactions. As the most luminous SNR in the local group, its well-established distance and minimal contamination make it possible to estimate the physical properties of molecular clouds associated with the remnant accurately. In this study, we analyzed new 12CO (J = 2-1, 3-2) data from ALMA (ID: 2021.2.00008.S), which offers a higher angular resolution (approximately <1 pc) and improved sensitivity compared to previous observations. This led to identifying twice as many molecular clouds associated with N132D. We also created a map of the 12CO (J = 3-2) / 12CO (J = 1-0) ratio at a pc-scale, revealing that the ratio is higher within the X-ray shell's interior and lower in the exterior regions. This trend aligns with Sano et al. (2020), indicating that clouds in the interior correspond to post-shock regions. Based on the spatial distribution, temperature, and density of the clouds, we plan to extract spectra from small regions and investigate the mechanisms behind both thermal and nonthermal X-ray emissions.

Correlations between the modulation of the observed radiation flux and the changes in the polarization degree and polarisation angle are expected in the orbiting spot scenario for X-ray/NIR flares from accreting black holes. We update our model (based on the KY code) and we confirm that the geometric shape of the emission region plays a significant role in the amplitudes and profiles of the model lightcurves. The emission region of an extended spiral wave can be distinguished from a simpler geometry of a localised, orbiting spot provided that the S/N is sufficiently good. We discuss the relevance of this scenario for the observed flares from the supermassive black hole in the Galactic center. Despite the fact that the hot spot model represents a phenomenological scheme to the flare origin, it helps to examine the role of model geometry. The KY code allows us to simulate the lightcurves for a wide range of parameters. The energy dependence of the changing degree and angle of polarization then allows, at least in principle, to discriminate between the cases of a rotating vs. non-rotating black hole.

Radio observations suggest that a class of radio sources characterized by compact, symmetric, edge-brightened, high-luminosity double radio structures (Compact Symmetric Objects Type 2; CSO 2s) may not evolve into larger jetted active galactic nuclei, but they may rather remain at the infant stage with sizes below a few hundred parsecs and ages between a few hundred to a few thousand years. Thus, a hypothesis has been put forward that transient fueling, e.g., via a star capture, could explain the relatively limited range of radio sizes and ages available to CSO 2s. We test this hypothesis by considering three observables: the supermassive black hole mass, the age of the CSO 2 radio structures, and the current time mass accretion rate. We discuss various methods of measuring the CSO BH masses; we associate the age of the CSO radio structures with the time elapsed since the postulated TDE; and we constrain the CSO mass accretion rates from the broadband radio-to-gamma-ray SED properties, whose characterization was facilitated by the high-energy observations with Chandra, XMM-Newton, NuSTAR, and Fermi-LAT. With these three observables, we explore the stellar mass vs. stellar radius parameter plane to uncover the fraction of CSO 2s with properties that are consistent with fueling via a star capture. Heavy black hole masses in the centers of CSO 2s, ~10^8-10^9 Msun, point at giant stars as the most likely tidally disrupted star candidates. On the one hand, population studies indicate that giant star tidal disruption events (TDEs) should be relatively rare compared to disruptions of solar type stars. On the other hand, multi-year radio campaigns revealed late radio emission in numerous TDEs. Thus, it is possible that a fraction of TDEs may evolve to become CSO 2s.

Chandra has observed the massive Black Hole + Wolf-Rayet star X-ray binary IC 10 X-1 repeatedly across its 25 year mission. Synergy between Chandra's unparalleled high-resolution X-ray optics and the other great observatories all operating at the same time, has enabling a detailed time domain study of this fascinating system and its host galaxy's X-ray binary population. IC 10 X-1 exhibits an interaction between the radiation field of the black hole, the wind of the accretion disk, and the stellar wind of the WR star. This manifests as an apparently stable phase-offset between the X-ray eclipse and the radial velocity curves traced by different ion species, which can confound traditional BH mass determinations. Contemporaneous pan-wavelength monitoring data from Gemini, HST (UV), Swift, XMM-Newton will hopefully soon be extended to the infrared by JWST, highlighting Chandra's increased value in the JWST era. IC 10 X-1 serves as a laboratory for studying the progenitors of the most massive black holes.

Stripped envelope supernovae (SESNe) include Type Ib/c and Type IIb SNe. Also related are the hydrogen-poor superluminous SNe (SLSNe-I). Type Ib/c SNe progenitors are thought to be massive Wolf-Rayet (W-R) stars (> 25-30 Msun at birth) that have lost their hydrogen (Type Ib) and possibly He envelope (Type Ic), before exploding as a SN; or lower mass W-R stars that may have lost their outer envelopes due to a binary companion. Type Ib/c SNe are surrounded by a low-density wind-blown medium, due to the fast winds (> 1000 km/s) from their progenitors. The density of a constant parameter wind should decrease with radius. Chandra observations of SESNe have however revealed that some, such as SN 2004dk and SN 2014c, show increasing X-ray emission with time). SN 1996cr, likely a stripped envelope SN, although early time observations to confirm the SN type were not available, also showed a sharp increase in X-ray luminosity, leading to a 500 ks deep Chandra observation. Increasing X-rays were not observed in SN 2001em, but are inferred from Chandra, XMM-Newton and Swift X-ray observations. Understanding the hydrodynamics of the interaction of the SN shock wave with this medium is key to evaluating the emission signatures from the SN. We study the interaction using analytic calculations as well as numerical simulations, followed by computation of the X-ray emission using non-equilibrium ionization calculations. We show how the temporal evolution of the X-ray emission depends on the parameters of the external medium, how the emission mechanism can change when the shock collides with the dense medium, and how the contribution of the forward and reverse shocks to the total X-ray emission will vary with time. Our results can be used to understand and interpret the X-ray emission from SESNe observed by Chandra, such as SN 1996cr and SN 2014C.

The sub-arcsecond resolution of the Chandra X-ray Telescope makes it an ideal instrument for crowded, active regions. Together with the >10keV capabilities of NuSTAR, we can probe the energetic coronal flare behavior of cool stars in the Orion Nebula Cluster (ONC), a dense, young star forming region. The flare properties of main-sequence (MS) stars are reasonably well understood, However, there is initial evidence that flares on young, pre-main sequence stars differ from their MS counterparts. As the hard X-rays generated in energetic flares can penetrate deep into the circumstellar disk, they strongly influence the disk chemistry with decisive consequences for planet formation. We report on 90 ks of simultaneous Chandra and NuStar observations of the ONC. Chandra resolves the sources in the dense core pin-point the flares, while NuSTAR allows us to directly measure the high energy tails. We report on the brightest of these flares, co-analyzing Chandra and NuSTAR, and present both light curve morphologies and spectra, in order to obtain an accurate and complete census of flares from young stars.

We briefly describe the Julia package for spectroscopy, SpectralFitting.jl. The ultimate aim is a single package to handle any spectral data across a broad variety of wavelengths. Julia is a language designed to take advantage of multiple CPUs, parallel programming, and multiple dispatch among other advantages. It is a high-level language that is inherently fast with no 'second language' problem. We illustrate some basic spectral fitting using XRISM and HETG grating data of SS Cyg.

Thanks to Chandra's high resolution capabilities, we have enjoyed unique resolved X-ray images of galaxies of all morphological types, allowing for population studies of X-ray binaries (XRBs), hot gas, and supernovae (SNe) in a variety of environments. XRBs probe the demographics of compact objects, close binaries, and massive stars, as well as the physics of accretion onto compact objects. Hot gas and SNe provide important tracers of stellar feedback in galaxy interstellar mediums (ISMs). As multiwavelength data sets on galaxies have vastly expanded over the last 25 years, Chandra data have been central to linking our understanding these X-ray emitting components to the properties of their host galaxies. I will discuss a number of galaxy studies and the insights that have been established over the years. I will further highlight how Chandra's archival and forthcoming PHANGS Legacy survey data will continue to be used to make headway in several highly active areas of astrophysics, complementing our understanding of stellar evolution models, ISM ionization in low-metallicity and high-redshift galaxies, formation pathways of other binary-related objects (e.g., GRBs, SNe, gravitational wave sources, etc.), and heating of the intergalactic medium during the epoch of heating at z > 8.

Based on the newly acquired dense gas observations from the JCMT MALATANG survey and high quality X-ray data from Chandra, we explore the correlation between X-ray emitting hot gas and HCN J=4→3, HCO+ J=4→3 emission for the first time at sub-kiloparsec scale of nearby star-forming galaxies. We find that both HCN J=4→3 and HCO+ J=4→3 line luminosity show a statistically significant correlation with the 0.5−2 keV X-ray emission of the diffuse hot gas. At the sub-kiloparsec scale, we find that the power-law index of the Lgas0.5−2keV − star formation rate (SFR) relation is log(Lgas0.5−2keV/ergs−1)=1.80log(SFR/M⊙yr−1)+39.16, deviated from previous linear relations at global scale. This implies that the global property of hot gas significantly differs from individual resolved regions, which is influenced by the local physical conditions close to the sites of star formation.

The established LX-SFR relation links X-ray luminosities (LX) to star-formation rates (SFRs) in star-forming galaxies. However, Luminous Infrared Galaxies (LIRGs) often appear under-luminous compared to predictions from this relation. Proposed explanations include significant intrinsic absorption, elevated metallicities, and young stellar populations. Using Chandra X-ray data and galaxy properties from the HECATE catalog, we analyze the X-ray spectra of ~60 LIRGs to test these theories. Our study focuses on disentangling the contributions of hot gas and X-ray binaries (XRBs) to LIRG X-ray emission, through spectral modeling. We present refined LX estimates for both hot gas and XRBs, highlighting their correlations with galaxy properties. Our results provide new insights into the observed X-ray deficit in LIRGs and the role of hot gas and XRBs in shaping this phenomenon.

The hot X-ray emitting interstellar medium has been extensively studied in nearby galaxies, and relationships between hot gas X-ray emission and galaxy SFR have been identified. However, the results of most studies of this gas in the literature have focused on galaxy-integrated properties, while the characteristics of hot gas are expected (and have been observed) to vary on sub-galactic scales. Here, we present preliminary work assessing how hot gas properties - such as absorption, temperature, and luminosity - vary with local SFR density in the nearby galaxy M83. We analyzed eleven archival Chandra observations totaling more than 750 ks of exposure time, giving us extremely deep coverage of the galaxy. With this, we present a new model for sub-galactic properties of the hot ISM as they relate to the star-formation activity within different regions in M83.

In the last two decades the number of millisecond pulsars (MSP; pulsars with intrinsic spin period, <20 millisecond) have more than tripled (from 150 to ~450). This boost in MSP discovery can be attributed to enhanced pulsation search algorithms combined with treasure maps of point sources from high-energy observational surveys such as Fermi-LAT catalogs, ROSAT, LOTASS 2.0 and the recent release of the eROSITA. Together, these developments have opened up the MSP searches to previously unexplored parts of the parameter search space. Chandra X-ray Observatory has been at the forefront of millisecond pulsar searches especially in crowded environments owing to its unmatched spatial and timing resolution. We systematically searched the Chandra Source Catalog (CSC 2.1) for millisecond pulsars in both accreting and non-accreting states. We also used X-ray properties of unique compact objects in the CSC 2.1, such as the X-ray spectra, variability and cross-matched these sources with other high energy catalogs mentioned above. In this poster we present findings from these searches.

We present a powerful, publicly available tool for extracting photon and exposure information from Chandra observations. Using only a list of sky positions, this tool extracts this information from all existing publicly available Chandra pointings, compiles a master catalog, and reports stacked photon counts and fluxes for these positions. Using only a list of sky positions, this tool will enable any researcher to quickly and easily calculate average X-ray properties and other advanced statistical metrics, which is extremely useful for multiwavelength studies. For bright/resovled sources, our algorithm calculates individual source fluxes which are as reliable as the fluxes from Chandra point source catalog 2.0. This program significantly increases the scope for studying the X-ray emission from the large populations of galaxies detected in wide-field surveys. The primary advantage of this technique is that the time consuming aspect - extraction and processing, occurs once, and then the list of photons allows the user to co-add the emission from any subset within the master catalog. We demonstrate some uses of this tool, including recent results on high-redshift AGN ("Little Red Dots") and highly obscured infrared-selected AGN.

Stripe 82X is a wide area (31.3 deg^2) survey overlapping the legacy Sloan Digital Sky Survey Stripe 82 field. Using X-ray data from Chandra and XMM-Newton, 6181 X-ray sources are significantly detected, 3457 of which have spectroscopic redshifts (56% spectroscopic completeness). In this catalog release, we publish 343 new spectroscopic redshifts and classifications and present black hole masses for 1297 Type 1 AGN. Using these black hole masses and estimating bolometric luminosities from X-ray spectral fitting, we find AGN at the highest X-ray luminosities are accreting at the highest Eddington ratios. This result is consistent with the paradigm that most black hole growth occurs in phase when the AGN is luminous.

Globular clusters (GCs) are ancient and massive star clusters on the outskirts of galaxies. Their density makes them important testbeds for modeling stellar evolution and cluster dynamical evolution, including formation channels of the numerous compact objects found within them. Over the last two decades, Chandra (CXO) has discovered large populations of active binaries (ABs), cataclysmic variables (CVs), millisecond pulsars (MSPs), and low-mass X-ray binaries (LMXBs) in over 60 GCs. This was possible due to the unprecedented subarcsecond angular resolution of CXO, enabling the correct identification of optical counterparts from HST. However, the crowded environment still makes assigning the correct multiwavelength counterpart challenging, and over one thousand CXO sources remain unclassified. We cross-matched the Chandra Source Catalog (CSC) to the HST UV Globular Cluster Survey (HUGS) and created a training dataset of ~270 reliably classified GC X-ray sources. Using this training dataset, we present the results of machine learning classification of hundreds of CSC sources in the globular clusters Omega Centauri and 47 Tuc based on X-ray and optical features. We find substantial numbers of classified AGNs, ABs, CVs, and a few MSPs. We use spectra, lightcurves, and other data to evaluate the accuracy of, and perform population studies on these nominal classifications. We discuss improvements to our machine learning pipeline compared to past works, as well as implications of our results on future machine learning classification efforts.

Traditional cross-matching algorithms in astronomy primarily rely on spatial information, such as the angular separation of the objects and how well their positions are determined. These methods do not capture the full scope of the intrinsic properties of the objects like magnitudes, colors, distance, etc. This limitation raises the possibility of errors and misidentifications, particularly when ambiguity exists in the matches. In the context of X-ray sources, we require a more robust multi-wavelength approach to catalog cross-matching. Using positional matches of X-ray and optical catalogs, we demonstrate that classical separation-based methods of cross-matching are inadequate, and propose a machine learning based method to match and resolve ambiguous matches using associated properties. When combined with existing spatial crossmatch algorithms, our model increases the overall reliability of the matches. We apply the approach to the Chandra Source Catalog 2.1 with Gaia DR3 and generate a final catalog of reliable matches. We acknowledge support from Chandra grant AR3-24002X and from the NASA Contract to the Chandra X-Ray Center NAS8-03060.

Recent work has shown that the emission from X-ray binary (XRB) populations in galaxies varies with stellar mass (M*), star-formation rate (SFR), and metallicity (Z). Such scaling relations are widely used in a variety of science applications to predict the XRB contributions to galaxy-integrated X-ray luminosities measured with Chandra (and other observatories). However, as galaxies approach low-SFR and low Z, the relatively shallow slope of the XRB luminosity function (XLF) can yield very large stochastic variations in the total X-ray luminosity expected from the XRB population, for fixed values of M*, SFR, and Z. We have created a procedure to conduct statistical sampling of any XLF to produce predictions for total XRB population spectral energy distributions (SEDs) and their stochastic uncertainties. We show the use of this procedure by studying galaxies that fall on the galaxy main-sequence relation (M*-SFR) and mass-metallicity relation (M*-Z). We are currently investigating how simply knowing the stellar mass of a population can translate into a predictable X-ray SED model for XRB populations. In the long term, we aim to incorporate our modeling procedures into SED fitting codes that use X-ray data to aid in constraining galaxy properties (e.g., Lightning).

We present a comprehensive search for periodic X-ray sources in the bulge of M31, utilizing approximately 2 Ms of archival Chandra observations collected over a span of 16 years. By applying the Gregory–Loredo algorithm, which is optimized for photon-counting and phase-folded light curves, we have identified seven periodic X-ray sources. Among these, four are newly discovered. Three of the sources are identified as novae, with periods ranging from 1.3 to 2.0 hours, likely corresponding to their orbital periods. The remaining four sources are classified as low-mass X-ray binaries, exhibiting periods between 0.13 and 19.3 hours, also presumed to be orbital due to evident eclipsing or dipping behaviors observed in their light curves. Our findings provide insights into the X-ray binary population within the M31 bulge. This study underscores the efficacy of leveraging archival X-ray data to systematically detect periodic X-ray sources in external galaxies, offering significant contributions to our understanding of exotic stellar populations.

On November 22, 2023, a Chandra DDT observation of the microquasar Cygnus X-3 (Cyg X-3) was taken during a quiescent state in the middle of an IXPE observation. With Chandra's superb energy resolution, we are able to resolve line features that are difficult to discern in the IXPE spectrum, aiding endeavors for spectropolarimetric fitting. In addition, the Chandra spectrum is rich with information about the interaction between the compact object and the stellar wind. By decomposing the dataset into phase bins, we can observe the orbital modulation of the lines as well as measure their velocity shifts with phase. We present the phase-resolved spectral analysis of Cyg X-3 from the 2023 Chandra observation and discuss interesting features in the spectrum and their possible physical implications.

Pile-up is a ubiquitous problem in X-ray CCD detectors that occurs when two or more X-ray photons strike a detector within the same readout time and are interpreted as a single “piled-up” event. In instruments such as the ACIS detector on the Chandra X-ray Observatory, pile-up can cause the registration of events as events of higher energies, a decrease in event count rate, and the migration of events to “grades” of poorer quality. Pile-up distorts the X-ray spectrum of bright objects and prevents us from accurately estimating the spectral parameters of sources such as X-ray binaries. We develop a data-driven neural network emulator to mimic the empirical effects of pile-up on the spectra emitted by accreting black hole systems. We train our emulator on simulated data generated with MARX (https://space.mit.edu/cxc/marx/) and validate it with both simulated data from MARX and observed data from Chandra/ACIS and Chandra/HETGS+ACIS-S (High Energy Transmission Grating Spectrometer + ACIS-S). We examine the ability of our emulator to accurately mimic pile-up in Chandra/ACIS and discuss potential refinements of the emulator that would improve analysis of archival Chandra/ACIS observations. We also discuss the prospects for generalizing our method to state-of-the-art and upcoming X-ray telescopes.

We report the detection of simultaneous QPO triplets, consisting of a low-frequency QPO at ~ 40 Hz and twin kHz QPOs at about 800 and 1000 Hz, from the AstroSat/LAXPC observation of 4U 1728-34. For the very first time, we report the detection of multiple sets of QPO triplets from a single observation in which the QPOs evolve with time, showing a remarkable correlation with each other and are consistent with their identification with the general-relativistic precession model. The observed correlation puts unprecedented constraints on the mass and moment of inertia of the neutron star to be M*⊙ = 1.92 ± 0.01 and I45/M*⊙ = 1.07 ± 0.01, respectively . We also estimate the mass and moment of inertia of neutron stars by solving the Tolman-Oppenheimer-Volkoff equations in conjunctions with the constraints from the Gravitational-wave event GW170817 and find that the parameters obtained from QPO observation favor relatively stiff equations of state (EOS) of the neutron star. Furthermore, the observed QPO frequencies show a tight correlation with the spectral with the spectral parameters of the source. The results provide insights into the nature of the QPOs and hold the promise of stringent constraints on the EOS of neutron stars

Cygnus X-3 (Cyg X-3) is a microquasar which is composed of a Compact Object (likely Black Hole) and a Wolf-Rayet Companion (the only known Wolf-Rayet X-ray binary in our Galaxy). It has a short orbital period of 4.8 hrs which imply an orbital separations of ~4 r_sun. The system routinely goes through a number of state changes (quiescent, quenched/hypersoft, major flaring, and minor flaring) which show correlated changes in the radio, X-ray, hard X-ray, and gamma-ray. It is know to produce relativistic jets which align close (< 10 deg.) to our line-of-sight. This presentation will review the discoveries made from Chandra observations of Cyg X-3. This will include X-ray imaging of Cyg X-3 and the discovery of the Cyg X-3's Little Friend (first X-ray detection of a Bok globule) and its large (~0.5 deg.) scattering halo. Also discussed will be the Chandra HETG observations made of Cyg X-3 in various states and orbital phases. These results will be linked to what is observed at other wavelengths and what has been seen with other observatories.

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. Models predict 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. The process that triggers the onset of an X-ray outburst has remained unconstrained observationally. Here, we report optical and X-ray monitoring of the early rise of the black hole X-ray binary, Swift J1753.5-0127, from the quiescent level, including the earliest Chandra detection of an outburst rise (20 photons). 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 from its quiescent level). We show that the ionization of hydrogen from a ring in the disc initiates the optical outburst. Initially the surface temperature is ~7,400 K and the ionized area is about 15-20 % the size of the full disc. The temperature then increases to ~14,000 K during the outburst rise and the ionized area increases to the full disc size. The mass accretion rate into the corona and onto the black hole do not increase until the X-rays brighten, which occurs when the optical flux has already increased by a factor of ~50-100 from pre-outburst levels, when the disc temperature was ~11,000 K. Once the X-rays start to brighten, signatures of irradiation appear on the disc surface (from optical emission lines), and synchrotron emission from the jet is also seen at radio and infrared wavelengths. The rapid then slower rise at optical and X-ray wavelengths suggests it is an outside-in outburst (the heating front starts in the outer disc), and we constrain the viscosity parameter to be alpha ~ 0.08. We conclude that as predicted by disc instability models, X-ray transient outbursts are caused by a heating front of ionized hydrogen in the disc, which propagates to the inner regions, causing matter to be accreted onto the black hole, associated with irradiation, and jets. This has implications for the switching on of AGN, and tidal disruption events. We demonstrate the ability of optical monitoring to be able to provide a few days lead time to the rise of X-ray transient outbursts.

We present, for the first time, broadband spectral and timing analysis of SMC X-1 during its super-orbital period excursion. SMC X-1 is a neutron star X-ray binary with a high mass companion, known to exhibit super-orbital period variability on the scale of 45-60 days, due to obscuration by a warped, precessing accretion disk. It undertakes excursions where the period decreases to ~45 days every ~10 years. We study SMC X-1 using observations from NuSTAR (Nuclear Spectroscopic Telescope Array) and XMM-Newton (X-ray Multi-Mirror Mission). We disentangle the high energy emission of the pulsar beam from the low energy emission of the accretion disk by creating energy-resolved pulse profiles. By modeling the shape of energy-resolved pulse profiles with a warped disk model, we investigate the geometry of SMC X-1’s accretion disk and search for changes to the disk shape. We also examine the broad-band X-ray spectrum to understand accretion rates during excursion. We find that the geometry of the disc does not change significantly during excursion. Studying the unstable accretion disk geometry of SMC X-1 allows us to study similar phenomena in other sources, particularly the ultra-luminous X-ray pulsars showing irregular changes in brightness with time.

In normal galaxies, hot gas and X-ray binaries (XRBs) dominate the X-ray emission spectrum. Thanks to high spatial resolution imaging with the Chandra X-ray Observatory, we can separate these emission components spatially and analyze the contributions to the galaxy-wide spectrum from XRB point-sources directly. Using Sherpa, we are generating grid spaces of XRB models that include intrinsic absorption and physical models of accretion disk and Comptonization emission. Our goal is to use X-ray color information from Chandra-detected sources in M83, along with our model grids, to identify a range of physically realistic models that are consistent with the data. The models can then be summed to build a galaxy-wide X-ray spectral model for XRBs in M83. We will discuss our procedure for constructing these models and using color information to constrain physical parameters of the XRBs.