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Swift detection of a new bright X-ray transient in M 31

ATEL # 1674; W. Pietsch (Max-Planck-Institut fuer extraterrestrische Physik, MPE), V. Burwitz (MPE), J. Greiner (MPE), M. Henze (MPE), H. Stiele (MPE)
on 22 Aug 2008; 14:47 UT
Password Certification: Wolfgang Pietsch (wnp@mpe.mpg.de)

Subjects: Optical, Ultra-Violet, X-ray, Transients

SWIFT J004320.5+411528 is detected as a new transient at RA(J2000) 00:43:20.56, Dec(J2000) +41:15:28.8 (1 sigma position error 3.7") with the X-Ray Telescope (XRT) on board the Swift satellite in ObsID 31255001 starting on 2008-08-21.31 UT. In five observation intervals spread over seven hours (effective exposure 5.8 ks) 41121 counts are detected in the 0.5-5 keV band (90% PSF). The Swift XRT spectrum is best fitted with a absorbed disk-black-body model with an absorption column of (1.10.8)e21 cm-2 and a inner disk temperature of (0.550.08) keV. The unabsorbed luminosity of the source corresponds to 1.8e38 erg s-1 (0.5-5 keV) for a M 31 distance of 780 kpc. These parameters are consistent with a neutron star low mass X-ray binary. However, a black hole as compact object can not be excluded.
The transient is not detected in simultaneous Swift UVOT uvw1 and uvw2 filter observations (3 sigma upper limit of 20.3 and 20.6 mag, respectively). The images and catalog of the Local Group Survey (Massey et al. 2006, AJ 131, 2478) show no counterpart brighter than Vmag 20.8 within the 3 sigma error radius.
No source is detected about one month earlier in the XMM-Newton observation starting on 2008-07-18.26 UT (see ATel #1647) with an EPIC pn upper limit for the unabsorbed luminosity of 1.4e35 erg s-1 (0.5-5 keV) assuming a spectrum as above.

The two new M 31 transients from ATels #1647 and #1658 were also in the field of view:
XMMU J004245.9+411036 is detected as a hard source with 82.510.2 counts (0.5-5 keV, 90% PSF) in the effective exposure time of 4.9 ks. This corresponds to an unabsorbed luminosity (0.5-5 keV) of 5.4e37 erg s-1, comparable to the luminosity found during the XMM-Newton detection about one month earlier (assuming the same absorbed power law with hydrogen column density of 1.9e21 cm-2 and photon index of 2.4 as reported in ATel #1647).
XMMU J004241.2+411821 is not detected (3 sigma upper limit of 6 counts, 0.5-5 keV, 50% PSF) in the effective exposure time of 6.0 ks. This corresponds to an unabsorbed luminosity (0.5-5 keV) of 5e36 erg s-1 assuming an absorbed power law with hydrogen column density of 6.6e20 cm-2 and photon index of 1.7. The upper limit is compatible to the brightness of the source measured during the XMM-Newton detection about one month earlier (ATel #1647).

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.


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R. E. Rutledge , Editor
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