CXOM31 J004253.1+411422 - a very bright new X-ray transient in M 31 discovered with Chandra and Swift

ATel #2356; M. Henze , W. Pietsch, F. Haberl, J. Greiner (all MPE) and the XMM-Newton/Chandra M 31 nova monitoring collaboration
on 22 Dec 2009; 15:01 UT
Credential Certification: Wolfgang Pietsch (wnp@mpe.mpg.de)

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On behalf of the XMM-Newton/Chandra M 31 nova monitoring collaboration (see http://www.mpe.mpg.de/~m31novae/xray/ao8/index.php ) we report the discovery of a new X-ray transient in M 31 in a 19 ks Chandra HRC-I observation which started on 2009 Dec 17.89 UT. The transient is detected with a count rate of 3.28 +- 0.04 ct/s (corrected for exposure and vignetting), making it by far the brightest object in the field of view. The source position is R.A.(J2000) = 00:42:53.15, Dec.(J2000) = +41:14:22.9, using the catalog of Kaaret (2002, ApJ, 578, 114) as reference. The source is designated CXOM31 J004253.1+411422.

CXOM31 J004253.1+411422 is in the field of view of a serendipitous 4.4 ks Swift observation which started on Dec 22.04. It is still the brightest source with a count rate of about 0.7 ct/s. The X-ray spectrum can be modelled (reduced chi2 of 0.9 with 89 d.o.f.) by an absorbed disk blackbody with an absorption column density of (6.6+-2.6)e20 cm-2 - consistent with the Galactic foreground absorption - and an inner disk temperature of (1.15+-0.08) keV. This model gives an unabsorbed 0.2-10.0 keV flux of (2.3+-0.1)e-11 erg cm-2 s-1 and a corresponding intrinsic source luminosity of (1.7+-0.1)e39 ergs s-1 (assuming a distance of 780 kpc). Applying the spectral model to the Chandra HRC-I count rate we derive an unabsorbed 0.2-10.0 flux of (6.3+-0.1)e-11 erg cm-2 s-1 and an unabsorbed luminosity of (4.6+-0.1)e39 ergs s-1. These spectral parameters and the very large X-ray luminosity indicate a black hole X-ray transient.

The luminosity makes CXOM31 J004253.1+411421 the brightest transient ever detected in M 31. The drop in luminosity by more than a factor of 2.5 in the four days between the Chandra and the Swift observation shows that the transient is rapidly variable. The source was not detected in a Chandra HRC-I observation on 2009 Dec 08.94 with a 3 sigma upper limit luminosity of 2.1e36 erg/s. No emission is detected in a 1.5 ks UVOT exposure during the Swift observation (3 sigma limiting magnitude of 20.1 mag in uvw2).