V458 Vul (Nova Vul 2007) becomes a highly-variable supersoft X-ray source

ATel #1721; J. J. Drake (Smithsonian Astrophysical Observatory), K. L. Page, J. P. Osborne, A. P. Beardmore (Leicester), J.-U. Ness, S. Starrfield (Arizona State), G. Schwarz (West Chester), M. Tsujimoto (Penn State), R. Wesson (University College London), M. Bode (Liverpool John Moores), P. Rodriguez-Gil (Instituto de Astrofisica de Canarias), B. Gaensicke, D. Steeghs (Warwick), C. Knigge (Southampton), D. Takei (Rikkyo), A. Zijlstra (Manchester)
on 17 Sep 2008; 16:53 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Jeremy J. Drake (jdrake@cfa.harvard.edu)

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Swift X-ray Telescope (XRT) monitoring observations of V458 Vul (Nova Vul 2007, S. Nakano, IAUC 8861 ) have found it to be entering a new phase characterised by a highly variable supersoft X-ray component accompanied by partially anti-correlated variations in the ultraviolet. An earlier report of entry into the supersoft phase (ATel #1246) has proven premature. XRT observations obtained from 2008 June 18 - September 1 found the nova to have declined in X-ray luminosity by a factor of 3 to an average of 0.02 count/s in the 0.3-10 keV band compared with the 2007 November-December period (ATel #1603). During an XRT observation on 2008 September 9, 397 days after the initial outburst, the X-ray flux was highly variable: an initial rate of 0.09+/-0.015 count/s declined to approximately 0.01 count/s and rose again to 0.1+/-0.012 count/s over a 12 hour interval. The Swift X-ray spectrum during the elevated state indicated the flux increase was due almost entirely to soft X-rays with energy < 0.5 keV. A campaign of monitoring at higher cadence was initiated and observations obtained during 2008 September 14-16 saw the source brighten from 0.05 to 0.5+/-0.05 count/s, again over a period of 12 hours. This was followed by a dramatic decline by a factor of about 50 on a timescale of only 3 hours. The source rose over the next 10 hours to 0.3+/-0.2 count/s and promptly faded back to approximately 0.01 count/s on a similar timescale. A day of steady behaviour ended in a dramatic rise to 0.65 count/s on 2008 September 16. The ultraviolet flux observed in the Swift UVW1 filter during this period varied by 1 mag about an average of 16 and was partially anti-correlated with the X-ray flux. Fits to the X-ray spectrum from the combined 2008 September 14-15 data using a simple absorbed blackbody spectrum indicated an effective temperature kT = 28.7 +3.5/-8.7 eV with absorption corresponding to a cosmic composition gas with neutral hydrogen column NH = (3.3 +2.0/-0.6)e21 cm^-2. The 0.3-10 keV unabsorbed flux is highly model-dependent and ranges from 10^-9 erg cm-2 s-1 for a blackbody spectrum (implying a super-Eddington luminosity for an assumed distance of 13kpc) to 10^-11 erg cm-2 s-1 for a white dwarf model atmosphere spectrum. The latter also suggests the absorbing column might be lower by a factor of about 4.