CXOM31 J004252.467+411631.17, a new M31 transient discovered by Chandra
ATel #5108; Robin Barnard (CfA), Mike Garcia (CfA), Steve Murray (JHU, CfA), Frank Primini (CfA)
on 5 Jun 2013; 19:27 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Robin Barnard (rbarnard@cfa.harvard.edu)
Subjects: X-ray, Binary, Black Hole, Transient
We report the discovery of a bright new transient X-ray source in the 5 ks, 2013 June 2 Chandra ACIS-I observation of the M31 center (ObsID 15324). CXOM31 J004252.467+411631.17 is located ~95" from the M31 nucleus. Many bright X-ray transients are thought to be black hole X-ray binaries.
We obtained 92 net source counts, which was not sufficient for spectral modeling. If we assume a disk blackbody model with a 1 keV inner disk temperature, and line of sight absorption 7E20 H atom/cm^2, then the 0.3--10 keV luminosity is 6.1+/-0.6 EE+37 erg/s. Alternatively, a hard state power law spectrum with photon index 1.7 yielded a 0.3--10 keV luminosity of 8.9+/-0.9 E+37 erg/s.
We will be following up with an HST/ACIS observation with the F435W filter, in order to search for a counterpart via difference imaging. We serendipitously observed the region containing the new transient in a 2007 January 10 observation; we found that the 4 sigma sensitivity limit for our counterpart will be B ~26.8. The optical to X-ray luminosity ratio will allow us to estimate the orbital period.
This transient is of particular interest because previous workers have found an excess of X-ray binaries within this region that may have been formed dynamically, in the high density bulge (Voss & Gilfanov, 2007). Since the stellar velocities in the bulge are considerably higher than in globular clusters, survivng X-ray binaries are expected to be short period systems with black hole accretors (Voss & Gilfanov, 2007). Finding an optical counterpart may yield a period that supports this theory.
