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A remarkable recurrent nova in M 31: the X-ray observations

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  • M. Henze
  • J.-u. Ness
  • M. J. Darnley
  • M. F. Bode
  • S. C. Williams
  • A. W. Shafter
  • M. Kato
  • I. Hachisu
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Article numberL8
<mark>Journal publication date</mark>1/03/2014
<mark>Journal</mark>Astronomy and Astrophysics
Volume563
Number of pages5
Publication StatusPublished
<mark>Original language</mark>English

Abstract

Context. Another outburst of the recurrent M 31 nova M31N 2008-12a was announced in late November 2013. Optical data suggest an unprecedentedly short recurrence time of approximately one year.
Aims: In this Letter we address the X-ray properties of M31N 2008-12a.
Methods: We requested Swift monitoring observations shortly after the optical discovery. We estimated source count rates and extracted X-ray spectra from the resulting data. The corresponding ultraviolet (UV) data were also analysed.
Results: The nova M31N 2008-12a was clearly detected as a bright supersoft X-ray source (SSS) only six days after the well-constrained optical discovery. It displayed a short SSS phase of two weeks' duration and an exceptionally hot X-ray spectrum with an effective black-body temperature of ~97 eV. During the SSS phase the X-ray light curve displayed significant variability that might have been accompanied by spectral variations. The very early X-ray variability was found to be anti-correlated with simultaneous variations in the UV flux.
Conclusions: The X-ray properties of M31N 2008-12a coherently point towards a high-mass white dwarf in the nova system. This object might be a promising Type Ia supernova progenitor. We rediscovered additional X-ray detections of M31N 2008-12a that are consistent with our data and increase the number of known nova outbursts to seven. This nova is an exceptional object that merits further attention in the future.