Multi-frequency Studies of Galactic X-ray Sources Populations. Hard   X-ray Galactic sources of low to intermediate Lx; A search for isolated   accreting black holes
  Our Galaxy harbours a large population of X-ray sources of intermediate to low X-ray luminosity (typically Lx from 10^27 to 10^34 erg/s). At energies below 2 keV, active coronae completely dominate the X-ray landscape. However, the nature and the properties of Galactic sources detected at energies > 2 keV is much less constrained. Optical follow-up spectroscopic observations show that in addition to cataclysmic variables (CVs) and very active stellar coronae, massive stars (colliding wind binaries, quiescent high-mass X-ray binaries and Gamma-Cas analogs) account for a sizable fraction of the Galactic hard X-ray sources at medium flux (Fx > 10^-13 erg/s/cm^2). Cross-correlations of the 2XMM-DR3 catalogue with 2MASS and GLIMPSE confirm the presence above 2 keV of a large population of coronally active binaries, probably of the BY Dra and RS CVn types, in addition to many distant and absorbed massive stars. We also report the results of a specific optical identification campaign aimed at studying the nature of the optically faint hard X-ray sources and at constraining the surface density of black holes (BHs), either isolated and accreting from the interstellar medium or in quiescent binaries. Not astonishingly, most of our sample of 14 optically faint and X-ray hard sources are identified with CVs and Me stars. We do not find any likely counterpart in only three cases. Our observations also allow us to put an upper limit of 0.2 BH deg^-2 at Fx = 1.3 10^-13 erg/s/cm^2 in directions toward the center of our Galaxy. This implies a combined Bondi-Hoyle and mass accretion rate to Lx efficiency of accretion onto black holes of less than 10^-3. 
C. Motch and M. W. Pakull
astro-ph.HE