CXOM31 J004220.9+4115205, a new X-ray transient in M31 discovered by Chandra
ATel #4887; Robin Barnard (CfA), Mike Garcia (CfA), Steve Murray (JHU, CfA), Frank Primini (CfA)
on 13 Mar 2013; 20:40 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Robin Barnard (rbarnard@cfa.harvard.edu)
Subjects: X-ray, Transient
We report the discovery of a new X-ray transient during the 2013 March 12, 5 ks Chandra ACIS-I observation of the M31 center (Obs ID 14931). CXOM31 J004220.9+411520 is located ~4.4' from the M31 nucleus. It yielded 38 counts in a region of very low background (~0--2 counts); most of the flux appears to be < ~1 keV, possibly suggesting a thermal spectrum.
There are no known objects within 10", and this transient is likely to be a low mass X-ray binary containing either a black hole or neutron star accretor.
Assuming a line-of-sight absorption equivalent to 7E+20 H atom /cm^2 and a multi-temperature disk blackbody with 1 keV inner disk yields a 0.3--10 keV flux 1.05+/-0.15 E-13 erg / cm^2 / s; this corresponds to a luminosity of 7.1+/-1.1 E+36 erg/s for an assumed distance of 780 kpc. However, the spectrum could just as easily be a power law with photon index 1.7 (typical of faint X-ray binaries); this yields a higher luminosity of 1.2+/-0.2 E+37 erg/s.
Looking back to our 2013 February 18 ACIS-I observation (Obs ID 14930) yielded 7 counts in the same region. Therefore, CXOM31 J004220.9+411520 may be increasing in luminosity.
Unfortunately, M31 will be unobservable for Chandra in the next ~2 months; however, if it continues to brighten, then we may observe it again with Chandra in May, possibly following up with HST observations to search for the counterpart.
