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Swift XRT detections of X-ray transients in M 31

ATel #1806; M. Henze, W. Pietsch, J. Greiner, V. Burwitz (Max-Planck-Institut fuer extraterrestrische Physik)
on 23 Oct 2008; 15:58 UT
Credential Certification: Wolfgang Pietsch (wnp@mpe.mpg.de)

Subjects: X-ray, Transient

Referred to by ATel #: 1815, 1859, 1870

This is a re-submission of ATel #1805 correcting formating of the table at the end.

We report Swift X-Ray Telescope (XRT) detections of outbursts of two recurrent X-ray transients in M 31 in October 2008 and give information about the luminosity evolution of two X-ray transients that were discovered in July and August 2008 (see ATels #1647 and #1674).

1.) A new outburst of XMMM31 J004224.5+412401 (VG2007 233) (Voss & Gilfanov 2007, A&A 468, 49; Stiele et al. 2008, A&A 480, 599) was detected with 12+-4 counts (corrected for exposure and vignetting) in a 2.5 ks Swift XRT observation starting on 2008-10-15.15 UT (ObsID 31255009). This corresponds to an unabsorbed luminosity of (1.9+-0.4)e37 erg/s, assuming an absorbed power law with hydrogen column density of 6.6e-20 cm^-2 and photon index of 1.7, for a M 31 distance of 780 kpc. The transient was still visible, with a similar luminosity (26+-6 counts in 6.0 ks), in a Swift XRT observation starting on 2008-10-21.63 UT (ObsID 31255010), but was not detected (3 sigma upper limit of 1.4e37 erg/s) in ObsID 31255006 (4.7 ks), starting on 2008-09-25.51. XMMM31 J004224.5+412401 was previously detected in a XMM-Newton observation starting on 2004-07-16.69 (ObsID 0202230201) and was classified as a X-ray binary candidate according to its time variability. The source was not known before as a recurrent transient.

2.) A new outburst of the recurrent transient XMMM31 J004205.8+411329 (r3-125) (Stiele et al. 2008, A&A 480, 599; Williams et al. 2004, ApJ 609, 735) was detected with 62+-9 counts (corrected for exposure and vignetting) on 2008-10-21.63 UT (ObsID 31255010, 6.0 ks). This corresponds to an unabsorbed luminosity of (4.5+-0.7)e37 erg/s. The transient was not detected during observations 31255006 (starting on 2008-09-25.51 UT) and 31255009 (starting on 2008-10-15.15 UT) with 3 sigma upper limits of 4.3e-3 cts/s (1.6e37 erg/s) and 8.4e-3 cts/s (3.13e37 erg/s), respectively. The source was also not detected during an XMM-Newton observation starting on 2008-07-18.26 UT (see ATel #1647) with an EPIC pn 3 sigma upper limit for the unabsorbed luminosity of 8.4e35 erg/s (0.5-5 keV). The spectral parameters and source distance assumed were the same as for XMMM31 J004224.5+412401 above.

3.) The transient XMMU J004245.9+411036 that was discovered in July 2008 (ATel #1647) was still active on 2008-10-21.63 UT (ObsID 31255010) with a count rate of (2.2+-0.2)e-2 cts/s which corresponds to an unabsorbed luminosity of (6.1+-0.7)e37 erg/s (0.5-5 keV), comparable to the luminosity reported in ATel #1674, assuming the same spectral parameters (absorbed power law with hydrogen column density of 1.9e21 cm^-2 and photon index of 2.4).

4.) The transient SWIFT J004320.5+411528 was discovered in August 2008 (ATel #1674). It shows a declining luminosity during the last four weeks as it is shown in the table below:

ObsID    date [UT]      count rate [cts/s]
31255001 2008-08-21.31 8.93e-02 ± 4.4e-03
31255002 2008-08-28.68 1.32e-01 ± 8.5e-03
31255003 2008-09-04.23 1.16e-01 ± 8.0e-03
31255004 2008-09-06.59 9.78e-02 ± 5.3e-03
31255005 2008-09-11.48 1.10e-01 ± 5.2e-03
31255006 2008-09-25.51 7.39e-02 ± 4.8e-03
31255008 2008-10-14.74 6.31e-02 ± 6.4e-03
31255009 2008-10-15.15 5.50e-02 ± 5.4e-03
31255010 2008-10-21.63 5.17e-02 ± 3.5e-03

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.