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We describe the highly variable X-ray and UV emission of V458 Vul (Nova Vul2007), observed by Swift between 1 and 422 days after outburst. Initiallybright only in the UV, V458 Vul became a variable hard X-ray source due tooptically thin thermal emission at kT=0.64 keV with an X-ray band unabsorbedluminosity of 2.3x10^{34} erg s^{-1} during days 71-140. The X-ray spectrum atthis time requires a low Fe abundance (0.2^{+0.3}_{-0.1} solar), consistentwith a Suzaku measurement around the same time. On day 315 we find a new X-rayspectral component which can be described by a blackbody with temperature ofkT=23^{+9}_{-5} eV, while the previous hard X-ray component has declined by afactor of 3.8. The spectrum of this soft X-ray component resembles thosetypically seen in the class of supersoft sources (SSS) which suggests that thenova ejecta were starting to clear and/or that the WD photosphere is shrinkingto the point at which its thermal emission reaches into the X-ray band. We finda high degree of variability in the soft component with a flare rising by anorder of magnitude in count rate in 0.2 days. In the following observations ondays 342.4-383.6, the soft component was not seen, only to emerge again on day397. The hard component continued to evolve, and we found an anticorrelationbetween the hard X-ray emission and the UV emission, yielding a Spearman rankprobability of 97%. After day 397, the hard component was still present, wasvariable, and continued to fade at an extremely slow rate but could not beanalysed owing to pile up contamination from the bright SSS component.

Swift X-ray and UV monitoring of the Classical Nova V458 Vul (Nova Vul 2007)