Discovery of a New Black Hole Transient within 100'' (400 pc) of M31*

We identified a new X-ray transient CXOM31 004252.457+411631.17 (T13) in M31 during a 2013 June Chandra observation. This system is particularly exciting because it is located within 100'' of M31*; it is thought that this region of the bulge is sufficiently dense to form X-ray binaries dynamically, but only systems with black hole accretors and/or short periods are expected to survive. A follow-up XMM-Newton observation yielded a soft spectrum, well described by a 0.39 ± 0.02 keV disk blackbody; applying this model to the Chandra observation yielded an observed 0.3-10 keV luminosity peak of 6.2 ± 0.6× 10³⁷ erg s^-1 (4.7× 10³⁶ erg s^-1 in the 2.0-10 keV band). Observing with Hubble Space Telescope/Advanced Camera for Surveys did not reveal an optical counterpart, but allowed us to place an upper limit of B > 26.9, corresponding to an absolute V band magnitude >2.0. From the 2-10 keV luminosity and absolute V magnitude, we estimate an orbital period <5 h from an empirical relation. Fitting a disk blackbody + blackbody model allows us to reject a neutron star accretor at a 14σ level.