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The transient X-ray source CXOM31 004205.77+411330.43 exhibited severaloutbursts during our long-term monitoring campaign of ~monthly observations ofthe M31 center with Chandra. However, the decay profile appears to be unlikethose observed from Galactic transients. We followed up the 2011 outburst withtwo ~B band HST/ACS observations, one in outburst and the other in quiescence,and used difference imaging to search for a counterpart; this would bedominated by re-processed X-ray emission from the disk. We found a counterpartwith B = 28.21+/-0.16. An XMM-Newton observation from a previous outburstyielded a spectrum that is well described by an absorbed power law withabsorption equivalent to ~2.6 E+21 H atom cm^(-2) and photon index ~1.8; thehighest quality Chandra spectrum yielded only $\sim$130 counts, and best fitsconsistent with the XMM-Newton results. We calculated an absolute V magnitudeof +1.9 during outburst for a typical disk spectrum. An empirical relationbetween the ratio of X-ray to optical flux and orbital period suggests a period< ~4 hr for a black hole accretor. Such a short period is expected to yield anasymmetric, precessing disk, and we propose that the observed decay lightcurveis due to modulation of the mass transfer rate due to the disk precession.

Chandra, XMM-Newton and HST observations of a transient in M31 with a possible asymmetric, precessing disk