Nova V2491 Cyg has become a bright super-soft source

ATel #1542; J P Osborne, K Page, P Evans, A Beardmore (U. Leicester, UK), E Kuulkers, A Ibarra (ESA/ESAC, Spain), J-U Ness, S Starrfield (Arizona State U., USA), M Bode (Liverpool John Moores U., UK), J J Drake (SAO/CfA, USA), M Orio (INAF-Padova, Italy), J Krautter (Heidelberg U, Germany), G Schwarz (West Chester U., USA). M Tsujimoto (PSU, USA), D Takei (Rikkyo U, Japan)
on 20 May 2008; 23:24 UT
Credential Certification: Julian P Osborne (julo@star.le.ac.uk)

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Continued monitoring of nova V2491 Cyg by the Swift X-ray Telescope has revealed the rapid emergence of a bright soft X-ray component. In addition a Suzaku observation shows that strong low energy X-ray emission lines have been present.

Recent Swift observations from day 36 after outburst (May 16) show a very strongly increasing XRT count rate, reaching ~5 c/sec on day 38. This factor of ten flux increase in approximately two days is in stark contrast to the prior increase by this factor which took 18 days (see ATel #1523). The recent rapid flux rise has not been monotonic.

The newly-emerged X-ray flux is very soft (E<0.6 keV), and can be roughly fit with a blackbody with kT~0.033 keV (in addition to a much fainter hard spectral component). A blackbody may not be a physically meaningful model, although it serves as an empirical description; other very soft spectral models may also fit.

An observation with the Suzaku XIS on day 29 (May 9), before the rapid flux increase reported here, shows strong emission lines from K shell N, O and Ne lines.

The rapid, non-monotonic soft X-ray flux increase of V2491 Cyg again appears very similar to the behaviour seen by Swift from the recurrent nova RS Oph 2006 at the start of its super-soft phase. The early emergence of the super-soft phase may indicate that the white dwarf in V2491 Cyg is similarly massive.

Swift continues to observe (at an increased rate). We thank the Swift PI and operations team for their support.