ATEL 2474; Optical ID of Bright Black Hole X-ray Nova in M31
We report the likely optical ID of the bright transient
recently discovered in M31, CXOM31 J004253.1+411422 (Henze etal, ATEL 2356).
With a peak X-ray luminosity of nearly 5e39 erg/sec, this is the brightest
X-ray transient yet found in M31.
An approximately 4000 second HST exposure on 21 Jan 2010,
using the ACS and F435W filter, finds an apparently new
object with m(f435w)=23.8. This object is not seen in an earlier
serendiptious 4000 second ACS exposure on 13 October 2004 using the F660N
filter. Finding charts are available at
http://hea-www.harvard.edu/~garcia/m31/J004253.html
We note that given the difference in the filters it is possible that
the object is not the counterpart, but mearly extremely blue. Followup
observations with the F435W filter are planned and will address this
possibility.
Taking the M31 distance modulus of 24.47 and assuming a typical
absorption at B of A(B) = 0.4, the absolute magnitude of the likely
counterpart is M(B) = -1.04. Assuming the X-ray luminosity was still
near its peak level on 21 January,
the Van Paradijs and McClintock 1994 relation
between luminosities and orbital periods for X-ray binaries suggests
an orbital period of 5.6 days, similar to that found for other
transients in M31 (ie, Williams etal 2005 ApJ 620 723).
