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Register a new userNew Hubble Space Telescope imaging of the counterparts to six ultraluminous X-ray sources
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T. P. Roberts Durhamn University
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We report the results of new Hubble Space Telescope imaging of the positions of six ultraluminous X-ray sources (ULXs). Using images in three ACS filters we detect good candidate counterparts to four out of six ULXs, with one more possible detection, and observed magnitudes in the range m ~ 22 - 26 in the F606W filter. The extinction-corrected colours and absolute magnitudes vary from source to source, even after correcting for additional extinction in the host galaxy, and only one counterpart is readily explained as an OB star. Nevertheless, these counterparts are decent candidates for future follow-up in pursuit of dynamical mass constraints on the likely black holes powering these sources.
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We present a systematic analysis of archival HST WFPC2 ``Association data sets that correlate with the Chandra positions of a set of 44 ultra-luminous X-ray sources (ULXs) of nearby galaxies. We have improved the Chandra-HST relative astrometry whenever possible. Disparate numbers of potential ULX counterparts are found, and in some cases none are found. The lack of or low number of counterparts in some cases may be due to insufficient depth in the WFPC2 images. Particularly in late-type galaxies, the HST image in the ULX region was often complex or crowded. We therefore address various scenarios for the nature of the ULX since it is not known which, if any, of the sources found are true counterparts. The optical luminosities of the sources are typically in the range 10^4-6 L_sun. In several cases color information is available, with the colors roughly tending to be more red in early-type galaxies. This suggests that, in general, the (potential) counterparts found in early-type galaxies are likely to be older stellar populations, and are probably globular clusters. Several early-type galaxy counterparts have blue colors, which may be due to younger stellar populations in the host galaxies, however these could also be background sources. In spiral galaxies the sources may also be due to localized structure in the disks rather than bound stellar systems. Alternatively some of the counterparts in late-type galaxies may be isolated supergiant stars. The observed X-ray/optical flux ratio is diluted by the optical emission of the cluster in cases where the system is an X-ray binary in a cluster, particularly in the case of a low-mass X-ray binaries in old cluster. (abridged)
We present a photometric survey of the optical counterparts of ultraluminous X-ray sources (ULXs) observed with the Hubble Space Telescope in nearby (< 5 Mpc) galaxies. Of the 33 ULXs with Hubble & Chandra data, 9 have no visible counterpart, placing limits on their M_V of ~ -4 to -9, enabling us to rule out O-type companions in 4 cases. The refined positions of two ULXs place them in the nucleus of their host galaxy. They are removed from our sample. Of the 22 remaining ULXs, 13 have one possible optical counterpart, while multiple are visible within the error regions of other ULXs. By calculating the number of chance coincidences, we estimate that 13 +/- 5 are the true counterparts. We attempt to constrain the nature of the companions by fitting the SED and M_V to obtain candidate spectral types. We can rule out O-type companions in 20 cases, while we find that one ULX (NGC 253 ULX2) excludes all OB-type companions. Fitting with X-ray irradiated models provides constraints on the donor star mass and radius. For 7 ULXs, we are able to impose inclination-dependent upper and/or lower limits on the black holes mass, if the extinction to the assumed companion star is not larger than the Galactic column. These are NGC 55 ULX1, NGC 253 ULX1, NGC 253 ULX2, NGC 253 XMM6, Ho IX X-1, IC342 X-1 & NGC 5204 X-1. This suggests that 10 ULXs do not have O companions, while none of the 18 fitted rule out B-type companions.
We review observations of ultraluminous X-ray sources (ULXs). X-ray spectroscopic and timing studies of ULXs suggest a new accretion state distinct from those seen in Galactic stellar-mass black hole binaries. The detection of coherent pulsations indicates the presence of neutron-star accretors in three ULXs and therefore apparently super-Eddington luminosities. Optical and X-ray line profiles of ULXs and the properties of associated radio and optical nebulae suggest that ULXs produce powerful outflows, also indicative of super-Eddington accretion. We discuss models of super-Eddington accretion and their relation to the observed behaviors of ULXs. We review the evidence for intermediate mass black holes in ULXs. We consider the implications of ULXs for super-Eddington accretion in active galactic nuclei, heating of the early universe, and the origin of the black hole binary recently detected via gravitational waves.
We present the results of our continued systematic search for near-infrared (NIR) candidate counterparts to ultraluminous X-ray sources (ULXs) within 10 Mpc. We observed 42 ULXs in 24 nearby galaxies and detected NIR candidate counterparts to 15 ULXs. Fourteen of these ULXs appear to have a single candidate counterpart in our images and the remaining ULX has 2 candidate counterparts. Seven ULXs have candidate counterparts with absolute magnitudes in the range between -9.26 and -11.18 mag, consistent with them being red supergiants (RSGs). The other eight ULXs have candidate counterparts with absolute magnitudes too bright to be a single stellar source. Some of these NIR sources show extended morphology or colours expected for Active Galactic Nuclei (AGN), strongly suggesting that they are likely stellar clusters or background galaxies. The red supergiant candidate counterparts form a valuable sample for follow-up spectroscopic observations to confirm their nature, with the ultimate goal of directly measuring the mass of the compact accretor that powers the ULX using binary Doppler shifts.
We use deep J and Ks images of the Antennae (NGC 4038/9) obtained with WIRC on the Palomar 200-inch telescope, together with the Chandra X-ray source list of Zezas et al. (2002a), to search for IR counterparts to X-ray point sources. We establish an X-ray/IR astrometric frame tie with 0.5 rms residuals over a ~4.3 field. We find 13 ``strong IR counterparts brighter than Ks = 17.8 mag and < 1.0 from X-ray sources, and an additional 6 ``possible IR counterparts between 1.0 and 1.5 from X-ray sources. The surface density of IR sources near the X-ray sources suggests only ~2 of the ``strong counterparts and ~3 of the ``possible counterparts are chance superpositions of unrelated objects. Comparing both strong and possible IR counterparts to our photometric study of ~220 Antennae, IR clusters, we find the IR counterparts to X-ray sources are ~1.2 mag more luminous in Ks than average non-X-ray clusters. We also note that the X-ray/IR matches are concentrated in the spiral arms and ``overlap regions of the Antennae. This implies that these X-ray sources lie in the most ``super of the Antennaes Super Star Clusters, and thus trace the recent massive star formation history here. Based on the N_H inferred from the X-ray sources without IR counterparts, we determine that the absence of most of the ``missing IR counterparts is because they are intrinsically less luminous in the IR, implying that they trace a different (possibly older) stellar population.
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