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Optical ID of CXOM31 J004253.1+411422, a bright Black Hole X-ray Nova in M31

ATEL # 2474; M. Garcia, S. Murray, F. Primini (SAO); P Callanan (UCC); N. Nooraee (DIAS,UCC)
on 11 Mar 2010; 21:26 UT
Password Certification: Michael R. Garcia (

Subjects: Optical, X-ray, Black Holes, Transients

We report the likely optical ID of the bright transient recently discovered in M31, CXOM31 J004253.1+411422 (Henze et al, 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 serendipitous 4000 second ACS exposure on 13 October 2004 using the F660N filter. Finding charts are available at We note that given the difference in the filters it is possible that the object is not the counterpart, but merely extremely blue. Followup observations with the F435W filter ~6 months from now 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 2010, 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 et al 2005 ApJ 620 723).

Optical ID of brightest BHXN in M31

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