XMMU J004243.6+412519 is a transient X-ray source in M31 , first discovered 2012 January 15 .
Different approaches to fitting the brightest follow-up observation gave luminosities 1.3–2.5 \times 10 ^ { 39 } erg s ^ { -1 } , making it the second ultraluminous X-ray source ( ULX ) in M31 , with a probable black hole accretor .
These different models represent different scenarios for the corona : optically thick and compact , or optically thin and extended .
We obtained Chandra ACIS and HST ACS observations of this object as part of our transient monitoring program , and also observed it serendipitously in a 120 ks XMM-Newton observation .
We identify an optical counterpart at J2000 position 00:42:43.70 +41:25:18.54 ; its F435W ( \sim B band ) magnitude was 25.97 \pm 0.03 in the 2012 March 7 observation , and > 28.4 at the 4 \sigma level during the 2012 September 7 observation , indicating a low mass donor .
We created two alternative lightcurves , using the different corona scenarios , finding linear decay for the compact corona and exponential decay for the extended corona ; linear decay implies a disk that is > 5 magnitudes brighter than we observed .
We therefore favor the extended corona scenario , but caution that there is no statistical preference for this model in the X-ray spectra alone .
Using two empirical relations between the X-ray to optical ratio and the orbital period , we estimate a period of \sim 9–30 hr ; this period is consistent with that of the first ULX in M31 ( 18 ^ { +5 } _ { -6 } hr ) .