Spectroscopy of a New Candidate Black Hole Discovered in Quiescence

We propose to study a possible new quiescent black hole X-ray Binary detected in the Galactic Bulge Survey, CX561=CXOGBS J174607.6-261547, with GMOS on Gemini-S. Our goal is to track disk emission lines and absorption lines from the donor to measure the binary period, radial velocity curve, and rotational broadening to derive the mass of the primary. Measurement of the radial velocity curve alone will yield a mass function, which may be high enough to rule out a white dwarf or neutron star companion. If the primary is revealed as a black hole, it will be the first dynamically confirmed black hole discovered while in quiescence. The present population of Galactic black holes has been found in outburst and followed into quiescence. Having not undergone a recent outburst, black hole X-ray binaries detected in quiescence could display different properties than those detected in outburst. As a possible new black hole binary discovered in quiescence, CX561 offers a new window into black hole X-ray Binaries as well as proof that the survey approach can detect new quiescent X-ray Binaries.