Chandra Detection of XTE J1650-500 in Quiescence and the Minimum Luminosity of Black Hole X-Ray Binaries

The Galactic black hole X-ray binary XTE J1650-500 entered a quiescent regime following the decline from the 2001-2002 outburst that led to its discovery. Here we report on the first detection of its quiescent counterpart in a 36 ks observation taken in 2007 July with the Chandra X-Ray Observatory. The inferred 0.5-10 keV unabsorbed flux is in the range (2.5-5.0) × 10^-15 erg s^-1 cm^-2. Notwithstanding large distance uncertainties, the measured luminosity is comparable to that of the faintest detected black hole X-ray binaries, all having orbital periods close to the expected bifurcation period between j- and n-driven low-mass X-ray binaries. This suggests that a few 10³⁰ erg s^-1 might be a limiting luminosity value for quiescent black holes.