Swift X-ray and ultraviolet monitoring of the classical nova V458 VUL (Nova Vul 2007)

Ness, J.-U., Drake, J.J., Beardmore, A.P., Boyd, D., Bode, M.F., Brady, S., Evans, P.A., Gaensicke, B.T., Kitamoto, S., Knigge, C., Miller, I., Osborne, J.P., Page, K.L., Rodriguez-Gil, P., Schwarz, G., Staels, B., Steeghs, D., Takei, D., Tsujimoto, M., Wesson, R. and Zijlstra, A. (2009) Swift X-ray and ultraviolet monitoring of the classical nova V458 VUL (Nova Vul 2007). The Astronomical Journal, 137 (5), 4160. (doi:10.1088/0004-6256/137/5/4160).

Abstract

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 2.3 × 1034 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 white dwarf 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 analyzed owing to pile-up contamination from the bright SSS component.

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