Publication Search Results

ID: 159347
Type: article
Authors: Lehmer, Bret D.; Eufrasio, Rafael T.; Basu-Zych, Antara; Doore, Keith; Fragos, Tassos; Garofali, Kristen; Kovlakas, Konstantinos; Williams, Benjamin F.; Zezas, Andreas; Santana-Silva, Luidhy
Abstract: In this work, we present detailed constraints on the metallicity dependence of the high-mass X-ray binary (HMXB) X-ray luminosity function (XLF). We analyze ≍5 Ms of Chandra data for 55 actively star-forming galaxies at D ≲ 30 Mpc, with gas-phase metallicities spanning $12+\mathrm{log}({\rm{O}}/{\rm{H}})$ ≍ 7-9.2. Within the galactic footprints, our sample contains a total of 1311 X-ray point sources, of which ≍49% are expected to be HMXBs, with the remaining sources likely to be low-mass X-ray binaries (LMXBs; ≍22%) and unrelated background sources (≍29%). We construct a model that successfully characterizes the average HMXB XLF over the full metallicity range. We demonstrate that the SFR-normalized HMXB XLF shows clear trends with metallicity, showing steadily increasing numbers of luminous and ultraluminous X-ray sources ( $\mathrm{log}L$ (erg s^-1) = 38-40.5) with declining metallicity. However, we find that the low-luminosity ( $\mathrm{log}L$ (erg s^-1) = 36-38) HMXB XLF appears to show a nearly constant SFR scaling and slope with metallicity. Our model provides a revised scaling relation of integrated L_X/SFR versus $12+\mathrm{log}({\rm{O}}/{\rm{H}})$ , and a new characterization of its SFR-dependent stochastic scatter. The general trend of this relation is broadly consistent with past studies based on integrated galaxy emission; however, our model suggests that this relation is driven primarily by the high-luminosity end of the HMXB XLF. Our results have implications for binary population synthesis models, the nature of super-Eddington accreting objects (e.g., ultraluminous X-ray sources), recent efforts to identify active galactic nucleus candidates in dwarf galaxies, and the X-ray radiation fields in the early universe during the epoch of cosmic heating at z ≳ 10.

ID: 159323
Type: article
Authors: Xu, Cong; Günther, Hans Moritz; Kashyap, Vinay L.; Lee, Thomas C. M.; Zezas, Andreas
Abstract: Many astrophysical phenomena are time-varying, in the sense that their intensity, energy spectrum, and/or the spatial distribution of the emission suddenly change. This paper develops a method for modeling a time series of images. Under the assumption that the arrival times of the photons follow a Poisson process, the data are binned into 4D grids of voxels (time, energy band, and x-y coordinates), and viewed as a time series of non-homogeneous Poisson images. The method assumes that at each time point, the corresponding multiband image stack is an unknown 3D piecewise constant function including Poisson noise. It also assumes that all image stacks between any two adjacent change points (in time domain) share the same unknown piecewise constant function. The proposed method is designed to estimate the number and the locations of all of the change points (in time domain), as well as all of the unknown piecewise constant functions between any pairs of the change points. The method applies the minimum description length principle to perform this task. A practical algorithm is also developed to solve the corresponding complicated optimization problem. Simulation experiments and applications to real data sets show that the proposed method enjoys very promising empirical properties. Applications to two real data sets, the XMM observation of a flaring star and an emerging solar coronal loop, illustrate the usage of the proposed method and the scientific insight gained from it.

ID: 156969
Type: article
Authors: Cappallo, R. C.; Laycock, S. G. T.; Christodoulou, D. M.; Roy, A.; Bhattacharya, S.; Coe, M. J.; Zezas, Andreas
Abstract: SXP 1062 is a long-period X-ray pulsar (XRP) with a Be optical companion located in the Small Magellanic Cloud. First discovered in 2010 from XMM-Newton data, it has been the target of multiple observational campaigns due to the seeming incongruity between its long spin period and recent birth. In our continuing modelling efforts to determine the inclination angle (I) and magnetic axis angle (θ) of XRPs, we have fitted 19 pulse profiles from SXP 1062 with our pulsar model, Polestar, including three consecutive Chandra observations taken during the trailing end of a Type I outburst. These fittings have resulted in most likely values of I = 76^○ ± 2^○ and θ = 40^○ ± 9^○. SXP 1062 mostly displays a stable double-peaked pulse profile with the peaks separated by roughly a third of a phase, but recently the pulsar has spun up and widened to a spacing of roughly half of a phase, yet the Polestar fits for I and θ remain constant. Additionally, we note a possible correlation between the X-ray luminosity and the separation of the peaks in the pulse profiles corresponding to the highest luminosity states.

ID: 155694
Type: article
Authors: Kopsacheili, M.; Zezas, Andreas; Leonidaki, I.
Abstract: We present new diagnostic tools for distinguishing supernova remnants (SNRs) from H II regions. Up to now, sources with flux ratio [S II]/H{α } higher than 0.4 have been considered as SNRs. Here, we present combinations of three or two line ratios as more effective tools for the separation of these two kinds of nebulae, depicting them as 3D surfaces or 2D lines. The diagnostics are based on photoionization and shock- excitation models (MAPPINGS III) analysed with support vector machine (SVM) models for classification. The line-ratio combination that gives the most efficient diagnostic is [O I]/H{α } - [O II]/H{β } - [O III]/H{β }. This method gives 98.95 per cent completeness in the SNR selection and 1.20 per cent contamination. We also define the [O I]/H{α } SNR selection criterion and measure its efficiency in comparison with other selection criteria.

ID: 157870
Type: article
Authors: Kouroumpatzakis, K.; Zezas, Andreas; Sell, P.; Kovlakas, K.; Bonfini, P.; Willner, Steven P.; Ashby, Matthew L. N.; Maragkoudakis, A.; Jarrett, T. H.
Abstract: X-ray luminosity (L_X) originating from high-mass X-ray binaries (HMXBs) is tightly correlated with the host galaxy's star formation rate (SFR). We explore this connection at sub-galactic scales spanning ∼7 dex in SFR and ∼8 dex in specific SFR (sSFR). There is good agreement with established relations down to SFR ≃ 10^-3 M $_{\odot }\, \rm {yr^{-1}}$ , below which an excess of X-ray luminosity emerges. This excess likely arises from low-mass X-ray binaries. The intrinsic scatter of the L_X-SFR relation is constant, not correlated with SFR. Different star formation indicators scale with L_X in different ways, and we attribute the differences to the effect of star formation history. The SFR derived from H α shows the tightest correlation with X-ray luminosity because H α emission probes stellar populations with ages similar to HMXB formation time-scales, but the H α-based SFR is reliable only for $\rm sSFR{\gt }10^{-12}$ M $_{\odot }\, \rm {yr^{-1}}$ /M_☉.

ID: 156291
Type: article
Authors: Politakis, Babis; Zezas, Andreas; Andrews, Jeff J.; Williams, Stephen J.
Abstract: We analyse the vertical distribution of high-mass X-ray binaries (HMXBs) in NGC 55, the nearest edge-on galaxy to the Milky Way (MW), based on X-ray observations by Chandra. Adopting a statistical approach, we estimate the difference between the scale height of the vertical distribution of HMXBs and the vertical distribution of star-forming activity between 0.33 and 0.57 kpc. The spatial offsets can be explained by a momentum kick the X-ray binaries receive during the formation of the compact object after a supernova explosion of the primary star. Determining the vertical distribution of HMXBs in the MW using Gaia DR2 astrometry, we find that the corresponding difference is considerably lower at 0.036 ± 0.003 kpc, attributed to the greater gravitational potential of the MW. We also calculate the centre-of-mass transverse velocities of HMXBs in NGC 55, using traveltime information from binary population synthesis codes and for different star formation histories (SFHs). For a flat SFH model (typical of spiral galaxies like NGC 55), we find that HMXBs are moving with a typical transverse velocity between 34 and 48 km s^-1, consistent with space velocities of MW HMXBs. For an exponentially declining SFH model, HMXBs are moving at a velocity of 21 km s^-1, consistent with the corresponding velocity of HMXBs in the Small Magellanic Cloud and Large Magellanic Cloud. Finally, we estimate the formation efficiency of HMXBs in NGC 55 at 299_{-46}^{+50} (systems/M_☉ yr^-1), consistent within the errors with the Magellanic Clouds but significantly higher than the MW, a difference that can be explained by the subsolar metallicity of NGC 55.

ID: 150486
Type: article
Authors: Anastasopoulou, K.; Zezas, Andreas; Gkiokas, V.; Kovlakas, K.
Abstract: We present results from Chandra observations of the X-ray starburst galaxies NGC 3310 and NGC 2276. We detect 27 discrete sources in NGC 3310, and 19 discrete sources in NGC 2276 with luminosities above 1.0 × 10³⁸ erg s^-1. 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; sources with L(0.3-10.0 keV) > 10³⁹ erg s^-1), 14 for NGC 3310 concentrated on the circumnuclear star-forming ring and north spiral arm and 11 for NGC 2276 with the brighter ones on the west side of the galaxy which is compressed due to harassment by the intra-group medium it is moving into. We find for both galaxies that the ULX-hosting areas are located above the general L_X-SFR scaling relations while other areas either follow or fall below the scaling relations. This indicates that sub-galactic regions follow the galaxy-wide scaling relations but with much larger scatter resulting from the age (and possibly metallicity) of their local stellar populations in agreement with recent theoretical and observational results. Such differences in age could be the origin of the scatter we observe in the low-SFR regime in the Lx-SFR scaling relations.

ID: 154539
Type: article
Authors: Antoniou, Vallia; Zezas, Andreas; Drake, Jeremy J.; Badenes, Carles; Haberl, Frank; Wright, Nicholas J.; Hong, Jaesub; Di Stefano, Rosanne; Gaetz, Terrance J.; Long, Knox S.; Plucinsky, Paul P.; Sasaki, Manami; Williams, Benjamin F.; Winkler, P. Frank; SMC XVP collaboration
Abstract: We have compiled the most complete census of high-mass X-ray binaries (HMXBs) in the Small Magellanic Cloud with the aim to investigate the formation efficiency of young accreting binaries in its low-metallicity environment. In total, we use 123 X-ray sources with detections in our Chandra X-ray Visionary Program (XVP), supplemented by 14 additional (likely and confirmed) HMXBs identified by Haberl & Sturm that fall within the XVP area, but are neither detected in our survey (nine sources) nor matched with any of the 127 sources identified in the XVP data (five sources). Specifically, we examine the number ratio of the HMXBs [N(HMXBs)] to (a) the number of OB stars, (b) the local star formation rate (SFR), and (c) the stellar mass produced during the specific star formation burst, all as a function of the age of their parent stellar populations. Each of these indicators serves a different role, but in all cases we find that the HMXB formation efficiency increases as a function of time (following a burst of star formation) up to ∼40─60 Myr, and then gradually decreases. The formation efficiency peaks at ∼30─40 Myr with average rates of {\text{}}{{N}}({HMXB})/{SFR}={339}_-83⁺⁷⁸ {({M}_ȯ /{yr})}^-1, and N(HMXB)/M \star =({8.74}_-0.92^+1.0)× {10}^-6 {M}_ȯ ^-1, in good agreement with previous estimates of the average formation efficiency in the broad ∼20─60 Myr age range.

ID: 155408
Type: article
Authors: Brightman, Murray; Harrison, Fiona A.; Bachetti, Matteo; Xu, Yanjun; Fürst, Felix; Walton, Dominic J.; Ptak, Andrew; Yukita, Mihoko; Zezas, Andreas
Abstract: Ultraluminous X-ray (ULX) pulsars are a new class of object powered by apparent super-critical accretion onto magnetized neutron stars. Three sources in this class have been identified so far; M82 X-2, NGC 5907 ULX-1, and NGC 7793 P13 have been found to have two properties in common; ̃1 s spin periods, and for NGC 5907 ULX-1 and NGC 7793 P13 periodic X-ray flux modulations on timescales of ̃60-80 days. M82 X-2 resides in a crowded field that includes the ULX M82 X-1 separated from X-2 by 5″, as well as other bright point sources. A 60 day modulation has been observed from the region, but the origin has been difficult to identify; both M82 X-1 and X-2 have been suggested as the source. In this paper we present the analysis of a systematic monitoring campaign by Chandra, the only X-ray telescope capable of resolving the crowded field. From a simple Lomb-Scargle periodogram analysis and a more sophisticated Gaussian Process analysis we find that only X-2 exhibits a periodic signal around 60 days, supporting previous claims that it is the origin. We also construct a phase-averaged flux profile of the modulations from higher-cadence Swift/XRT data and find that the flux variations in the Chandra data are fully consistent with the flux profile. Since the orbit of the neutron star and its companion is known to be 2.5 days, the ̃60 day period must be super-orbital in origin. The flux of the modulations varies by a factor of ̃100 from the minimum to the maximum, with no evidence for spectral variations, making the origin difficult to explain.

ID: 154150
Type: article
Authors: Cappallo, R.; Laycock, S. G. T.; Christodoulou, D. M.; Coe, M. J.; Zezas, Andreas
Abstract: The X-ray source SXP348 is a high-mass X-ray binary system in the Small Magellanic Cloud. Since its 1998 discovery by BeppoSAX, this pulsar has exhibited a spin period of ̃340-350 s. In an effort to determine the orientation and magnetic geometry of this source, we used our geometric model Polestar to fit 71 separate pulse profiles extracted from archival Chandra and XMM-Newton observations over the past two decades. During 2002, pulsations ceased being detectable for nine months despite the source remaining in a bright state. When pulsations resumed, our model fits changed, displaying a change in accretion geometry. Furthermore, in 2006, detectable pulsations again ceased, with 2011 marking the last positive detection of SXP348 as a point source. These profile fits will be released for public use as part of the database of Magellanic Cloud pulsars.

ID: 154709
Type: article
Authors: Lazzarini, M.; Williams, B. F.; Hornschemeier, A. E.; Antoniou, Vallia; Vasilopoulos, G.; Haberl, F.; Vulic, N.; Yukita, M.; Zezas, Andreas; Bodaghee, A.; Lehmer, B. D.; Maccarone, T. J.; Ptak, A.; Wik, D.; Fornasini, Francesca M.; Hong, Jaesub; Kennea, J. A.; Tomsick, J. A.; Venters, T.; Udalski, A.; Cassity, A.
Abstract: We present a source catalog from the first deep hard X-ray (E > 10 keV) survey of the Small Magellanic Cloud (SMC), the Nuclear Spectroscopic Telescope Array (NuSTAR) Legacy Survey of the SMC. We observed three fields, for a total exposure time of 1 Ms, along the bar of this nearby star-forming galaxy. Fields were chosen for their young stellar and accreting binary populations. We detected 10 sources above a 3σ significance level (4-25 keV) and obtained upper limits on an additional 40 sources. We reached a 3σ limiting luminosity in the 4-25 keV band of ̃10³⁵ erg s^-1, allowing us to probe fainter X-ray binary (XRB) populations than has been possible with other extragalactic NuSTAR surveys. We used hard X-ray colors and luminosities to constrain the compact-object type, exploiting the spectral differences between accreting black holes and neutron stars at E > 10 keV. Several of our sources demonstrate variability consistent with previously observed behavior. We confirmed pulsations for seven pulsars in our 3σ sample. We present the first detection of pulsations from a Be-XRB, SXP 305 (CXO J005215.4-73191), with an X-ray pulse period of 305.69 ± 0.16 s and a likely orbital period of ̃1160-1180 days. Bright sources (≳5 × 10³⁶ erg s^-1) in our sample have compact-object classifications consistent with their previously reported types in the literature. Lower-luminosity sources (≲5 × 10³⁶ erg s^-1) have X-ray colors and luminosities consistent with multiple classifications. We raise questions about possible spectral differences at low luminosity between SMC pulsars and the Galactic pulsars used to create the diagnostic diagrams.

ID: 154152
Type: article
Authors: Lehmer, Bret D.; Eufrasio, Rafael T.; Tzanavaris, Panayiotis; Basu-Zych, Antara; Fragos, Tassos; Prestwich, Andrea; Yukita, Mihoko; Zezas, Andreas; Hornschemeier, Ann E.; Ptak, Andrew
Abstract: We present new Chandra constraints on the X-ray luminosity functions (XLFs) of X-ray binary (XRB) populations, as well as their scaling relations, for a sample of 38 nearby galaxies (D = 3.4-29 Mpc). Our galaxy sample is drawn primarily from the Spitzer Infrared Nearby Galaxies Survey (SINGS) and contains a wealth of Chandra (5.8 Ms total) and multiwavelength data, allowing for star formation rates (SFRs) and stellar masses (M _⋆) to be measured on subgalactic scales. We divided the 2478 X-ray-detected sources into 21 subsamples in bins of specific SFR (sSFR ≡ SFR/M _⋆) and constructed XLFs. To model the XLF dependence on sSFR, we fit a global XLF model, containing contributions from high-mass XRBs (HMXBs), low-mass XRBs (LMXBs), and background sources from the cosmic X-ray background that respectively scale with SFR, M _⋆, and sky area. We find an HMXB XLF that is more complex in shape than previously reported and an LMXB XLF that likely varies with sSFR, potentially due to an age dependence. When applying our global model to XLF data for each individual galaxy, we discover a few galaxy XLFs that significantly deviate from our model beyond statistical scatter. Most notably, relatively low-metallicity galaxies have an excess of HMXBs above ≈10³⁸ erg s^-1, and elliptical galaxies that have relatively rich populations of globular clusters (GCs) show excesses of LMXBs compared to the global model. Additional modeling of how the XRB XLF depends on stellar age, metallicity, and GC specific frequency is required to sufficiently characterize the XLFs of galaxies.

ID: 150444
Type: article
Authors: Mahajan, Smriti; Ashby, Matthew L. N.; Willner, Steven P.; Barmby, P.; Fazio, Giovanni G.; Maragkoudakis, A.; Raychaudhury, S.; Zezas, A.
Abstract: We present multiwavelength global star formation rate (SFR) estimates for 326 galaxies from the Star Formation Reference Survey in order to determine the mutual scatter and range of validity of different indicators. The widely used empirical SFR recipes based on 1.4 GHz continuum, 8.0 μm polycyclic aromatic hydrocarbons (PAHs), and a combination of far-infrared (FIR) plus ultraviolet (UV) emission are mutually consistent with scatter of ≲0.3 dex. The scatter is even smaller, ≲0.24 dex, in the intermediate luminosity range 9.35 × 10⁹ L_⊙ or with implied far-UV extinction <1 mag, the UV spectral slope gives extinction corrections with 0.22 dex uncertainty.

ID: 151873
Type: article
Authors: Stampoulis, Vasileios; van Dyk, David A.; Kashyap, Vinay L.; Zezas, Andreas
Abstract: We propose a new soft clustering scheme for classifying galaxies in different activity classes using simultaneously four emission-line ratios: log ([N II]/H α), log ([S II]/H α), log ([O I]/H α), and log ([O III]/H β). We fit 20 multivariate Gaussian distributions to the four-dimensional distribution of these lines obtained from the Sloan Digital Sky Survey in order to capture local structures and subsequently group the multivariate Gaussian distributions to represent the complex multidimensional structure of the joint distribution of galaxy spectra in the four-dimensional line ratio space. The main advantages of this method are the use of all four optical-line ratios simultaneously and the adoption of a clustering scheme. This maximizes the use of the available information, avoids contradicting classifications, and treats each class as a distribution resulting in soft classification boundaries and providing the probability for an object to belong to each class. We also introduce linear multidimensional decision surfaces using support vector machines based on the classification of our soft clustering scheme. This linear multidimensional hard clustering technique shows high classification accuracy with respect to our soft clustering scheme.

ID: 154599
Type: article
Authors: Strantzalis, A.; Hatzidimitriou, D.; Zezas, A.; Antoniou, Vallia; Lianou, S.; Tsilia, S.
Abstract: We present the results of the photometric analysis of a large part of the main body of the Small Magellanic Cloud. Using the 6.5m Magellan Telescope at the Las Campanas Observatory in Chile, we have acquired deep B and I images in four fields (0.44 deg each in diameter), yielding accurate photometry for 1068 893 stars down to 24th magnitude, with a spatial resolution of 0.20 arcsec per pixel. Colour-magnitude diagrams and (completeness-corrected) luminosity functions have been constructed, yielding significant new results that indicate at least two discrete star formation events over a period from 2.7 to 4 Gyr ago. Also, we have derived star formation rates as a function of look-back time and have found enhancements of SF between 4 and 6 Gyr and at younger ages.

ID: 147900
Type: article
Authors: Andrews, Jeff J.; Zezas, Andreas; Fragos, Tassos
Abstract: By employing Monte Carlo random sampling, traditional binary population synthesis (BPS) offers a substantial improvement in efficiency over brute force, grid-based studies. Even so, BPS models typically require a large number of simulation realizations, a computationally expensive endeavor, to generate statistically robust results. Recent advances in statistical methods have led us to revisit the traditional approach to BPS. In this work we describe our publicly available code dart_board, which combines rapid binary evolution codes, typically used in traditional BPS, with modern Markov chain Monte Carlo methods. dart_board takes a novel approach that treats the initial binary parameters and the supernova kick vector as model parameters. This formulation has several advantages, including the ability to model either populations of systems or individual binaries, the natural inclusion of observational uncertainties, and the flexible addition of new constraints, which are problematic to include using traditional BPS. After testing our code with mock systems, we demonstrate the flexibility of dart_board by applying it to three examples: (i) a generic population of high-mass X-ray binaries (HMXBs); (ii) the population of HMXBs in the Large Magellanic Cloud (LMC), in which the spatially resolved star formation history is used as a prior; and (iii) one particular HMXB in the LMC, Swift J0513.4-6547, in which we include observations of the system's component masses and orbital period. Although this work focuses on HMXBs, dart_board can be applied to a variety of stellar binaries, including the recent detections by gravitational wave observatories of merging compact object binaries.

ID: 145740
Type: article
Authors: Bonfini, P.; Bitsakis, T.; Zezas, A.; Duc, P. -A; Iodice, E.; González-Martín, O.; Bruzual, G.; González Sanoja, A. J.
Abstract: Deep exposure imaging of early-type galaxies (ETGs) are revealing the second-order complexity of these objects, which have been long considered uniform, dispersion-supported spheroidals. 'Fine structure' features (e.g. ripples, plumes, tidal tails, rings) as well as depleted stellar cores (i.e. central light deficits) characterize a number of massive ETG galaxies, and can be interpreted as the result of galaxy-galaxy interactions. We discuss how the time-scale for the evolution of cores and fine structures are comparable, and hence it is expected that they develop in parallel after the major interaction event which shaped the ETG. Using archival data, we compare the 'depleted stellar mass' (i.e. the mass missing from the depleted stellar core) against the prominence of the fine structure features, and observe that they correlate inversely. This result confirms our expectation that, while the supermassive black hole (SMBH) binary (constituted by the SMBHs of the merger progenitors) excavates the core via three-body interactions, the gravitational potential of the newborn galaxy relaxes, and the fine structures fade below detection levels. We expect the inverse correlation to hold at least within the first Gyr from the merger which created the SMBH binary; after then, the fine structure evolves independently.

ID: 150063
Type: article
Authors: Dietrich, Jeremy; Weiner, Aaron S.; Ashby, Matthew L. N.; Hayward, Christopher C.; Martínez-Galarza, Juan Rafael; Ramos Padilla, Andrés F.; Rosenthal, Lee; Smith, Howard A.; Willner, S. P.; Zezas, Andreas
Abstract: Galaxy mergers are key events in galaxy evolution, often causing massive starbursts and fueling active galactic nuclei (AGNs). In these highly dynamic systems, it is not yet precisely known how much starbursts and AGNs, respectively, contribute to the total luminosity, at what interaction stages they occur, and how long they persist. Here we estimate the fraction of the bolometric infrared (IR) luminosity that can be attributed to AGNs by measuring and modeling the full ultraviolet to far-IR spectral energy distributions (SEDs) in up to 33 broad bands for 24 merging galaxies with the Code for Investigating Galaxy Emission. In addition to a sample of 12 confirmed AGNs in late-stage mergers, found in the Infrared Astronomical Satellite Revised Faint Source Catalog, our sample includes a comparison sample of 12 galaxy mergers from the Spitzer Interacting Galaxies Survey, mostly early stage. We also perform SED modeling of merger simulations to validate our methods, and we supplement the SEDs with mid-IR spectra of diagnostic lines obtained with Spitzer's InfraRed Spectrograph. The estimated AGN contributions to the IR luminosities vary from system to system from 0 per cent up to ˜91 per cent but are significantly greater in the later-stage, more luminous mergers, consistent with what is known about galaxy evolution and AGN triggering.

ID: 149160
Type: article
Authors: Fornasini, Francesca M.; Civano, Francesca; Fabbiano, Giuseppina; Elvis, Martin; Marchesi, Stefano; Miyaji, Takamitsu; Zezas, Andreas
Abstract: We present an X-ray stacking analysis of ∼75,000 star-forming galaxies between 0.1 ⁴⁰–10⁴¹ erg s^‑1, a regime in which X-ray binaries (XRBs) can dominate the X-ray emission. Comparing the measured luminosities to established XRB scaling relations, we find that the redshift evolution of the luminosity per star formation rate (SFR) of XRBs depends sensitively on the assumed obscuration and may be weaker than previously found. The XRB scaling relation based on stacks from the Chandra Deep Field South overestimates the XRB contribution to the COSMOS high specific SFR stacks, possibly due to a bias affecting the CDF-S stacks because of their small galaxy samples. After subtracting the estimated XRB contribution from the stacks, we find that most stacks at z > 1.3 exhibit a significant X-ray excess indicating nuclear emission. The AGN emission is strongly correlated with stellar mass but does not exhibit an additional correlation with SFR. The hardness ratios of the high-redshift stacks indicate that the AGN are substantially obscured (N _H ∼ 10²³ cm^‑2). These obscured AGN are not identified by IRAC color selection and have L _X ∼ 10⁴¹–10⁴³ erg s^‑1, consistent with accretion at an Eddington rate of ∼10^‑3 onto 10⁷–10⁸ M _⊙ black holes. Combining our results with other X-ray studies suggests that AGN obscuration depends on stellar mass and an additional variable, possibly the Eddington rate.

ID: 147903
Type: article
Authors: Lazzarini, M.; Hornschemeier, A. E.; Williams, B. F.; Wik, D.; Vulic, N.; Yukita, M.; Zezas, A.; Lewis, A. R.; Durbin, M.; Ptak, A.; Bodaghee, A.; Lehmer, B. D.; Antoniou, V.; Maccarone, T.
Abstract: We present 15 high-mass X-ray binary (HMXB) candidates in the disk of M31 for which we are able to infer compact object type, spectral type of the donor star, and age using multiwavelength observations from NuSTAR, Chandra, and the Hubble Space Telescope. The hard X-ray colors and luminosities from NuSTAR permit the tentative classification of accreting X-ray binary systems by compact object type, distinguishing black hole from neutron star systems. We find hard-state black holes, pulsars, and non-magnetized neutron stars associated with optical point-source counterparts with similar frequency. We also find nine non-magnetized neutron stars coincident with globular clusters and an equal number of pulsars with and without point-source optical counterparts. We perform spectral energy distribution (SED) fitting for the most likely optical counterparts to the HMXB candidates, finding seven likely high-mass stars and one possible red helium-burning star. The remaining seven HMXB optical counterparts have poor SED fits, so their companion stars remain unclassified. Using published star formation histories, we find that the majority of HMXB candidates---X-ray sources with UV-bright point-source optical counterpart candidates---are found in regions with star formation bursts less than 50 Myr ago, and three are associated with young stellar ages (<10 Myr). This is consistent with similar studies of HMXB populations in the Magellanic Clouds, M33, NGC 300, and NGC 2403.