A Soft X-Ray Transient in the M31 Bulge

We have examined a probable soft X-ray transient source in the M31 bulge at R.A. = 0h42m41.ˢ814 ± 0.″08, decl. = 41°16′35.″86 ± 0.″07. On the three occasions we observed the source, its spectrum was soft (kTin ≈ 1 keV). The brightest detection of the source was on 2004 July 17, with a 0.3-7 keV luminosity of ~5 × 1037 ergs s-1. The only previous detection of the source was in 1979 by the Einstein observatory. The multiple detections over 25 years suggest that the duty cycle of the source is in the range 0.02-0.06. Coordinated HST ACS imaging before, during, and after the outburst revealed no variable optical source within the position errors of the X-ray source. The optical data place a firm upper limit on the brightness of the counterpart of the X-ray outburst of B > 24.7, suggesting that the binary has a period ≲5.2 days. The X-ray spectrum and lack of bright stars at the source location indicate that the source was a soft transient event occurring in a low-mass X-ray binary, making this source a good black hole candidate in M31.