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Volume 442, Number 3, November II 2005
Page(s) 879 - 894
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20053127

A&A 442, 879-894 (2005)
DOI: 10.1051/0004-6361:20053127

Optical novae: the major class of supersoft X-ray sources in M 31

W. Pietsch1, J. Fliri2, M. J. Freyberg1, J. Greiner1, F. Haberl1, A. Riffeser1, 2 and G. Sala1, 3

1  Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85741 Garching, Germany
    e-mail: wnp@mpe.mpg.de
2  Universitätssternwarte München, Scheinerstraße, 81679 München, Germany
3  Institut d'Estudis Espacials de Catalunya (ICE-CSIC), Campus UAB, Facultat de Ciencies, 08193 Bellaterra, Spain

(Received 24 March 2005 / Accepted 19 June 2005)

We searched for X-ray counterparts of optical novae detected in M 31 and M 33 . We combined an optical nova catalogue from the WeCAPP survey with optical novae reported in the literature and correlated them with the most recent X-ray catalogues from ROSAT, XMM-Newton, and Chandra, and - in addition - searched for nova correlations in archival data. We report 21 X-ray counterparts for novae in M 31 - mostly identified as supersoft sources (SSS) by their hardness ratios - and two in M 33 . Our sample more than triples the number of known optical novae with a supersoft X-ray phase. Most of the counterparts are covered in several observations allowing us to constrain their X-ray light curves. Selected brighter sources were classified by their XMM-Newton EPIC spectra. We use the well-determined start time of the SSS state in two novae to estimate the hydrogen mass ejected in the outburst to ~ $10^{-5}~M_{\odot}$ and ~ $10^{-6}~M_{\odot}$, respectively. The supersoft X-ray phase of at least 15% of the novae starts within a year. At least one of the novae shows a SSS state lasting 6.1 years after the optical outburst. Six of the SSSs turned on between 3 and 9 years after the optical discovery of the outburst and may be interpreted as recurrent novae. If confirmed, the detection of a delayed SSS phase turn-on may be used as a new method to classify novae as recurrent. At the moment, the new method yields a ratio of recurrent novae to classical novae of 0.3, which is in agreement (within the errors) with previous works.

Key words: galaxies: individual: M 31 -- galaxies: individual: M 33 -- stars: novae, cataclysmic variables -- X-rays: galaxies -- X-rays: binaries

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© ESO 2005

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