Optical Constraints on an X-ray Transient Source in M31

Abstract

We have detected a transient X-ray source in the M31 bulge through a continuing monitoring campaign with the Chandra ACIS-I camera. The source was detected at R.A.=00^h42^m33.428s+/-0.11", decl.=+41^deg17^'03.37"+/-0.11" in only a single observation taken 2004 May 23. Fortuitous optical HST ACS imaging of the transient location prior to the X-ray outburst, along with follow-up HST ACS imaging during and after the outburst, reveals no transient optical source brighter than B(equivalent)=25.5. The location of the source and its X-ray properties suggest that it is a low-mass X-ray binary (LMXB). Assuming the transient is similar to many Galactic X-ray novae, the X-ray luminosity of (3.9+/-0.4)×10³⁷ ergs s^-1 and the upper limit on the optical luminosity provide a prediction of <=1.6 days for the orbital period of the binary system.