Swift J004249.5+411212, a new X-ray transient in M 31

ATel #7048; C. Delvaux, P. Schady, W. Pietsch, F. Haberl, J. Greiner (MPE)
on 11 Feb 2015; 11:48 UT
Credential Certification: Wolfgang Pietsch (wnp@mpe.mpg.de)

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In recent monitoring observations of the Andromeda Galaxy with the X-Ray Telescope (XRT) onboard the Swift satellite from 2015-01-25 to 2015-02-10, we find a new X-ray transient (Swift J004249.5+411212) at RA=00:42:49.50 Dec=+41:12:12.3 (J2000, ePOS=3.5"), at 4.10' of the center of Andromeda Galaxy. There is no X-ray source present at this position in previous M 31 X-ray catalogues.
The following table gives the observation ID, the exposure time, the date and start time of the observations, count rates and and 2-sigma statistical uncertainties:

 
ObsID        Exp      Date           Time        Count rate 
             [ks]                    (MJD)       (/s) 
00031283028  4038     2015-01-17     57039.46      <0.003 
00031283029  3912     2015-01-25     57047.31	 0.004±0.001 
00031283030  4110     2015-02-02     57055.24	 0.009±0.002 
00080007001  12374    2015-02-06     57059.97	 0.007±0.001 
00080007002  3586     2015-02-08     57061.02	 0.008±0.002 
00031283031  3219     2015-02-10     57063.55      <0.002 

By fitting an absorbed power law to the 54 photons detected in ObsID 00080007001, we estimate an N_H=8.7±5.8 ×10²⁰cm^-2 and Γ=1.7±0.2, as well as an average flux of 3.5 × 10^-13erg cm^-2 s^-1 in the 0.3-5.0 keV band.
For a source in M 31 (780 kpc distance), this corresponds to an unabsorbed luminosity of 3.2 × 10³⁸erg s^-1 in the 0.3-5.0 keV band. Such a luminosity could be consistent with a LMXB transient containing a neutron star or a black hole.
In a stack of all UVOT/uvm2 filter observations on 2015-02-06 we do not detect any counterpart candidate within the X-ray position down to a 2-sigma limit of m2_AB>23.1.
The last observations suggest that the outburst has reached its maximum brightness and has faded down to the detection threshold of Swift/XRT after 16 days.
We strongly encourage further multi-wavelength observations of Swift J004249.5+411212, in particular deeper optical imaging and radio observations, to identify the counterpart of this new X-ray transient.
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.