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Detection of supersoft X-ray emission from the He/N nova M31N 2007-06b in the globular cluster in M 31

ATel #1294; W. Pietsch (Max-Planck-Institut fuer extraterrestrische Physik, MPE), V. Burwitz (MPE), J. Greiner (MPE), F. Haberl (MPE), M. Henze (MPE), G. Sala (MPE)
on 21 Nov 2007; 14:37 UT
Credential Certification: Wolfgang Pietsch (wnp@mpe.mpg.de)

Subjects: X-ray, Nova

Referred to by ATel #: 1296, 1306, 1333, 1335, 1647, 1671

On behalf of the XMM-Newton/Chandra M31 nova monitoring collaboration (see http://www.mpe.mpg.de/~m31novae/xray/ao6/index.php ) we searched in a 20ks Chandra HRC-I observation of M31 starting on 2007-11-07.64 UT for X-ray emission from optical novae. We detected a new source consistent with the position of the M 31 globular cluster Bol 111 at the edge of the HRC-I field of view. In the following 20ks observation of the HRC-I monitoring campaign starting on 2007-11-17.76 UT (with different roll angle the source was again detected with 329+-56 cts at an off-axis angle of 15.9 arcmin in the Chandra HRC-I at coordinates RA(J2000) 00:42:33.29, Dec(J2000) +41:00:26.4, with centroid position errors of 0.5" and 0.4", respectively (subject to the standard bore-sight correction and systematic shifts at the very far off-axis angle) confirming the identification with Bol 111. No X-ray source was known before in this cluster.

Quimby et al. (ATel #1118) reported the first optical nova in a globular cluster in M 31 (from Bol 111) that was discovered on 2007-06-19.4 UT. It was spectroscopically identified as a He/N nova (see Shafter & Quimby 2007arXiv0711.0378S). He/N novae tend to be faster than Fe II novae and often show Ne lines, which suggests that they occur on relatively massive ONe white dwarfs. We proposed Swift XRT observations to determine the spectrum of the new X-ray source in Bol 111 which was first detected 140 days after the nova outburst. A 7.1ks Swift observation started on 2007-11-18.40 UT. A soft X-ray source with about 60 cts is detected at the position of the HRC-I source with all photons below 580 eV. The temperature of a blackbody fit to the source spectrum can be constrained to 45 eV < kT < 65 eV (90% confidence) limiting the luminosity to below 3.5^38 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). A source with such a spectrum is consistent with the source counts detected in the Chandra HRC.

Such supersoft source spectra are typical for counterparts of optical novae and indicate that hydrogen burning is going on in the remaining white dwarf envelope. We therefore identify the supersoft transient in Bol 111 with the nova reported in ATel #1118.

We would like to thank the Swift team for the scheduling of the observations.