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Swift detection of X-ray transients in M 31

ATEL # 1671; W. Pietsch (Max-Planck-Institut fuer extraterrestrische Physik, MPE), M. Freyberg (MPE), M. Henze (MPE), H. Stiele (MPE), S. Immler (NASA/CRESST/GSFC)
on 21 Aug 2008; 14:05 UT
Password Certification: Wolfgang Pietsch (wnp@mpe.mpg.de)

Subjects: X-ray, Nova, Transients

The Ultraviolet/Optical Telescope (UVOT) and X-Ray Telescope (XRT) on board the Swift satellite mapped M 31 during a series of pre-planned observation with XRT integration times of up to 5 ks from 2008 May 25 to July 26 (PI Immler). We searched in these observations for X-ray transients.

SWIFT J004420.1+413702 is detected as a new transient at RA(J2000) 00:44:20.12, Dec(J2000) +41:37:02.1 (1 sigma position error 4.5") in ObsID 37721001 starting on 2008 May 28.11 UT. In seven observation intervals spread over 17 hours (total exposure 4.38 ks) 50+-7 counts are detected in the 0.5-5 keV band (90% PSF). This corresponds to a unabsorbed flux of (4.6+-0.7)e-13 erg cm-2 s-1 assuming an absorbed power law model with Galactic foreground absorption (6.6e20 cm-2 and a photon index of 1.7). If the source is located in M 31 (distance 780 kpc), the unabsorbed luminosity (0.5-5 keV) is 3.3e37 erg s-1. No source was found at this position in the XMM-Newton survey (Pietsch et al. 2005, A&A 434, 483) and in the second ROSAT PSPC survey of M 31 (Supper et al. 2001, A&A 373, 63). However, the first ROSAT PSPC survey (Supper et al. 1997, A&A 317, 328), WGACAT (White et al. 2000, http://wgacat.gsfc.nasa.gov/wgacat/wgacat.html) and the ROSAT HRI 1RXH catalogue (http://www.xray.mpe.mpg.de/rosat/rra/roshri/) contain sources consistent in position that clearly indicate variability. In an outburst in January 1992 the source reached similar brightness as during the Swift observation. Optical and IR images show a red star-like object within 3.5" of the X-ray position with 14.6 and 12.8 mag in R and K in the UCAC2 and NOMAD Catalogues, respectively (Zacharias et al. 2003, 2005). This object seems to be in the Galactic foreground and may not be related to the X-ray source.

The two new M 31 transients from ATels #1647 and #1658 were detected already 5 days before the first Chandra detection at comparable brightness. Note that the XMM-Newton luminosities for these transients are interchanged by mistake in ATel #1658.

XMMU J004245.9+411036 is detected in ObsID 37718001 starting on 2008 May 26.30 UT as a hard source with 28.0+-6.0 counts (0.5-5 keV, 90% PSF) in the effective exposure time of 3.8 ks. This corresponds to an unabsorbed luminosity (0.5-5 keV) of 2.7e37 erg s-1 assuming an absorbed powerlaw 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 detected in ObsID 37719001 starting on 2008 May 26.70 UT as a hard source with 37.6+-6.4 counts (0.5-5 keV, 50% PSF) in the effective exposure time of 3.7 ks. This corresponds to an unabsorbed luminosity (0.5-5 keV) of 5.4e37 erg s-1 assuming an absorbed powerlaw with hydrogen column density of 6.6e20 cm-2 and photon index of 1.7.

The supersoft transient in the globular cluster Bol 111 (identified with the first optical nova detected in a M 31 globular cluster, M31N 2007-06b, see ATels #1118, #1294, #1333, #1344) is detected in ObsID 37718001 as a supersoft source with 20 counts, all below 700 eV. Comparing count rates, it is a factor of 1.5 to 2 fainter than during the Swift observation in Nov 2007 (ATel #1294). This detection was about 2 months before the XMM-Newton observation (see ATel #1647).


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R. E. Rutledge , Editor
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