Swift X-ray and UV monitoring of the Classical Nova V458 Vul (Nova Vul
  2007)
  We describe the highly variable X-ray and UV emission of V458 Vul (Nova Vul
2007), observed by Swift between 1 and 422 days after outburst. Initially
bright only in the UV, V458 Vul became a variable hard X-ray source due to
optically thin thermal emission at kT=0.64 keV with an X-ray band unabsorbed
luminosity of 2.3x10^{34} erg s^{-1} during days 71-140. The X-ray spectrum at
this time requires a low Fe abundance (0.2^{+0.3}_{-0.1} solar), consistent
with a Suzaku measurement around the same time. On day 315 we find a new X-ray
spectral component which can be described by a blackbody with temperature of
kT=23^{+9}_{-5} eV, while the previous hard X-ray component has declined by a
factor of 3.8. The spectrum of this soft X-ray component resembles those
typically seen in the class of supersoft sources (SSS) which suggests that the
nova ejecta were starting to clear and/or that the WD photosphere is shrinking
to the point at which its thermal emission reaches into the X-ray band. We find
a high degree of variability in the soft component with a flare rising by an
order of magnitude in count rate in 0.2 days. In the following observations on
days 342.4-383.6, the soft component was not seen, only to emerge again on day
397. The hard component continued to evolve, and we found an anticorrelation
between the hard X-ray emission and the UV emission, yielding a Spearman rank
probability of 97%. After day 397, the hard component was still present, was
variable, and continued to fade at an extremely slow rate but could not be
analysed owing to pile up contamination from the bright SSS component.
