Matching microlensing events with X-ray sources
  The detection of old neutron stars and black holes in isolation is one of the cornerstones of compact object astrophysics. Microlensing surveys may help on this purpose since the lensing mechanism is independent of the emission properties of the lens. Indeed, several black hole candidates deriving through microlensing observations have been reported in the literature. The identification of counterparts, especially in the X-rays, would be a strong argument in favor of the compact nature of these lenses.   We perform a cross-correlation between the catalogs of microlensing events by the OGLE, MACHO and MOA teams, and those of X-rays sources from XMM-Newton and Chandra satellites. Based on our previous work, we select only microlensing events longer than 100 days, which should contain a large fraction of lenses as compact objects. Our matching criterion takes into account the positional coincidence in the sky.   We find a single match between a microlensing event OGLE 2004-BLG-81 and the X-ray source 2XMM J180540.5-273427. The angular separation is \sim 0.5 arc-seconds, well inside the 90% error box of the X-ray source. The hardness ratios reported in the 2XMM catalog suggest a hard spectrum with a peak between 2 and 4.5 keV or a softer but highly absorbed source. Moreover the microlensing event is not fully constrained, and possible association with a flaring CV or a RS CVn star have been suggested as well.   The small angular separation is a strong indicator that 2XMM J180540.5-273427 is the X-ray counterpart of the OGLE event. However, the uncertainties on both the nature of lensed system and the lens itself challenge the interpretation of 2XMM J180540.5-273427 as the first confirmed isolated black hole identified so far. In any case, it probes the diagnostic capacity of microlensing surveys and open the path for further identifications of black hole or neutron star candidates. 
N. Sartore, A. Treves
astro-ph.HE