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New X-Ray Transient in M31: CXOM31 J004059.2+411551

ATEL # 1147; J.L. Galache, M.R. Garcia, D. Steeghs, M.A.P. Torres, S.S. Murray, F.A. Primini (CfA) and B.F. Williams (U. of Washington)
on 19 Jul 2007; 15:44 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: Jose Luis Galache (

Subjects: Binaries, Black Holes, Transients

We report the discovery of a previously unseen X-ray point source detected in a 5ks observation starting on 2007-07-05.54 UT as part of our ongoing Chandra/HST M31 transient program. The source was detected on the edge of ACIS-S at coordinates RA(J2000) 00:40:59.2, Dec(J2000) +41:15:51.2, with centroid position errors of 2.4" and 3.0", respectively (subject to the standard bore-sight correction). The 0.7-3.0 keV spectrum is well modelled using an absorbed disk blackbody (after ascertaining that an added powerlaw made a negligible contribution to the flux). Leaving all parameters free we find the best fit with nH = 2.3e21 atoms/cm2 and Tin ~ 0.46 keV. We note that this nH is substantially larger than the galactic contribution along the line of sight. The equivalent unabsorbed luminosity (assuming a distance of 780 kpc to M31) is Lx ~ 7.5e37 erg/s in the 0.5-10 keV range. The presence of a very soft component in the spectrum and the X-ray luminosity are consistent with the source being a black hole transient in a thermal (high-soft) state. Further optical and X-ray observations of this source are encouraged.

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