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XMM-Newton detection of X-ray transients in M 31

ATEL # 1647; W. Pietsch (Max-Planck-Institut fuer extraterrestrische Physik, MPE), M. Freyberg (MPE), F. Haberl (MPE), M. Henze (MPE), H. Stiele (MPE)
on 6 Aug 2008; 12:49 UT
Password Certification: Wolfgang Pietsch (

Subjects: X-ray, Nova, Transients

We report on a 22 ks target of opportunity (TOO) XMM-Newton observation (ObsID 0560180101, starting on 2008-07-18.26 UT) of the field of the optical nova M31N 2007-12b (see ATels #1324, #1329, #1332) which was identified with a transient bright supersoft X-ray source (SSS) in January 2008 by Swift (see ATel #1360). While this nova is no longer detected with the XMM-Newton EPIC cameras, the supersoft transient in the globular cluster Bol 111 was still active. This source was reported in November 2007 and identified with the first optical nova detected in a M 31 globular cluster (M31N 2007-06b, see ATels #1118, #1294, #1333, #1344). The blackbody temperature and absorbing column of a fit to the source spectrum (0.2-1 keV) can be constrained to 47 eV < kT < 50 eV , 1.6e21 cm-2 < NH < 1.8e21 cm-2 (90% confidence) limiting the luminosity to below 3.5e38 erg/s (using the Eddington luminosity of a white dwarf at the Chandrasekhar mass, and assuming a He-rich atmosphere for a conservative upper luminosity limit, assuming a M 31 distance of 780 kpc). The temperature range is consistent with values determined in November 2007 (ATel #1294).

In addition we found two new and two recurrent transient X-ray sources in the field:

XMMU J004245.9+411036 is detected as a new transient at RA(J2000) 00:42:45.97, Dec(J2000) +41:10:36.6 (1 sigma position error 0.6" mainly determined by the systematic error). The 0.5-5.0 keV EPIC pn spectrum can be fitted by a power law model with best fit parameters of NH = 1.9e21 cm-2 and a photon index of 2.4. The unabsorbed luminosity (0.5-5 keV) is 4.2e37 erg s-1.

XMMU J004241.2+411821 is detected as a new transient at RA(J2000) 00:42:41.27, Dec(J2000) +41:18:21.5 (1 sigma position error 1.0"). We determine a EPIC pn count rate of 0.024 ct s-1, which would correspond to an unabsorbed luminosity (0.5-5 keV) of 6.2e36 erg s-1 (assuming an absorbed power law model with photon index 1.7 and galactic foreground absorption of NH = 6.6e20 cm-2). The luminosity presents a lower limit as the source spectrum shows a strong soft component.

We detect the recurrent supersoft transient [PFH2005] 430 = r3-8 (see e.g. Pietsch et al. 2005, A&A 434, 483; Williams et al. 2006, ApJ 643, 356) in outburst. We determine a lower limit for the EPIC pn count rate of 0.019 ct s-1. The source only shows photons below 750 eV in EPIC PN and MOS2, confirming its SSS nature. The position of the source close to CCD gaps in both cameras and at the position of the missing CCD in MOS1 prevents a more detailed analysis.

We also detect the hard recurrent transient [PFH2005] 329 = r2-16 (see e.g. Pietsch et al. 2005; Williams et al. 2006) in outburst. We determine an EPIC pn count rate of 0.081 ct s-1, which would correspond to an unabsorbed luminosity (0.5-5 keV) of 2.1e37 erg s-1 (assuming an absorbed power law model as above).

We checked the XMM-Newton optical monitor UVW1, UVM2 images taken during the TOO observation. No counterparts are evident for any of these transients. While the two new transients and [PFH2005] 329 most likely are low mass X-ray binaries, [PFH2005] 430 seems to be a classical transient SSS as identified as a class in ROSAT observations of the Magellanic Clouds (Trümper et al. 1991, Nature 349, 579).

We would like to thank the XMM-Newton Team for making the TOO observation possible.

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