X-ray Source Population Research Articles

IGR J00234+6141: a new INTEGRAL source identified as an intermediate polar

Context. Following an extensive survey of the galactic plane by the INTEGRAL satellite, new hard X-ray sources are discovered with a significant fraction of Cataclysmic Variables (CVs) among them. Aims. We report here the identification of one of these hard X-ray sources, IGR J00234+6141, as an accreting magnetic white dwarf of intermediate polar type. Methods. We analyse the high energy emission of the INTEGRAL source using all available data and provide complementary optical photometric and spectroscopic data obtained in August and October 2006, respectively. Results. Based on a refined INTEGRAL position, we confirm the proposed optical identification. We clearly detect the presence of a 564 s periodic optical modulation that we identify as the rotation of the white dwarf. The analysis of the optical spectrum also demonstrates that the emission lines show a modulation in radial velocity with an orbital period of Porb = (4.033 ± 0.005) h. Conclusions. The two periodicities indicate that IGR J00234+6141 is a magnetic CV of the intermediate polar type. This is one of the faintest and hardest sources of this type detected by INTEGRAL. This confirms earlier conclusions that intermediate polars contribute significantly to the population of galactic X-ray sources and represent a significant fraction of the high energy background.

Populations of Bright X-ray Sources in the Starburst Galaxies NGC 4038/4039

Assuming a naive star formation history, we construct synthetic X-ray source populations, using a population synthesis code, for comparison with the observed X-ray luminosity function (XLF) of the interacting galaxies NGC 4038/4039. We have included high- and intermediate-mass X-ray binaries, young rotation-powered pulsars and fallback disk-fed black holes in modeling the bright X-ray sources detected. We find that the majority of the X-ray sources are likely to be intermediate-mass X-ray binaries, but for typical binary evolution parameters, the predicted XLF seems to be steeper than observed. We note that the shape of the XLFs depends critically on the existence of XLF break for young populations, and suggest super-Eddington accretion luminosities or the existence of intermediate-mass black holes to account for the high luminosity end and the slope of the XLF in NGC 4038/4039.

ChandraMonitoring Observations of the Antennae Galaxies. II. X‐Ray Luminosity Functions

We present the X-ray luminosity functions (XLFs) of the X-ray source population detected in the Chandra monitoring observations of NGC 4038/4039 (the Antennae). The seven individual XLFs are well described by a flat power law with a cumulative slope α ~ 0.5-0.8. A similar slope (α = 0.48) is measured for the sources detected in the co-added observation, which reaches a limiting luminosity of ~1037 erg s-1. In our analysis we account for observational biases by deriving incompleteness functions and including them in the fitting process. We do not detect significant variations between the shape of the XLF of the seven observations. The two shorter exposures appear to have steeper XLFs, but these are still consistent with the other observations. These results indicate that the XLFs of star-forming galaxies are indeed flatter than those of more evolved stellar populations, even down to the typical luminosities of X-ray binaries. Based on this, as well as the X-ray variability and spectral properties of the X-ray sources, we suggest that the observed population down to our detection limit consists predominantly of X-ray binaries accreting close to their Eddington limit, similar to the high or very high states of Galactic X-ray binaries. In the case of ultraluminous X-ray sources (LX > 1039 erg s-1), we cannot rule out the contribution of a beamed component (because of either mechanical focusing or Doppler boosting) in their observed emission. However, even without beaming, we estimate that the maximum observed luminosity (LX ~ 1040 erg s-1) could be produced by a ~ 80 M☉ black hole accreting at its Eddington limit; such black holes can be the result of regular stellar evolution of double stellar systems.

High-mass X-ray binaries and the spiral structure of the host galaxy

We investigate the manifestation of the spiral structure in the distribution of high-mass X-ray binaries (HMXBs) over the host galaxy. We construct the simplest kinematic model. It shows that the HMXBs should be displaced relative to the spiral structure observed in such traditional star formation rate indicators as the Hα and far-infrared emissions because of their finite lifetimes. Using Chandra observations of M51, we have studied the distribution of X-ray sources relative to the spiral arms of this galaxy observed in Hα. Based on K-band data and background source number counts, we have separated the contributions from high-mass and low-mass X-ray binaries and active galactic nuclei. In agreement with model predictions, the distribution of HMXBs is wider than that of bright H II regions concentrated in the region of ongoing star formation. However, the statistical significance of this result is low, as is the significance of the concentration of the total population of X-ray sources to the spiral arms. We also predict the distribution of HMXBs in our Galaxy in Galactic longitude. The distribution depends on the mean HMXB age and can differ significantly from the distributions of such young objects as ultracompact H II regions.

ChandraMultiwavelength Project X‐Ray Point Source Number Counts and the Cosmic X‐Ray Background

We present the Chandra Multiwavelength Project (ChaMP) X-ray point source number counts and cosmic X-ray background (CXRB) flux densities in multiple energy bands. From the ChaMP X-ray point source catalog, � 5500 sourcesare selected, covering 9.6deg 2 in sky area. To quantitativelycharacterize the sensitivity and completeness of the ChaMP sample, we perform extensive simulations. We also include the ChaMP+CDFs (Chandra Deep Fields) number counts to cover large flux ranges from 2 ;10 � 17 to 2:4 ; 10 � 12 (0.5Y2 keV) and from 2 ; 10 � 16 to 7:1 ; 10 � 12 (2Y8 keV) ergs cm � 2 s � 1 . The ChaMP and the ChaMP+CDFs differential number counts are well fitted with a broken power law. The best-fit faint and bright power indices are 1:49 � 0:02 and 2:36 � 0:05 (0.5Y2 keV), and 1:58 � 0:01 and 2:59 þ0:06 � 0:05 (2Y8 keV), respectively. We detect breaks in the differential number counts that appear at differentfluxes in different energybands. Assuming a single power-law model for asource spectrum,wefind that the same population(s) of soft X-ray sources causes the break in the differential number counts for all energy bands. We measure the resolved CXRB flux densities from the ChaMP and the ChaMP+CDFs number counts with and without bright target sources. ByaddingtheknownunresolvedCXRBtotheChaMP+CDFresolvedCXRB,wealsoestimate totalCXRBfluxdensities.Thefractionsof theresolvedCXRBwithouttargetsourcesare78% � 1%and81% � 2% in the 0.5Y2 and 2Y8 keV bands, respectively, somewhat lower than but generally consistent with earlier numbers because of their large errors. These fractions increase by � 1% when target sources are included. Subject headingg cosmology: observations — methods: data analysis — surveys — X-rays: diffuse background — X-rays: general

ChandraX‐Ray Sources in the Collapsed‐Core Globular Cluster M30 (NGC 7099)

We report the detection of six discrete, low-luminosity (LX < 1033 ergs s-1) X-ray sources, located within 12'' of the center of the collapsed-core globular cluster M30 (NGC 7099), and a total of 13 sources within the half-mass radius, from a 50 ks Chandra ACIS-S exposure. Three sources lie within the very small upper limit of 1.9'' on the core radius. The brightest of the three core sources has a luminosity of LX(0.5-6 keV) ≈ 6 × 1032 ergs s-1 and a blackbody-like soft X-ray spectrum, which are both consistent with it being a quiescent low-mass X-ray binary (qLMXB). We have identified optical counterparts to four of the six central sources and a number of the outlying sources, using deep Hubble Space Telescope and ground-based imaging. While the two proposed counterparts that lie within the core may represent chance superpositions, the two identified central sources that lie outside of the core have X-ray and optical properties consistent with being cataclysmic variables (CVs). Two additional sources outside of the core have possible active binary counterparts. We discuss the X-ray source population of M30 in light of its collapsed-core status.

X-ray selected starbursts in the Great Observatories Origins Deep Survey-North

We investigate claims that recent ultra-deep X-ray surveys are detecting starbursts at cosmologically interesting redshifts (z= 0–1). We combine X-ray data from the 2 Ms Chandra Deep Field-North and multiwavelength observations obtained as part of the Great Observatories Origins Deep Survey (GOODS)-North to build the Spectral Energy Distributions (UV, optical, infrared) of X-ray sources in this field. These are fit with model templates providing an estimate of the total infrared luminosity (3–1000 μm) of each source. We then exploit the tight correlation between infrared and X-ray luminosities for star-forming galaxies, established in the local Universe, to select sources that are dominated by star formation rather than supermassive black hole accretion. This approach is efficient in discriminating normal galaxies from active galactic nuclei (AGN) over a wide range of star formation rates, from quiescent systems to starbursts. The above methodology results in a sample of 45 X-ray selected star-forming systems at a median redshift z≈ 0.5, the majority of which (60 per cent) are either luminous or ultraluminous infrared galaxies. This sample is least affected by incompleteness and residual AGN contamination and is therefore well suited for cosmological studies. We quantify the X-ray evolution of these sources by constructing their differential X-ray counts, dN/dS, and comparing them with evolving luminosity function models. The results are consistent with luminosity evolution of the form (1 +z)p with p≈ 2.4. This is similar to the evolution rate of star-forming galaxies selected at other wavelengths, suggesting that the deep X-ray surveys, like the Chandra Deep Field-North, are indeed finding the starburst galaxy population that drives the rapid evolution of the global star formation rate density in the range z≈ 0–1. Our analysis also reveals a separate population of infrared-faint X-ray sources at moderate z. These include old galaxies but also systems that are X-ray luminous for their stellar mass compared to local ellipticals. We argue that these may be post-starbursts that will, over time, become fainter at X-ray wavelengths and will eventually evolve into early-type systems (i.e. E/S0).

Map of the Galaxy in the 6.7-keV emission line

Abstract We study the two-dimensional surface brightness distribution of the Galactic X-ray background emission outside the central degree around Sgr A* in the 6.7-keV line as measured by the Proportional Counter Array spectrometer of the Rossi X-ray Timing Explorer observatory. The use of the emission line instead of continuum (3–20 keV) radiation and application of time-variability filtering to the long data set allows us to strongly suppress the contamination of the Galactic ridge X-ray emission (GRXE) map by bright point-sources. The surface brightness in the 6.7-keV line demonstrates very good correspondence with the near-infrared surface brightness over the whole Galaxy, supporting the notion that the GRXE consists mostly of integrated emission from weak Galactic X-ray sources. We find compatible linear correlations between near-infrared and 6.7-keV surface brightness for the bulge and disc of the Galaxy. This indicates that the populations of weak X-ray sources making up the GRXE in the disc and in the bulge are not significantly different.

New pre-main sequence candidates in the Taurus-Auriga star forming region

Aims. We have studied the X-ray source population of the Taurus Molecular Cloud (TMC) to search for new members of the Taurus-Auriga star forming region. Methods. Candidate members have been selected among the X-ray sources detected in 24 elds of the XMM-Newton Extended Survey of the Taurus Molecular Cloud, having an IR counterpart in the 2MASS catalog, based on color-magnitude and color-color diagrams. Their X-ray spectral properties have been compared with those of known members and other X-ray sources in the same elds but without a NIR counterpart. A search for are-lik e variability in the time series of all new candidates and the analysis of the X-ray spectra of the brightest candidates have been used to identify sources with a high probability of membership. Results. We have found that 347 of 1909 detected X-ray sources have an infrared counterpart in the 2MASS catalogue. Among them, we have selected 57 sources that are consistent with being new pre-main sequence star candidates at the distance of the Taurus-Auriga star forming region; the X-ray spectral properties of this sample are, on the whole, similar to the properties of known TMC members and di erent from those of X-ray sources without an IR counterpart, most of which are likely to be of extragalactic origin. For 12 such candidates, the likelihood of membership is very high, based on the relatively high plasma temperatures derived from their X-ray spectra and/or the observation of powerful ares in their light curves.

IGR J17544-2619: a new supergiant fast X-ray transient revealed by optical/infrared observations

One of the most recent discoveries of the INTEGRAL observatory is the existence of a previously unknown population of X-ray sources in the inner arms of the Galaxy. IGR J17544-2619, IGR J16465-4507 and XTE J1739-302 are among these sources. Although the nature of these systems is still unexplained, the investigations of the optical/NIR counterparts of the two last sources, combined with high energy data, have provided evidence of them being highly absorbed high mass X-ray binaries with blue supergiant secondaries and displaying fast X-ray transient behaviour. In this work we present our optical/NIR observations of IGR J17544-2619, aimed at identifying and characterizing its counterpart. We show that the source is a high mass X-ray binary at a distance of 2-4 kpc with a strongly absorbed O9Ib secondary, and discuss the nature of the system.

The X-ray properties of the dwarf Magellanic-type galaxy NGC 55

We present an analysis of the X-ray properties of the Magellanic-type galaxy NGC 55 based on two contiguous XMM-Newton observations. We detect a total of 137 X-ray sources in the field of view, 42 of which are located within the optical confines of the galaxy. On the basis of X-ray colour classification and after correcting for background objects, we conclude that our source sample includes ~ 20 X-ray binaries, 5 supernova remnants and 7 very soft sources (including 2 good candidate supersoft sources) associated with NGC 55. Detailed spectral and timing analysis was carried out on 4 of the brightest X-ray sources. One of these objects is identified with a Galactic foreground star and is a possible new RS CVn system. The other three are consistent with accreting X-ray binaries, though further evidence of short term variability is required to confirm this. We also find evidence of an underlying component, which is concentrated on the bar region but has an extent of at least 6' (3 kpc) in the plane of the galaxy and +/- 1' (+/- 500 pc) perpendicular to it. This emission is best fitted by a thermal plasma plus power-law model but with high intrinsic absorption consistent with its location in the central disc of the galaxy. We interpret the soft component as diffuse thermal emission linked to regions of current star formation, whilst the hard power-law component may originate in unresolved X-ray binary sources. The intrinsic luminosity of this residual disc emission may exceed Lx ~ 6e38 ergs/s (0.3-6 keV). A comparison with other Magellanic systems confirms that, in terms of both its discrete X-ray source population and its extended emission, NGC 55 has X-ray properties which are typical of its class.

A population synthesis study on the faint X-ray sources in the Galactic center region

Context.Recent Chandra observations of the Galactic center region (GCR) have uncovered a population of faint discrete X-ray sources. A few theoretical works have already investigated the nature of these sources.Aims.We examine the contributions and luminosity functions of various kinds of candidate objects that have been proposed either by previous authors or by ourselves.Methods.We conduct a population synthesis calculation based on the Hurley et al. rapid binary evolution code. Several candidate models, i.e. wind-accreting neutron stars, intermediate polars, low mass X-ray binaries, young pulsars, and massive stars with strong winds, are incorporated into our calculation. We also take the geometric effect of the accretion disk into account for Roche lobe overflow X-ray binaries.Results.Our results show that neutron star low-mass X-ray binaries contribute significantly to the observed sources. We also point out that wind-accreting neutron stars contribute negligible amount to these sources due to the propeller effect, and the intermediate polars play a minor role in accounting for the faint X-ray sources in both the Wang et al. and Muno et al. survey. It should be mentioned that the majority of the sources in the survey field of Wang et al. are still beyond what we can expect to find.

Pulsar Wind Nebulae and the Nonthermal X‐Ray Emission of Millisecond Pulsars

The nonthermal, nonpulsed X-ray emission of MSPs is investigated. As in young pulsars, MSPs emit a relativistic wind, which in interacting with the ISM and/or a binary companion can significantly contribute to the nonpulsed emission of these pulsars. An application and extension of a simple model developed for young pulsars is applied to the old recycled MSP B1957+20. It is found that the pulsar wind can indeed contribute to both the resolved and unresolved X-ray emission. For other MSPs in the Galactic field for which the spectral index of the nonpulsed component has been measured (i.e., PSR B1937+21, PSR J0218+4232) the contribution of the pulsar wind to the nonpulsed X-ray luminosity is estimated. For the MSPs in the core regions of globular clusters, the pulsar wind nebula is likely affected by its interaction with the dense stellar environment, possibly leading to a diminished contribution to the total X-ray emission. In this case, the existence of nonthermal nonpulsed X-ray emission is more likely for binary than for isolated MSPs, with the emission arising from the interaction of the relativistic pulsar wind and a binary companion. Our study suggests that the magnetization parameter in the pulsar wind nebulae of MSPs is significantly larger than that of the Crab Nebula, by about a factor of 10. The nebulae powered by rapidly rotating neutron stars either in isolation or in the quiescent state of soft X-ray transients can contribute to the faint X-ray source population associated with nonaccreting neutron stars. The emission from MSPs moving at high velocities (100 km s-1) through regions of the ISM characterized by magnetic field strengths 0.1 mG may appear spatially extended with a tail-like morphology. Thus, MSPs may also contribute to the faint filamentary X-ray source subpopulation in the Galaxy.

Optical Counterparts of Ultraluminous X-Ray Sources

We present the optical identification and characterization of counterparts of four objects previously cataloged as ultraluminous X-ray (ULX) sources. The objects were selected from the E. Colbert & A. Ptak catalog. The optical counterparts are identified as pointlike objects with magnitudes in the range of ~17-19. The optical spectra of three of the sources (IXO 32, 37, and 40) show the presence of emission lines typical of quasars. The position of these lines allows a precise estimation of their redshifts (2.769, 0.567, and 0.789 for IXO 32, 37, and 40, respectively). The fourth X-ray source, IXO 35, is associated with a red object that has a spectrum typical of an M star in our Galaxy. These identifications are useful for building clean samples of ULX sources, selecting suitable targets for future observations, and performing statistical studies on the different populations of X-ray sources.

The ultraluminous X-ray sources in the high-velocity system of NGC 1275

We report the results of a study of X-ray point sources coincident with the high-velocity system (HVS) projected in front of NGC 1275. A very deep X-ray image of the core of the Perseus cluster, made with the Chandra X-ray Observatory, has been used. We find a population of ultraluminous X-ray sources [ULXs; seven sources with L X (0.5 - 7.0 keV) > 7 x 10 39 erg s -1 ]. As with the ULX populations in the Antennae and Cartwheel galaxies, those in the HVS are associated with a region of very active star formation. Several sources have possible optical counterparts found on the Hubble Space Telescope (HST) images, although the X-ray brightest one does not. Absorbed power-law models fit the X-ray spectra, with most having a photon index between 2 and 3.

X-ray source populations in nearby star-forming galaxies

We present results on the X-ray source populations of nearby star-forming galaxies based on Chandra observations. First we discuss the monitoring campaigns on the Antennae and M82 galaxies. In both cases we find that the majority of the X-ray sources exhibit intensity and/or spectral variability. However, despite of this variability, we do not find any statistically significant variations of their X-ray luminosity functions (XLFs). We also find that the majority of the X-ray sources are associated with star-forming regions, although we do not always identify optical counterparts to the X-ray sources. Especially in the case of M82 we find that the most luminous sources are clustered in the central region of the galaxy. Finally, we present the first results from a study of a sample of nearby star-forming galaxies which form a starburst age sequence: although their XLFs to first order are represented by power-laws with consistent slopes, there is an indication for small variations, suggesting a change of their X-ray binary populations.

X-ray populations in galaxies

High-resolution Chandra observations have allowed the detection of populations of X-ray sources in galaxies of all morphological types. The X-ray Luminosity Functions (XLFs) of these X-ray source populations have been derived and studied to uncover the drivers for the formation and evolution of binaries in different stellar populations and environments. These XLFs also provide a tool for identifying the nature of the X-ray source population, since different XLFs characterize X-ray sources belonging to young and old stellar populations. Similarly, X-ray colors can be used for identifying different types of X-ray sources. Ultra-Luminous X-ray sources (ULXs, L X > 10 39 ergs s −1) are found to be associated with star-forming stellar populations. The study of the ULX population of the Antennae galaxies points to compact accreting binaries.

Populations of X-Ray Sources in Galaxies

Today's sensitive X-ray observations allow the study of populations of X-ray binaries in galaxies as distant as 20–30 Mpc. Photometric diagrams and luminosity functions applied to these populations provide a direct probe of the evolved binary component of different stellar populations. The study of the X-ray populations of E and S0 galaxies has revamped the debate on the formation and evolution of low-mass X-ray binaries (LMXBs) and on the role of globular clusters in these processes. Though overall stellar mass drives the amount of X-ray binaries in old stellar populations, the amount of sources in star-forming galaxies is related to the star-formation rate. Short-lived, luminous, high-mass X-ray binaries (HMXBs) dominate these young X-ray populations. The most luminous sources in these systems are the debated ultraluminous X-ray sources (ULXs). Observations of the deep X-ray sky, and comparison with deep optical surveys, are providing the first evidence of the X-ray evolution of galaxies.

Constraining the Nature of the Galactic Center X-Ray Source Population

We searched for infrared counterparts to the cluster of X-ray point sources discovered by Chandra in the Galactic Center Region (GCR). While the sources could be white dwarfs, neutron stars, or black holes accreting from stellar companions, their X-ray properties are consistent with magnetic Cataclysmic Variables, or High Mass X-ray Binaries (HMXB) at low accretion-rates. A direct way to decide between these possibilities and hence between alternative formation scenarios is to measure or constrain the luminosity distribution of the companions. Using infrared (J, H, K, Br-gamma) imaging, we searched for counterparts corresponding to typical HMXB secondaries: spectral type B0V with K<15 at the GCR. We found no significant excess of bright stars in Chandra error circles, indicating that HMXBs are not the dominant X-ray source population, and account for fewer than 10% of the hardest X-ray sources.

XMM-Newton 13H deep field - I. X-ray sources

We present the results of a deep X-ray survey conducted with XMM-Newton, centred on the UK ROSAT 13H deep field area. This region covers 0.18 deg^2 and is the first of two areas covered with XMM-Newton as part of an extensive multi-wavelength survey designed to study the nature and evolution of the faint X-ray source population. We have produced detailed Monte-Carlo simulations to obtain a quantitative characterisation of the source detection procedure and to assess the reliability of the resultant sourcelist. We use the simulations to establish a likelihood threshold above which we expect less than 7 (3%) of our sources to be spurious. We present the final catalogue of 225 sources. Within the central 9 arcmin, 68 per cent of source positions are accurate to 2 arcsec, making optical follow-up relatively straightforward. We construct the N(>S) relation in four energy bands: 0.2-0.5 keV, 0.5-2 keV, 2-5 keV and 5-10 keV. In all but our highest energy band we find that the source counts can be represented by a double powerlaw with a bright end slope consistent with the Euclidean case and a break around 10^-14 cgs. Below this flux the counts exhibit a flattening. Our source counts reach densities of 700, 1300, 900 and 300 deg^-2 at fluxes of 4.1x10^-16, 4.5x10^-16, 1.1x10^-15 and 5.3x10^-15 cgs in the 0.2-0.5, 0.5-2, 2-5 and 5-10 keV energy bands respectively. We have compared our source counts with those in the two Chandra deep fields and Lockman hole and find our source counts to be amongst the highest of these fields in all energy bands. We resolve >51% (>50%) of the X-ray background emission in the 1-2 keV (2-5 keV) energy bands.