  Another outburst of the recurrent M31 nova M31N 2008-12a was announced in
late November 2013. Optical data suggest an unprecedentedly short recurrence
time of approximately one year. In this Letter we address the X-ray properties
of M31N 2008-12a. 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 was also
analysed. 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 blackbody 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. 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 re-discovered 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.
