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Nova M31N 2012-05c found as a supersoft X-ray source with XMM-Newton

ATel #4511; M. Henze, W. Pietsch, F. Haberl (Max Planck Institute for extraterrestrial Physics), M. Middleton (University of Amsterdam)
on 23 Oct 2012; 09:11 UT
Credential Certification: Martin Henze (mhenze@mpe.mpg.de)

Subjects: X-ray, Nova

We report the serendipitous discovery of a supersoft X-ray source (SSS) counterpart to the recent optical nova M31N 2012-05c in M 31. The optical outburst of the object (alternative name: PNV J00423149+4126138) was discovered by K. Hornoch (ATel#4096) and it was subsequently confirmed as a nova using Hα observations (Hornoch, ATel#4181). We detected the X-ray counterpart with (7.6±0.2) × 10-2 ct s-1 in a 25 ks XMM-Newton target of opportunity (ToO) observation of a region north of the M 31 centre on 2012-08-08.96 UT.

Analysis of the XMM-Newton EPIC pn and MOS spectra showed the object to be a SSS with an effective (blackbody) best-fit temperature of (35±2) eV and NH = (4.6±0.5) × 1021 cm-2 (1σ confidence ranges). However, the high absorption in this model (Galactic foreground at NH ~ 0.7 × 1021 cm-2) might be caused by inadequacies of the blackbody parametrisation, because a spectral fit using an NLTE atmosphere model with solar metallicities (from Rauch 2003, A&A, 403, 709) produced considerably different best-fit parameters: kT = (61±1) eV and NH = (2.8±0.4) × 1021 cm-2. These parameters still suggest additional intrinsic absorption, because the (projected) position of the nova lies in a relatively low-extinction area between the spiral arms of M 31 (compare Montalto et al. 2009, A&A, 507, 283).

In an earlier 13 ks XMM-Newton ToO observation of the same field on 2012-07-28.64 UT the SSS was not detected with a 3σ upper limit of 1.7 × 10-3 ct s-1. This allows us to put tight constraints on the SSS turn-on time of the nova: 90±6 days, with respect to the discovery date reported in ATel #4096. Based on the correlation between SSS turn-on and turn-off times found for a sample of M 31 novae by Henze et al. (2011, A&A, 533, A52) we predict, on the 95% confidence level, the SSS counterpart of M31N 2012-05c to disappear within 269+147-95 days after outburst (between 2012-10-26 and 2013-06-25). There is an additional uncertainty on this estimate, because the nova was discovered after the visibility window for M 31 re-opened. Therefore, the last non-detection was in Dec 2011 (K. Hornoch, private communication) and the date of the optical outburst is not well constrained. However, the fast SSS turn on of the nova indicates that also its optical light curve may have decayed rapidly (see Henze et al. 2011). As the object was discovered at a relatively bright R ~ 17.4 mag, we assume that only a few days could have elapsed since its maximum light. Such a small offset would not change our prediction dramatically.

We would like to thank the XMM-Newton Team for making the ToO observations possible.