A Chandra/ACIS observation of the transient M31 ULX XMMU J004243.6+412519

ATel #3937; R. Barnard (CfA), M. R. Garcia (CfA), S. S. Murray (Johns Hopkins)
on 22 Feb 2012; 14:11 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Robin Barnard (rbarnard@cfa.harvard.edu)

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We report on our ~4 ks Chandra ACIS observation of XMMU J004243.6+412519, an X-ray transient first reported by Henze et al. (2012a, Atel #3890) after its appearance in their 2012-01-15 XMM-Newton observation. Its luminosity exceeded 1E+39 during a follow-up XMM-Newton observation, and Henze et al. (2012b, Atel #3921) classified it as the second ultra-luminous X-ray source (ULX) to be observed in M31. XMMU J004243.6+412519 yielded ~4,000 net source photons during our Chandra ACIS observation on 2012-2-19; it was not piled up due to its high off-axis angle, and the brightest pixel in the image registered only 0.04 counts per CCD frame. The 0.3-7.0 keV spectrum was best fitted by an absorbed disk blackbody model, with column density equivalent to 3.2+/-0.5 E+21 H atom cm^{-2}, and temperature, KT = 0.95+/-0.05 keV; chi^2/dof = 130/127. The best fit 0.3-10 keV unabsorbed luminosity was 1.08+/-0.04 E+39 erg/s assuming a distance to M31 of 780 kpc; this is consistent with the scenario proposed by Henze et al. (2012b), where XMMU J004243.6+412519 contains a stellar mass black hole accreting at a high rate. We measured a position of R.A.(J2000) = 00:42:43.683, Dec.(2000) = +41:25:18.53, with 1 sigma error of 0.20" in RA and 0.14" in Dec after registering X-ray bright globular clusters with an image from the Local Group Galaxy Survey of Massey et al. (2006, AJ, 13, 2478). As part of our ongoing program to measure orbital periods of BH novae in M31, follow-up HST observations are being scheduled in an attempt to find the optical counterpart.