ULX Candidates Research Articles

2XMM ultraluminous X-ray source candidates in nearby galaxies

Ultraluminous X-ray sources (ULXs) are some of the most enigmatic X-ray bright sources known to date. It is generally accepted that they cannot host black holes as large as those associated with active galaxies, but they appear to be significantly more luminous than their better understood Galactic X-ray binary (XRB) cousins, while displaying an intriguing combination of differences and similarities with them. Through studying large, representative samples of these sources we may hope to enhance our understanding of them. To this end, we derive a large catalogue of 650 X-ray detections of 470 ULX candidates, located in 238 nearby galaxies, by cross correlating the 2XMM Serendipitous Survey with the Third Reference Catalogue of Bright Galaxies. The presented dedicated catalogue offers a significant improvement over those previously published both in terms of number and the contribution of background contaminants, e.g. distant quasars, which we estimate to be at most 24 per cent, but more likely ~17 per cent. To undertake population studies, we define a `complete' sub-sample of sources compiled from observations of galaxies with sensitivity limits below 10^39 erg/s. The luminosity function of this sample is consistent with a simple power-law. Although we do not find any statistical requirement for a high luminosity cut-off, as has been reported previously, we are not able to rule out its presence. Also, we find that the number of ULXs per unit galaxy mass decreases with increasing galaxy mass for ULXs associated with spiral galaxies, in broad agreement with previous results, but is consistent with being constant with galaxy mass for sources associated with elliptical galaxies.

A large catalogue of ultraluminous X‐ray source candidates in nearby galaxies

AbstractSince their discovery, Ultraluminous X‐ray sources (ULXs) have attracted attention due to their combination of extreme luminosities and extra‐nuclear locations. However, they are a fairly rare phenomenon, and attempts to investigate the general properties of the population have been hindered by a relative lack of known sources. Here, we present a large catalogue of ULX candidates including 655 detections of 475 discrete sources, based on the 2XMM Serendipitous Survey. To demonstrate the potential of such a resource, we present some scientific analysis of this population, focusing on the spectral turnover seen, often at ∼6 keV, in the highest quality ULX data. We also demonstrate how the recent reflection and Comptonisation interpretations of this feature may be distinguished observationally in the future, specifically using NGC 4517 ULX1, a previously unanalysed source with high quality data, as an example case (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

XMM-Newtonobservations of the hot spot galaxy NGC 2903

We report on the first deeper X-ray broad-band observation of the hot spot galaxy NGC 2903 obtained with XMM-Newton. X-ray imaging and spectra of the spiral barred galaxy were obtained from XMM-Newton archival data to study its X-ray population and the conditions of the hot gas in its central region. We investigate the spectral properties of the discrete point-source population and give estimates of their X-ray spectral parameters. By analysing the RGS spectra, we derive temperature and abundances for the hot gas located in its central region. A total of six X-ray point sources (four of them ULX candidates) were detected in the energy range of 0.3-10.0 keV located within the galaxy D25 optical disk. Three of these sources are detected for the first time, and one of them with a luminosity of higher than 10^39 erg/s. After fitting three different models, we were able to estimate their luminosities, which are compatible with those of binaries with a compact object in the form of black holes (BHs) rather than neutron stars (NSs). We extracted the combined first-order RGS1 and RGS2 spectra of its central region, which display several emission lines. Both O\,{\sc vii} $f$ and $r$ lines seem to be of similar strength, which is consistent with the presence of the collisionally ionized gas that is typical of starburst galaxies. We fitted the spectrum to a model for a plasma in collisional ionization equilibrium (CIE) and the continuum was modelled with a power law, resulting in a plasma temperature of T = 0.31 \pm 0.01 keV and an emission measure EM \equiv n_Hn_eV =6.4_{-0.4}^{+0.5}\times 10^{61}$~cm$^{-3}. We also estimated abundances that are consistent with solar values.

ULTRALUMINOUS X-RAY SOURCE CORRELATIONS WITH STAR-FORMING REGIONS

Maps of low-inclination nearby galaxies in Sloan Digitized Sky Survey u-g, g-r and r-i colors are used to determine whether Ultraluminous X-ray sources (ULXs) are predominantly associated with star-forming regions of their host galaxies. An empirical selection criterion is derived from colors of HII regions in M81 and M101 that differentiates between the young, blue stellar component and the older disk and bulge population. This criterion is applied to a sample of 58 galaxies of Hubble type S0 and later and verified through an application of Fisher's linear discriminant analysis. It is found that 60% (49%) of ULXs in optically-bright environments are within regions blueward of their host galaxy's HII regions compared to only 27% (0%) of a control sample according to the empirical (Fisher) criterion. This is an excess of 3-sigma above the 32% (27%) expected if the ULXs were randomly distributed within their galactic hosts. This indicates a ULX preference for young, approximately <10 Myr, OB associations. However, none of the ULX environments have the morphology and optical brightness suggestive of a massive young super star cluster though several are in extended or crowded star-forming (blue) environments that may contain clusters unresolved by Sloan imaging. Ten of the 12 ULX candidates with estimated X-ray luminosities in excess of 3e39 erg/s are equally divided among the group of ULX environments redward of HII regions and the group of optically faint regions. This likely indicates that the brightest ULXs turn on at a time somewhat later than typical of HII regions; say 10-20 Myr after star formation has ended. This would be consistent with the onset of an accretion phase as the donor star ascends the giant branch if the donor is a <20 solar-mass star.

Keck Spectroscopy of ROSAT Ultraluminous X-Ray Source Candidates

ABSTRACTWe present results of optical follow-up observations of ultraluminous X-ray source (ULX) candidates. A published ROSAT catalog of 87 ULX candidates named 19 of these as having optical counterparts in Digitized Sky Survey (DSS) images. Using Keck optical spectroscopy, we classify 16 ULX candidates in this subset as well as one other ULX candidate from the catalog with a counterpart in the DSS. This is one of the largest sets of optical identifications of such objects thus far. Fifteen are background active galactic nuclei (AGNs); two are foreground stars in our Galaxy. These findings are consistent with background and foreground object expectations, as derived from log N - log S relations. The logarithmic X-ray to optical flux ratios of these objects are also typical of background and foreground objects. Finally, we discuss the nature of ULX candidates outside the optical extent of their apparent host galaxies.

The Lack of Halo Ultraluminous X-Ray Sources

The premise that Ultraluminous X-ray sources (ULXs) exist beyond the optical extent of nearby galaxies is investigated. A published catalog containing 41 ULX candidates located between 1 and ~3 times the standard D_{25} isophotal radius of their putative host galaxies is examined. Twenty-one of these sources have spectroscopically-confirmed distances. All 21 are background objects giving a 95% probability that at least 37 of the 41 candidates are background sources. Thirty-nine of the 41 sources have X-ray-to-optical flux ratios, -1.6<log(F_{X}/F_{O})<+1.3, consistent with those of background active galactic nuclei. (The remaining two are not detected in optical to a weak limit of m_{B}~21.5$ mag corresponding to log(F_{X}/F_{O})\gtrsim 1.6.) The uniform spatial distribution of the sample is also consistent with a background population. This evidence suggests that ULXs rarely, if at all, exist beyond the distribution of luminous matter in nearby galaxies and, as a consequence, there is no correlation between the population of ULXs and halo objects such as old globular clusters or Pop~III remnants.

Search for Serendipitous ULX Candidates in XMM-Newton Observations

We report the serendipitous discovery of several ULX candidates in XMM-Newton observations. Such discoveries suggest that ULXs are not a negligible component of the extragalactic X-ray source population.

Ultraluminous X‐Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis

X-ray observations have revealed in other galaxies a class of extranuclear X-ray point sources with X-ray luminosities of 1039-1041 ergs s-1, exceeding the Eddington luminosity for stellar mass X-ray binaries. These ultraluminous X-ray sources (ULXs) may be powered by intermediate-mass black holes of a few thousand M☉ or stellar mass black holes with special radiation processes. In this paper, we present a survey of ULXs in 313 nearby galaxies with D25 > 1' within 40 Mpc with 467 ROSAT High Resolution Imager (HRI) archival observations. The HRI observations are reduced with uniform procedures, refined by simulations that help define the point source detection algorithm employed in this survey. A sample of 562 extragalactic X-ray point sources with LX = 1038-1043 ergs s-1 is extracted from 173 survey galaxies, including 106 ULX candidates within the D25 isophotes of 63 galaxies and 110 ULX candidates between 1D25 and 2D25 of 64 galaxies, from which a clean sample of 109 ULXs is constructed to minimize the contamination from foreground or background objects. The strong connection between ULXs and star formation is confirmed based on the striking preference of ULXs to occur in late-type galaxies, especially in star-forming regions such as spiral arms. ULXs are variable on timescales over days to years and exhibit a variety of long term variability patterns. The identifications of ULXs in the clean sample show some ULXs identified as supernovae (remnants), H II regions/nebulae, or young massive stars in star-forming regions, and a few other ULXs identified as old globular clusters. In a subsequent paper, the statistic properties of the survey will be studied to calculate the occurrence frequencies and luminosity functions for ULXs in different types of galaxies to shed light on the nature of these enigmatic sources.

ChandraObservations of Diffuse Gas and Luminous X‐Ray Sources around the X‐Ray–bright Elliptical Galaxy NGC 1600

We observed the X-ray-bright E3 galaxy NGC 1600 and nearby members of the NGC 1600 group with the Chandra X-Ray Observatory ACIS-S3 to study their X-ray properties. Unresolved emission dominates the observation; however, we resolved some of the emission into 71 sources, most of which are low-mass X-ray binaries associated with NGC 1600. Twenty-one of the sources have LX > 2 ? 1039 ergs s-1 (0.3-10.0 keV; assuming they are at the distance of NGC 1600), marking them as ultraluminous X-ray point source (ULX) candidates; we expect that only 11 ? 2 are unrelated foreground/background sources. NGC 1600 may have the largest number of ULX candidates in an early-type galaxy to date; however, cosmic variance in the number of background active galactic nuclei cannot be ruled out. The spectrum and luminosity function (LF) of the resolved sources are more consistent with sources found in other early-type galaxies than with sources found in star-forming regions of galaxies. The source LF and the spectrum of the unresolved emission both indicate that there are a large number of unresolved point sources. We propose that these sources are associated with globular clusters (GCs) and that NGC 1600 has a large GC specific frequency. Observations of the GC population in NGC 1600 would be very useful for testing this prediction. Approximately 50%-75% of the unresolved flux comes from diffuse gaseous emission. The spectral fits, hardness ratios, and X-ray surface brightness profile all point to two gas components. We interpret the soft inner component (a 25'', kT ~ 0.85 keV) as the interstellar medium of NGC 1600 and the hotter outer component (a 25'', kT ~ 1.5 keV) as the intragroup medium of the NGC 1600 group. The X-ray image shows several interesting structures. First, there is a central region of excess emission that is roughly cospatial with H? and dust filaments immediately west of the center of NGC 1600. There appear to be holes in the X-ray emission to the north and south of the galaxy center that are roughly coincident with the lobes of the NGC 1600 radio source. On larger scales, there is excess emission to the northeast, which we suggest may indicate the center of the group potential. The group galaxy NGC 1603 shows a tail of X-ray emission to its west that is probably due to ram pressure stripping.