Gaia DR2 Distances and Peculiar Velocities for Galactic Black Hole Transients

We report on a first census of Galactic black hole X-ray binary (BHXRB) properties with the second data release (DR2) of {\em Gaia}, focusing on dynamically confirmed and strong candidate black hole transients. DR2 provides five-parameter astrometric solutions including position, parallax and proper motion for 11 of a sample of 24 systems. Distance estimates are tested with parallax inversion as well as Bayesian inference. We derive an empirically motivated characteristic scale length of $L$=2.17$\pm$0.12 kpc for this BHXRB population to infer distances based upon an exponentially decreasing space density prior. Geometric DR2 parallaxes provide new, independent distance estimates, but the faintness of this population in quiescence results in relatively large fractional distance uncertainties. Despite this, DR2 estimates generally agree with literature distances. The most discrepant case is BW Cir, for which detailed studies of the donor star have suggested a distant location at >~25 kpc. A large DR2 measured parallax and relatively high proper motion instead prefer significantly smaller distances, suggesting that the source may instead be amongst the nearest of XRBs. However, both distances create problems for interpretation of the source, and follow-up data are required to resolve its true nature. DR2 also provides a first distance estimate to one source, MAXI J1820+070, and novel proper motion estimates for 7 sources. Peculiar velocities relative to Galactic rotation exceed $\sim$ 50 km s$^{-1}$ for the bulk of the sample, with a median system kinetic energy of peculiar motion of $\sim$ 5 $\times$ 10$^{47}$ erg. BW Cir could be a new high-velocity BHXRB if its astrometry is confirmed. A putative anti-correlation between peculiar velocity and black hole mass is found, as expected in mass-dependent BH kick formation channels, but this trend remains weak in the DR2 data.Comment: MNRAS in pres

Luminous [O III] and [N II] from Tidally Disrupted Horizontal Branch Stars

We model the emission lines generated in the photoionised debris of a tidally disrupted horizontal branch star. We find that at late times, the brightest optical emission lines are [N II] \lambda\lambda 6548,6583 and [O III] \lambda\lambda 4959,5007. Models of a red clump horizontal branch star undergoing mild disruption by a massive (50 -- 100 M_\sun) black hole yield an emission line spectrum that is in good agreement with that observed in the NGC 1399 globular cluster hosting the ultraluminous X-ray source CXOJ033831.8 - 352604. We make predictions for the UV emission line spectrum that can verify the tidal disruption scenario and constrain the mass of the BH.Comment: 8 pages, 6 figures, Accepted for publication in MNRA

Some Constraints On the Effects of Age and Metallicity on the Low Mass X-ray Binary Formation Rate

We have studied the low mass X-ray binary (LMXB) populations within and outside globular clusters (GC) in NGC 4365 and NGC 3115. Using published age and metallicity constraints from optical and IR observations of their GCs, we do not find any evidence for an increase in the LMXB formation rate in the intermediate age cluster population of NGC 4365, as has been proposed in some scenarios of dynamical LMXB formation in GCs. The old, metal-rich, red population of GCs in NGC 3115 on the other hand is {\it at least} three times as efficient at creating LMXBs as the old, metal-poor, blue clusters. These data suggest that the higher formation efficiency of LMXBs in the red GC subsystems of many galaxies is largely a consequence of their higher metallicity. A comparison of the densities of field LMXBs in different galaxies does not reveal an obvious correlation with the age of the field stars as predicted by models in which the LMXB formation rate in the field drops monotonically with time after an initial burst. This suggests that either a significant fraction of the field LMXBs are created in GCs and subsequently injected into the field, or the LMXB formation rate has a more complex time evolution pattern.Comment: Accepted for publication in ApJ Letters. 7 pages including 5 figure

An X-ray emitting black hole in a globular cluster

We present optical and X-ray data for the first object showing strong evidence for being a black hole in a globular cluster. We show the initial X-ray light curve and X-ray spectrum which led to the discovery that this is an extremely bright, highly variable source, and thus must be a black hole. We present the optical spectrum which unambiguously identifies the optical counterpart as a globular cluster, and which shows a strong, broad [O III] emission line, most likely coming from an outflow driven by the accreting source.Comment: 5 pages, 2 figures, to appear in the proceedings of IAUS 246, "Dynamical Evolution of Dense Stellar Systems", ed. Vesperini, Giersz and Sill