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

The transient X-ray source CXOM31 004205.77+411330.43 exhibited several outbursts during our long-term monitoring campaign of approximately monthly observations of the M31 centre with Chandra. However, the decay profile appears to be unlike those observed from Galactic transients. We followed up the 2011 outburst with two ∼B band HST/ACS observations, one in outburst and the other in quiescence, and used difference imaging to search for a counterpart; this would be dominated by reprocessed X-ray emission from the disc. We found a counterpart with B = 28.21 ± 0.16. An XMM-Newton observation from a previous outburst yielded a spectrum that is well described by an absorbed power law with absorption equivalent to ∼2.6 × 10²¹ H atom cm^-2 and photon index ∼1.8; the highest quality Chandra spectrum yielded only ∼130 counts, and best fits consistent with the XMM-Newton results. We calculated an absolute V magnitude of +1.9 during outburst for a typical disc spectrum. An empirical relation between the ratio of X-ray to optical flux and orbital period suggests a period ≲ 4 h for a black hole accretor. Such a short period is expected to yield an asymmetric, precessing disc, and we propose that the observed decay light curve is due to modulation of the mass transfer rate due to the disc precession.