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XMM J004243.6+412519 is a transient X-ray source in M31, first discovered2012 January 15. Different approaches to fitting the brightest follow-upobservation gave luminosities 1.3--2.5E+39 erg/s, making it the secondultraluminous X-ray source (ULX) in M31, with a probable black hole accretor.These different models represent different scenarios for the corona: opticallythick and compact, or optically thin and extended. We obtained Chandra ACIS and{\em HST} ACS observations of this object as part of our transient monitoringprogram, and also observed it serendipitously in a 120 ks XMM-Newtonobservation. We identify an optical counterpart at J2000 position 00:42:43.70+41:25:18.54; its F435W (~ B band) magnitude was 25.97+/-0.03 in the 2012 March7 observation, and >28.4 at the 4 sigma level during the 2012 September 7observation, indicating a low mass donor. We created two alternativelightcurves, using the different corona scenarios, finding linear decay for thecompact corona and exponential decay for the extended corona; linear decayimplies a disk that is >5 magnitudes brighter than we observed. We thereforefavor the extended corona scenario, but caution that there is no statisticalpreference for this model in the X-ray spectra alone. Using two empiricalrelations between the X-ray to optical ratio and the orbital period, weestimate a period of ~9--30 hr; this period is consistent with that of thefirst ULX in M31 (18 +5 -6 hr).

The second ULX transient in M31: Chandra, HST and XMM observations, and evidence for an extended corona