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 V458Vul (Nova Vul 2007), observed by Swift between 1 and 422 days after outburst. Initially bright only in the UV, V458Vul 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 2.3×10 erg s during days 71-140. The X-ray spectrum at this time requires a low Fe abundance (0.2 −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 −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. Subject headings: novae, cataclysmic variables stars: individual (V458 Vul) 1 European Space Astronomy Centre, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain: [email protected] 2 School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA 3 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 4 Department of Physics & Astronomy, University of Leicester, Leicester, LE1 7RH, UK 5 BAAVSS, 5 Silver Lane, West Challow, Wantage, OX12 9TX, UK 6 Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK 7 AAVSO, 5 Melba Drive, Hudson, NH 03051, USA 8 Department of Physics, University of Warwick, Coventry CV4 7AL, UK 9 Department of Physics, Rikkyo University, 3-34-1 NishiIkebukuro, Toshima, Tokyo 171-8501, Japan 10 School of Physics & Astronomy, University of Southampton, Southampton SO17 1BJ, UK 11 BAAVSS, Furzehill House, Ilston, Swansea, SA2 7LE, UK 12 Isaac Newton Group, PO Ap. de Correos 321, 38700 Sta. Cruz de la Palma, Spain 13 Instituto de Astrof́ısica de Canarias, Vı́a Láctea s/n, La Laguna, E-38205 Santa Cruz de Tenerife, Spain 14 Department of Geology and Astronomy, West Chester University, West Chester, PA 19383, USA 15 CBA Flanders, Alan Guth Observatory, Koningshofbaan 51, Hofstade, Aalst, Belgium 16 Department of Astronomy & Astrophysics, Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802, USA 17 Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Sagamihara, Kanagawa 229-0015, Japan 18 Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK 19 Jodrell Bank Center for Astrophysics, Alan Turing Building, School of Physics and Astronomy, The University of Manchester, Oxford Street, Manchester, M13 9PL, UK