The observed relation between the X-ray and radio properties of low-luminosity accreting black holes (BHs) has enabled the identification of multiple candidate black hole X-ray binaries (BHXBs) in globular clusters (GCs). Here, we report an identification of the radio source VLA J213002.08+120904 (aka M15 S2), recently reported in Kirsten et al., as a BHXB candidate. They showed that the parallax of this flat-spectrum variable radio source indicates a -+ 2.2 0.3 0.5 kpc distance, which identifies it as lying in the foreground of the GC M15. We determine the radio characteristics of this source and place a deep limit on the X-ray luminosity of ∼4 × 10 29 erg s −1 . Furthermore, we astrometrically identify a faint red stellar counterpart in archival Hubble images with colors consistent with a foreground star; at 2.2 kpc, its inferred mass is 0.1-0.2 M e . We rule out that this object is a pulsar, neutron star X-ray binary, cataclysmic variable, or planetary nebula, concluding that VLA J213002.08+120904 is the first accreting BHXB candidate discovered in quiescence outside of a GC. Given the relatively small area over which parallax studies of radio sources have been performed, this discovery suggests a much larger population of quiescent BHXBs in our Galaxy, 2.6 × 10 4 -1.7 × 10 8 BHXBs at 3σ confidence, than has been previously estimated (∼10 2 -10 4 ) through population synthesis.

Precise and accurate measurements of distances to Galactic X-ray binaries (XRBs) reduce uncertainties in the determination of XRB physical parameters. We have cross-matched the XRB catalogues of Liu et al. (2006, 2007) to the results of Gaia Data Release 2. We identify 86 X-ray binaries with a Gaia candidate counterpart, of which 32 are low-mass X-ray binaries (LMXBs) and 54 are high-mass X-ray binaries (HMXBs). Distances to Gaia candidate counterparts are, on average, consistent with those measured by Hipparcos and radio parallaxes. When compared to distances measured by Gaia candidate counterparts, distances measured using Type I X-ray bursts are systematically larger, suggesting that these bursts reach only 50% of the Eddington limit. However, these results are strongly dependent on the prior assumptions used for estimating distance from the Gaia parallax measurements. Comparing positions of Gaia candidate counterparts for XRBs in our sample to positions of spiral arms in the Milky Way, we find that HMXBs exhibit mild preference for being closer to spiral arms; LMXBs exhibit mild preference for being closer to inter-arm regions. LMXBs do not exhibit any preference for leading or trailing their closest spiral arm. HMXBs exhibit a mild preference for trailing their closest spiral arm. The lack of a strong correlation between HMXBs and spiral arms may be explained by star formation occurring closer to the midpoint of the arms, or a time delay between star formation and HMXB formation manifesting as a spatial separation between HMXBs and the spiral arm where they formed.

We present near-simultaneous Chandra/HST observations of the very faint (L x < 10 36 erg s −1 ) X-ray transient source M15 X-3, as well as unpublished archival Chandra observations of M15 X-3. The Chandra observations constrain the luminosity of M15 X-3 to be < 10 34 erg s −1 in all observed epochs. The X-ray spectrum shows evidence of curvature, and prefers a fit to a broken power-law with break energy E break = 2.7 +0.4 −0.6 keV, and power law indices of Γ 1 = 1.3 +0.1 −0.2 and Γ 2 = 1.9 +0.2 −0.2 over a single power law. We fit our new F438W (B), F606W (broad V ), and F814W (I) HST data on the blue optical counterpart with a model for an accretion disk and a metal-poor main sequence star. From this fit, we determine the companion to be consistent with a main sequence star of mass 0.440 +0.035 −0.060 M ⊙ in a ∼4-hour orbit. X-ray irradiation of the companion is likely to be a factor in the optical emission from the system, which permits the companion to be smaller than calculated above, but larger than 0.15 M ⊙ at the 3σ confidence level. M15 X-3 seems to be inconsistent with all suggested hypotheses explaining very faint transient behavior, except for magnetospherically inhibited accretion.

We present an unprecedented, deep study of the primordial low-mass X-ray binary population in an isolated, lower-metallicity environment. We perform followup observations of previously-identified X-ray binary candidates in the Sculptor Dwarf Galaxy by combining a second Chandra observation with Spitzer and Gemini photometry, as well as Gemini spectroscopy of selected targets. Of the original nine bright X-ray sources identified, we are able to classify all but one as quasars, active galactic nuclei, or background galaxies. We further discover four new X-ray sources in the secondepoch Chandra observation. Three of these new sources are background sources and one is a foreground flaring star. We have found that Sculptor is effectively devoid of X-ray sources above a few 10 34 erg s −1 . If Sculptor is able to retain primordial binaries at a similar rate to globular clusters, this implies that bright X-ray binaries observed in globular clusters in the present epoch are all formed dynamically.

Identifying X-ray binary (XRB) candidates in nearby galaxies requires distinguishing them from possible contaminants including foreground stars and background active galactic nuclei. This work investigates the use of supervised machine learning algorithms to identify high-probability X-ray binary candidates. Using a catalogue of 943 Chandra X-ray sources in the Andromeda galaxy, we trained and tested several classification algorithms using the X-ray properties of 163 sources with previously known types. Amongst the algorithms tested, we find that random forest classifiers give the best performance and work better in a binary classification (XRB/non-XRB) context compared to the use of multiple classes. Evaluating our method by comparing with classifications from visible-light and hard X-ray observations as part of the Panchromatic Hubble Andromeda Treasury, we find compatibility at the 90% level, although we caution that the number of source in common is rather small. The estimated probability that an object is an X-ray binary agrees well between the random forest binary and multiclass approaches and we find that the classifications with the highest confidence are in the X-ray binary class. The most discriminating X-ray bands for classification are the 1.7-2.8, 0.5-1.0, 2.0-4.0, and 2.0-7.0 keV photon flux ratios. Of the 780 unclassified sources in the Andromeda catalogue, we identify 16 new high-probability X-ray binary candidates and tabulate their properties for follow-up.