M 31 in X-rays with Swift - a new transient and an old source gone

ATel #3443; M. Henze, W. Pietsch, F. Haberl (Max-Planck-Institut fuer extraterrestrische Physik)
on 21 Jun 2011; 10:33 UT
Credential Certification: Martin Henze (mhenze@mpe.mpg.de)

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We report on the discovery of a new X-ray transient in M 31 found in four 4 ks ToO observations of the central region of this galaxy with the Swift X-ray telescope (XRT) in June 2011. The source is located at R.A.(J2000) = 00:42:49.6, Dec.(J2000) = +41:17:16 (uncertainty 3.8 arcseconds). No X-ray source was known at this position before. The XRT spectrum of the transient, designated SWIFT J004249.6+411716, only contains about 80 counts in the four observations combined. Assuming an absorption column density of 6.7x10²⁰ cm^-2, the Galactic foreground, the spectrum can be fit using a powerlaw model with a photon index of 2.5+-0.9. Below we give the light curve of the source (luminosities are unabsorbed), corresponding to this spectral model and a M 31 distance of 780 kpc.

 
   ObsID       MJD     Exposure  Count rate  Luminosity   
               [d]       [ks]    [E-3 ct/s]  [E37 erg/s] 
 
00032008001  55715.02     4.5     6.4+-2.4     2.3+-0.9 
00032009001  55719.04     4.1    10.8+-2.7     4.0+-1.0 
00032010001  55723.19     2.5    10.4+-3.8     3.9+-1.4 
00032011001  55727.79     3.8     6.8+-2.8     2.5+-1.0 

Furthermore, we report on the apparent disappearance of a prominent X-ray source in the M 31 central region. The object r2-11 in the catalogue of Kong et al. (2002,ApJ,577,738) was already discovered with the Einstein satellite (source 71 in Trinchieri & Fabbiano,1991,ApJ,382,82). It was subsequently present as a hard X-ray source in observations with ROSAT, XMM-Newton and Chandra at a luminosity of around 6x10³⁷ erg/s. In our first Swift observation, this source was only marginally detected at a luminosity of (1.5+-0.7)x10³⁷ erg/s, assuming Galactic foreground absorption and a powerlaw spectrum with photon index of 1.7. In the following Swift observations r2-11 could not be found any more, with a 3 sigma luminosity upper limit of 1.5x10³⁷ erg/s for the three observations combined. This indicates a drop in luminosity for r2-11 by at least a factor of four, a behaviour which was unparalleled during the more than 30 years the source has been known.

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.