2005ApJ...631L..65M

MACHO-96-BLG-5 was a microlensing event of exceptionally long duration, lasting ∼800 days, which is suggestive of a stellar-mass black hole for the lens. With a 9.8 ks Chandra ACIS-S observation, we detected no X-ray events associated with the position of MACHO-96-BLG-5. An upper limit is estimated as 4.6 counts at the 99% confidence level. By assuming an absorbed power law spectrum with a photon index Γ=1.4-2.0, the upper limit on the 0.3-8 keV luminosity is estimated as (7-8)x10²⁹ ergs/s at an assumed distance of 1 kpc. The corresponding Eddington ratio is (8-9)x10^–10 for a black hole of 6 M_☉. The luminosity is lower than observed for quiescent black hole binaries in our Galaxy and is consistent with a scenario, previously suggested by Bennett et al., in which the lens for MACHO-96-BLG-5 is an isolated stellar-mass black hole, although other possibilities are not ruled out.