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Swift XRT discovery of the third supersoft X-ray transient in a M 31 globular cluster

ATel #3013; W. Pietsch, M. Henze, F. Haberl, V. Burwitz (Max-Planck-Institut fuer extraterrestrische Physik, MPE)
on 8 Nov 2010; 15:04 UT
Credential Certification: Wolfgang Pietsch (wnp@mpe.mpg.de)

Subjects: X-ray, Transient

Referred to by ATel #: 3019, 3074

We report the discovery of a new transient X-ray source in M 31 which was found in a 7.7 ks Swift XRT observation starting on 2010-11-07.06 UT (ObsID 00031861001). The observation was part of a Swift ToO monitoring that we requested for the recurrent nova M31N 1963-09c (see ATels #3001, #3006) and the new transient serendipitously was in the XRT field of view. The source is clearly detected with a count rate of (2.0+-0.2)e-2 ct/s at the position RA = 00:42:43.92, Dec = +41:12:41.7 (J2000; 90% error of 3.8 arcsec). No X-ray source has been found at this position before in various deep X-ray surveys (e.g. XMM-Newton: Stiele et al. 2008, A&A 480, 599; Chandra ACIS-I: Kong et al. 2002, ApJ 577, 738; Chandra HRC: Williams et al. 2004, ApJ 609, 735). All source counts are below 1 keV. Therefore, we classify the new transient as a supersoft X-ray source (SSS). We checked archival Swift data, and found the SSS to be present already in two previous observations with the following count rates: (1.2+-0.2)e-2 ct/s in ObsID 00031851002 (4.8 ks, starting on 2010-11-03.04 UT; PI: M. Garcia) and (6.4+-1.9)e-3 ct/s in ObsID 00031861002, the first observation of our ToO monitoring (3.9 ks, starting on 2010-11-06.20 UT). No source was found in a 2.6 ks observation on 2010-10-27.56 UT (ObsID 00031851001; PI: M. Garcia) with a 3 sigma upper limit of 6.3e-3 ct/s. This shows that the SSS was already active for four days before our discovery and that its light curve was variable by a factor of about 3 during this time. We will continue to monitor the X-ray light curve of this interesting object with Swift.

The position of the new SSS is consistent, within the errors, with the position of the M 31 globular cluster (GC) Bol 126 which is located at RA = 00:42:43.583, Dec = + (J2000; Galetti et al. 2007, A&A 471, 127). The discovery of SSSs in GCs is a very rare event. Only three such objects were known previously. Two of them have been found by us in the M 31 GCs Bol 111 (identified with the optical nova M31N 2007-06b in Bol 111) and Bol 194 (see Henze et al. 2009, A&A 500, 769).

We would like to thank the Swift Team for making these observations possible, in particular N. Gehrels, the duty scientists as well as the science planners.