We present a detailed analysis of the X-ray point source population of M81. By identifying HST counterparts to Chandra sources, taking into account the chance coincidence probability, we classify a large fraction of the X-ray point source population with unique counterparts: high-mass X-ray binaries and low-mass X-ray binaries in globular clusters. We then compare the shapes of the uncontaminated X-ray luminosity functions and the X-ray properties of sources of different classes to models and other existing work. We also calculate scaling relations with the star formation rate and stellar mass between the different classes of sources in global and sub-galactic scales. One initial, primary result is that the more massive and dense globular clusters are more likely to be associated with X-ray binaries.
A soft X-ray emission in excess of the extrapolation of the hard X-ray continuum is detected in many Seyfert 1 galaxies below 1 keV. To understand the uncertain nature of this soft excess, which could be due to warm Comptonization or to blurred ionized reflection, we consider the different behaviors of these models above 10 keV. I will first present the results of a study done on about one hundred Seyfert 1s for which we performed a joint spectral analysis of XMM-Newton and Swift/BAT data in order to get a hard X-ray view of the soft excess. I will then discuss recent results obtained by performing a similar analysis using simultaneous Chandra and/or XMM-Newton, and NuSTAR observations of about ten objects of our previous sample.
I present an analysis of Chandra/LETGS and Chandra/HETGS observations of the ultracompact X-ray binary 4U 1626-67. This system, hosting a neutron star accreting from a white dwarf companion, is one of the most compact in its class, with an orbital period of 41 minutes. The neutron star is also a pulsar with a period of 7.7 s and a magnetic field of around 4×1012 Gauss. A recent Chandra/HETGS study identified a collisional plasma as the origin for strong Ne and O diskline emission. These disklines imply a source inclination of 38 degrees, suggesting a carbon/oxygen composition for the white dwarf donor. The Chandra/LETGS observations were designed to identify emission from H- and He-like carbon in order to verify the expectation that the accreted plasma is mostly composed of carbon and oxygen, and allow us to establish a low temperature limit for the collisionally ionized plasma. The previously reported detection of a narrow Fe line also provides a challenge to interpretations of the nature of the system, as this line was detected in observations in 2010 and 2015, but not in 2014. I will discuss our findings in the context of the origins of the collisional ionized plasma as well as the nature of the Fe K line emission.
We present Chandra and XMM-Newton X-Ray observations of the Abell 3391/Abell 3395 intercluster filament. The galaxy clusters Abell 3395, Abell 3391, to the north and south respectively, and the galaxy group ESO-161 located in between the two subclusters are in alignment along a large-scale intercluster filament. We find a global projected temperature kT = 4.91-0.7+1.4 keV, and electron density ne=5.4+7.3-1.3×10-5 cm-3 for the intercluster filament. These values are consistent with intracluster medium from Abell 3395 and Abell 3391 being tidally pulled into the filamentary region. We present temperature, density, entropy, and abundance profiles across the filament, as well as simulated profiles for Lynx exposures of the same amount of time as the Chandra exposures.
Plasma pervades the observable Universe. The intracluster medium (ICM) is the largest reservoir of astrophysical plasma—ionized hot gas, emitting X-ray via bremsstrahlung, that is subject to electromagnetic forces. Thanks to its unprecedented spatial resolution, Chandra has revealed a wealth of substructure in the ICM. Of particular interest is the ubiquitous presence of cold fronts, AGN bubbles, and gaseous stripped tails. Lynx, with its tremendous increase in sensitivity over existing instruments, will greatly expand our knowledge of cluster plasma physics by revealing the development of Kelvin-Helmholtz Instability, Rayleigh-Taylor instability, and magnetic draping. The ICM provides a unique laboratory with which to study plasma physics under extreme conditions. At the same time, the application of plasma physics to the ICM provides insights into the thermalization and evolution of galaxy clusters, specifically into how energy is transported and dissipated.
Deep observations of nearby galaxy clusters with Chandra have revealed concave 'bay' structures in a number of systems (Perseus, Centaurus and Abell 1795), which have similar X-ray and radio properties. These bays have all the properties of cold fronts, where the temperature rises and density falls sharply, but are concave rather than convex. By comparing to simulations of gas sloshing, we find that the bay in the Perseus cluster bears a striking resemblance in its size, location and thermal structure, to a giant (≈ 50 kpc) roll resulting from Kelvin-Helmholtz instabilities. If true, the morphology of this structure can be compared to simulations to put constraints on the initial average ratio of the thermal and magnetic pressure, β = pth/pB, throughout the overall cluster before the sloshing occurs, for which we find β=200 to best match the observations. Simulations with a stronger magnetic field (β=100) are disfavoured, as in these the large Kelvin-Helmholtz rolls do not form, while in simulations with a lower magnetic field (β=500) the level of instabilities is much larger than is observed. We find that the bay structures in Centaurus and Abell 1795 may also be explained by such features of gas sloshing. These results are only possible thanks to the unrivaled subarcsecond spatial resolution of Chandra. The future impact that Lynx will have on the study of subtle surface brightness features resulting from gas sloshing and the development of Kelvin-Helmholtz instabilities will be discussed.
Black holes are simple, and accretion is accretion, right? Analogies between accretion and emission processes in X-ray binaries and quasars are expected to scale appropriately by factors of up to a billion in black hole mass, mapping accretion states in both temporal and spectral domains. Several such analogies have been predicted, but the relative dearth of observed quasar state transitions on timescales of an astronomer lifetime have stymied progress. Our dedicated new searches are discovering many "changing look quasars" (CLQs) that precipitously drop or regain UV/Balmer continuum and broad line emission, effectively switching between quasar and galaxy states, which seem to provide quasar accretion state changes on tractable timescales. We describe ongoing and planned optical spectroscopic surveys to find and characterize CLQs. Multiwavelength studies of CLQs have just begun as well; X-ray observations promise to distinguish between theories competing to explain the CLQ phenomenon, but also to confirm and/or extend the analogy to X-ray binaries. At the dawn of the upcoming era of all-sky time domain surveys, we can expect thousands of such transitions to be detected and studied. More powerful X-ray observatories like Lynx can track luminosity and spectral changes around supermassive black holes of even modest transitions with high fidelity.
The obscuring torus is one of the main components of the basic unified model of active galactic nuclei (AGN), needed to create anisotropy in obscuration as a function of the viewing angle. We present the first study of the geometrical properties of the AGN torus in a large and representative sample of type II Seyfert nuclei. The sample consists of 124 AGN selected in the hard X-ray band from the Swift/BAT 70-month catalog and observed simultaneously with NuSTAR and Swift/XRT. These data enable us to explore the constraints that observed spectra place on the properties of the obscuring torus in individual AGN and in the local population of Seyfert II nuclei. We make use of empirically motivated spectral models for X-ray reprocessing in approximately toroidal geometry for constraining the distribution of the average column density of the torus, and the distribution of the torus covering factor within this sample. We find that the torus-averaged column density is independent of the line-of-sight column density, with typical column density that is borderline Compton-thick, i.e., around the unity optical depth for Compton scattering. The distribution of torus covering factors is broad but shows a preference for high covering, peaking around the covering factor of 90%, with the median at 70%, in agreement with recent sample studies in the infrared band. We also examine the dependence of the covering factor on intrinsic luminosity, finding that the median covering factor peaks around the intrinsic X-ray luminosity of 1042.5 erg/s and decreases toward both lower and higher luminosities.
Numerous studies on the connection between X-ray AGN and their host galaxies are building up a detailed picture of the physical mechanisms regulating AGN activity and star formation as a function of redshift. Most of these works have focused on moderate and high-luminosity AGN with LX>1042 erg/s, and only recently the lower-luminosity AGN population in dwarf and early-type galaxies was investigated using stacking techniques. We present the redshift evolution of low-luminosity AGN (LX < 1042 erg/s) in star-forming galaxies at z=0.1-5 in the COSMOS field, using a sample of 123,000 star-forming galaxies and the Chandra COSMOS Legacy survey data providing 160 ks of exposure for each source. The large area of the COSMOS survey enables the study of galaxies with a wide range of physical properties, allowing us to probe not just the locus of the star-forming main-sequence but also galaxies with higher and lower than average specific star formation rates (sSFRs). Stacking the X-ray data of galaxies grouped into z, mass (M), and star formation rate (SFR) bins, we are sensitive to average X-ray luminosities between 1040 erg/s at z=0.2 and 1042 erg/s at z=3, two orders of magnitude fainter than X-ray detected sources. At these luminosities, X-ray binaries (XRBs) make a significant, or even dominant, contribution to the total X-ray emission, and some studies suggest that their emission evolves with redshift, but different redshift dependences for LX/SFR and LX/M of XRBs have been measured. Our stacking analysis reveals that the average LX of star-forming galaxies exceeds the luminosity expected based on local XRB relations, and that this luminosity excess is higher for stacks of galaxies at higher redshifts and with lower sSFR. The variations of X-ray hardness ratios with redshift and sSFR suggest that the LX of galaxies with high sSFR is dominated by XRBs at all redshifts, while the LX of low sSFR galaxies above z∼1 is dominated by AGN that may be highly obscured. We find that the redshift dependences of XRB luminosity derived by previous studies overestimate the LX contribution of XRBs for our sample. We discuss the implications of these results for studies of XRB evolution and obscured AGN populations with Lynx.
Chandra's High Energy Transmission Grating Spectrometer (HETGS) has revolutionized high-resolution x-ray absorption and emission line spectroscopy of astrophysical sources, but it is limited by its small effective area (<60 cm2 below 1 keV). Its resolving power R (λ/Δ λ) of ∼1000 has helped to provide a wealth of information over the last 18 years, but it is insufficient to address many current high priority science questions related to the evolution of clusters, galaxies, and stars. Examples requiring R∼5000 are the state and abundance of baryons in galactic halos, feedback measurements in high-z galaxy clusters and from supermassive black holes, young stars and stellar lifecycles, and interstellar dust. Even exoplanet atmospheres could be studied with a powerful spectrograph. The transmission grating technology to provide R>5000 with thousands of cm2 effective area on Lynx exists today: Critical-angle transmission (CAT) gratings are blazed soft x-ray diffraction gratings that combine the advantages of traditional transmission gratings (low mass, relaxed alignment tolerances, transparency at high energies) and blazed reflection gratings (high efficiency, high resolving power through the use of higher orders). We have demonstrated R>10,000 and efficiency >30%. CAT gratings have passed vibration and thermal testing and have been vetted at Technology Readiness Level 4 in 2016. We have fabricated numerous gratings 32 mm x 32 mm in size. CAT gratings are baselined for the currently proposed Arcus x-ray spectroscopy Explorer mission. We will give an overview over the current state in CAT grating technology and instrument design for a Lynx CAT grating spectrometer.
One of the most interesting classes of objects in the X-ray universe are the ultraluminous X-ray sources (ULXs) as they probe the accretion physics at the most extreme accretion rates. Their reported contribution in the heating of the universe during the epoch of reionization, and their connection with exotic systems (e.g. accreting pulsars at high accretion rates, or intermediate mass black holes) further underlines their importance. The difficulties in directly measuring compact objects' properties or identifying donors have led to statistical studies of ULXs, by means of correlating their populations with their host galaxy properties. Such studies have been made possible by the wealth of data acquired with the Chandra and XMM-Newton observatories.
In order to study ULX populations in the local universe and their connection with the environment, we have compiled a volume-limited catalog of galaxies including robust estimates of their distances, star-formation rates, stellar masses, metallicities etc. based on several multiwavelength indicators (ranging from FIR to UV). By cross-matching with the Chandra Source Catalog, we provide the most up-to-date catalog of nearby ULXs. We discuss the population of ULXs based on this analysis and their association with galaxies of different types.
We present results from Chandra observations of the X-ray starburst galaxies NGC 3310 and NGC 2276. Previous studies showed that these spiral galaxies exhibit interesting morphologies. NGC 3310 shows a circumnuclear star-forming ring triggered from a minor interaction while NGC 2276, which is also member of a small group of galaxies, exhibits a compressed side (west) as it moves supersonically through the intragroup medium of the NGC 2300 group. We detect 30 discrete sources with luminosities above 2.0× 1038 erg s-1 in NGC 3310, and 17 discrete sources with luminosities above 5.0× 1038 erg s-1 in NGC 2276. The majority of the sources have photon indices of 1.7-2.0, typical for X-ray binaries. Both galaxies have large numbers of Ultra Luminous X-ray sources (ULXs), 14 for NGC 3310 and 11 for NGC 2276. The majority of ULXs in NGC 3310 lay in the star-forming ring and the north spiral arm of the galaxy. It also has a variable nucleus with no sign of an AGN. Based on these observations we investigate whether the morphologically distinct regions of the galaxy (rings, spiral arms) and subgalactic regions at different galactic radii follow the existing scaling relations that correlate X-ray binary luminosity, SFR and the stellar mass. Any existing trends will give us valuable information on distant galaxies with the same behaviour. In NGC 2276 we see an intriguing excess of ULXs in the west (compressed) side of the galaxy. More interestingly the overall luminosity of the ULXs is ∼5 times larger on the west side while the star formation rate and metallicity on the two sides of the galaxy are comparable. We argue that this tentative excess of ULXs in the shocked side of the galaxy most likely reflects the difference in the age of the stellar populations which makes it a perfect laboratory to study the connection between ULXs and the age of the stellar populations.
We present a library of spectral energy distributions (SEDs) for Galactic black-hole X-ray binaries (XRBs) as a function of Eddington ratio, based on RXTE spectra. We discuss the evolution of the SED shape as a function of the Eddington ratio and the BH mass. These SEDs can be used in population synthesis models to compute bolometric luminosities in order to model XRB populations. By combining this library with broad-band spectra of Ultraluminous X-ray sources (ULXs), based on NuSTAR observations, we calculate the synthetic integrated X-ray spectrum of a galaxy. We achieve this by folding these spectra through the observed XLF for its XRB population above the 1036 erg s-1. We find that the contribution of ULXs changes significantly the shape of the SED as their spectrum shows a turnover at ∼10 keV.
Galaxy formation simulations predict that a large fraction of the baryonic mass of a spiral galaxy resides in the warm-hot circumgalactic medium (CGM). We have made deep Suzaku observations to probe the CGM of a nearby galaxy NGC3221 in emission. Unresolved point sources, mostly AGN, contaminate the weak emission from the diffuse plasma, so need to be removed. The superb sensitivity of Chandra allowed us to detect point sources in our Suzaku fields. Here we present preliminary results on X-ray point sources detected in our recent (March 2017) Chandra observations of NGC 3221 Off-field 1 and NGC 3221 Off-field 2.
We present a survey of X-ray point sources in the nearest and dynamically young galaxy cluster, Virgo, using archival Chandra observations that sample the vicinity of 80 early-type member galaxies. We detect a total of 1046 point sources (excluding galactic nuclei) out to a projected galactocentric radius of 40 kpc and down to a limiting 0.5-8 keV luminosity of 2×1038 erg/s. Based on the cumulative spatial and flux distributions of these sources, we statistically identify ∼120 excess sources that are not associated with the main stellar content of the individual galaxies, nor with the cosmic X-ray background. This excess is significant at a 3.5σ level, when Poisson error and cosmic variance are taken into account. On the other hand, no significant excess sources are found in the halos of a control sample of field galaxies, suggesting that at least some fraction of the excess sources around the Virgo galaxies are truly intra-cluster X-ray sources. Assisted with ground-based and HST optical imaging of Virgo, we discuss the origins of these intra-cluster X-ray sources, in terms of supernova-kicked low-mass X-ray binaries (LMXBs), globular clusters, LMXBs associated with the diffuse intra-cluster light, stripped relic of galactic nuclei and free-floating massive black holes. We also discuss the Lynx perspective of studying intra-cluster X-ray sources.
We report new Chandra X-ray observations of the shell supernova remnants (SNRs) Kes 75 (G29.7-0.3) and G310.6-1.6, both containing a pulsar and pulsar-wind nebula (PWN). For Kes 75, expansion of both shell and PWN is apparent across the three epochs, 2000, 2006, and 2016, but brightness and morphology changes of the PWN make a quantitative measurement difficult. One image comparison method gives an expansion rate between 2006 and 2016 of the NW edge of the PWN of about (0.2-0.25)% yr-1, for an expansion age R/(dR/dt) of 400-500 yr. Consistent results are obtained between 2000 and 2016. Since 2008, the pulsar has had a period of 328 ms and a braking index n of 2.19 (Archibald et al. 2015), giving a spindown age tsd = P/((n - 1) dP/dt) of 1230 yr, an upper limit to the true age under the normal assumptions of magnetic-dipole energy loss with constant n (though n has changed from 2.65 to its current value for this pulsar following a bright magnetar-like outburst in 2006). Our result indicates that the initial spindown time τ = tsd - t is of order t, the true age. For t < τ, simple models predict the PWN radius to grow as R∝6/5, so that the true age is 1.2 times the expansion age, or about 500-600 yr. For the current braking index, the pulsar's initial luminosity was larger than the current value by a factor of 4-6, while the initial period was within a factor of 2 of its current value. We confirm directly that Kes 75 contains the youngest known PWN in the Galaxy, independent of assumptions about the pulsar spindown. The PWN contains a jet whose structure and brightness have evolved significantly since 2009. The brighter northern part of the jet at the center of the PWN has faded by 13% in 2009 and by 23% in 2016. In 2016, the outermost northwest part of the PWN has faded by about 10%. We attribute this PWN fading to a sudden decrease in the pulsar's wind luminosity (and an associated drop in n) following the bright 2006 outburst. An incomplete shell of Kes 75 expands at an average rate of 0.05% yr-1, implying a significant deceleration of the blast wave. Supernova ejecta must be colliding with a much denser than average ambient medium south and east of the pulsar where we measured expansion of the shell. A complex pattern of measured proper motions in the east provides us with a unique opportunity to study this collision in an unprecedented detail. G310.6-1.6 might be a young (∼1000 yr) SNR, since its PWN is surrounded by a very circular, but nonthermal, shell. But we detect no expansion of the shell by comparing Chandra observations in 2010 and 2016. This rules out a very young age for this remnant. This is puzzling since we found no trace of thermal emission is the deep (200 ks) Chandra spectrum of the shell. We will discuss the evolutionary status of this mysterious PWN/SNR system based on constraints derived from our shell expansion measurements. We will also describe variations that we found within the PWN, including an apparent outward motion of a bright knot south of the pulsar. Kes 75 and G310.6-1.6 are highly dynamic objects, with changes in shape and variations in brightness detectable with Chandra over time baselines as short as several years. We have much more to learn about them by studying motions and variations in brightness of their shells and PWNe over longer time baselines and with deeper Chandra (and also Lynx) observations.
Previous XMM-Newton observations of the active galactic nucleus (AGN) PG 1211+143 have provided evidence for ionized outflows with velocities of 0.07c and 0.129c. In April 2015, we have carried out simultaneous multiwavelength observations of this AGN, including X-ray Chandra High Energy Transmission Gratings (HETG), Hubble Space Telescope UV Cosmic Origins Spectrograph (HST-COS), and Jansky Very Large Array (JVLA) radio. The aim of these multiwavelength observations was to confirm the presence of ultras-fast outflows, and to provide new insights into their physical properties. High-resolution Chandra HETG spectroscopy, together with HST-COS and JVLA, has enabled us to constrain the physical conditions of ionized outflows, and determine the kinematic characteristics of X-ray absorption lines in this AGN. Our studies of X-ray Chandra observations confirm the existence of an ultra-fast outflow with of 0.06c in PG 1211+143. Moreover, our new UV observations with HST-COS reveal the presence of a broad blue-shifted Lyman-alpha absorption line with the same outflow velocity, which could be a potential counterpart to the X-ray outflow. Finally, we explore how the future observations with Chandra's successor, Lynx, will allow deeper insights into nearly relativistic outflows in sources like PG 1211+113.
The stellar magnetic fields dominate the environment around late-type stars. They are responsible for driving the coronal high-energy radiation, the development of stellar winds, and transient events such as flares and coronal mass ejections (CMEs). While considerable progress has been made for the first two processes, our understanding on the eruptive behavior of active stars is still limited. This information is critical as these phenomena can have a strong or even catastrophic impact on planetary systems, particularly, during the early stages of evolution where they can be the dominant factor in determining the properties around late-type stars. In this context, we present here the results of a numerical project aimed at studying the properties of CMEs in active stars, and their possible observable signatures to be detected with future instrumentation. A state-of-the-art 3D MHD code is considered for this purpose, which incorporates one of the latest models employed for space weather forecast in the solar system. An initial exploration of the parameter space for the formation and evolution of this eruptive phenomenon is performed, including the observed properties of the stellar magnetic fields in which they develop.
Gas motions play an important role in shaping the thermodynamic and chemical properties of the X-ray emitting intracluster medium (ICM). The deviation from the Gaussian line shape is a novel method for measuring gas motions. Specifically, it allows us to probe the turbulent driving scales, and detect velocity substructures along the line-of-sight. In this presentation, using mock X-ray spectra of galaxy clusters extracted from the Omega500 cosmological hydrodynamic simulations, we investigate the nature of non-gaussian X-ray line shapes, highlighting the relative importance of AGN feedback versus accretion-driven gas motions. We will discuss the prospect of mapping the 3D ICM velocity field using non-Gaussian line shapes with the high-resolution X-ray micro-calorimeter in the proposed Lynx mission.
Recent studies of high mass binaries have traditionally employed population synthesis codes to link evolutionary models with observed populations. However for certain types of binaries, particularly those that are rare or short-lived, traditional population synthesis is an inefficient technique; only a small fraction of simulated systems appear similar to those observed. I will introduce a statistical wrapper for use around pre-existing rapid binary evolution codes that efficiently explores the region of parameter space using a Markov chain Monte Carlo technique. The code can flexibly match a combination of observables such as the compact object mass and orbital period in high mass X-ray binaries while naturally accounting for uncertainties in the measurements. Using several test cases, I will demonstrate how this code can be applied to populations of systems such as the merging black hole binaries that produce gravitational waves as well as specific, individual systems such as Swift J0513.4-6547, a high mass X-ray binary in the Large Magellanic Cloud.
Our understanding of the connection between AGN and their host galaxies and the underlying properties of the full AGN population is presently limited by complex observational biases that are difficult to untangle using conventional methods and theoretical models. To more completely explore these selection effects, we use a semi-numerical galaxy formation simulation along with a universal Eddington ratio distribution, as determined by Jones et al. (2016) from SDSS data, to describe the multi-wavelength AGN population. In particular, we explicitly model selection effects to produce the "observed" AGN population for comparison with both theoretical and observational X-ray data. We investigate the impact on the "observed" population of selecting AGN in the X-rays based on thresholds in luminosity (as they are selected in most surveys). We find that we can broadly reproduce the host galaxies and halos of the X-ray AGN population, and that different AGN selection techniques yield samples with very different host galaxy properties. Furthermore, we discuss the capabilities of using this technique to build synthetic SEDs in order to explore the synthesis of the X-ray background.