2011ApJ...729...67J

Estimation of the black hole mass in bright X-ray sources of nearby galaxies is crucial to the understanding of these systems and their formation. However, the present allowed black hole mass range spans five orders of magnitude (10 M_☉< M < 10⁵ M_☉) with the upper limit obtained from dynamical friction arguments. We show that the absence of a detectable optical counterpart for some of these sources can provide a much more stringent upper limit. The argument is based only on the assumption that the outer regions of their accretion disks are a standard one. Moreover, such optically dark X-ray sources cannot be foreground stars or background active galactic nuclei, and hence must be accreting systems residing within their host galaxies. As a demonstration we search for candidates among the point-like X-ray sources detected with Chandra in 13 nearby elliptical galaxies. We use a novel technique to search for faint optical counterparts in the Hubble Space Telescope images whereby we subtract the bright galaxy light based on isophotal modeling of the surface brightness. We show that for six sources with no detectable optical emission at the 3σ level, their black hole masses M_BH< 5000 M_☉. In particular, an ultra-luminous X-ray source in NGC 4486 has M_BH< 1244 M_☉. We discuss the potential of this method to provide stringent constraints on the black hole masses, and the implications on the physical nature of these sources.