Swift X-ray and UV evolution of nova V2491 Cyg 7-34 days after eruption

ATel #1523; K. L. Page, J. P. Osborne, P. A. Evans (U. Leicester, UK), E. Kuulkers, A. Ibarra (ESA/ESAC, Spain), J. J. Drake (SAO/CfA, USA), J.-U. Ness (Arizona State U., USA), A. Beardmore (U. Leicester, UK), M. Bode (Liverpool John Moores U., UK), M. Orio (INAF-Padova, Italy & U. of Wisconsin, USA), G. Schwarz (West Chester U., USA)
on 14 May 2008; 14:13 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

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Swift has continued to monitor the nova V2491 Cyg, which erupted on 2008 April 10 (IAUC # 8934 , CBET #1334), after its two observations on April 11 and 15 (ATel #1480). We report here on Swift data collected every 1-3 days over the interval April 17 to May 14 (7-34 days after eruption).

The X-ray emission initially peaked at ~9 count ks^-1 on April 15, and then declined to ~3 count ks^-1 on April 23. The total spectrum over this interval was hard (see ATel #1480); the Baysian (1.5-10keV)/(0.3-1.5keV) hardness ratio was 7.3 +2.3/-1.4.

Since April 27, V2491 Cyg has continuously increased in brightness, reaching ~200 count ks^-1 on May 14. The X-ray spectrum has been softening substantially during this interval, with most of the rise in brightness occurring at energies below 1.5 keV. April 27 corresponds approximately to the start of the decline from the secondary peak in the AAVSO V band light curve.

A 7.9 ks observation was obtained on May 10. The spectrum extracted from these data can be fitted using 2 Mekal components (kT_1 = 1.15+0.23/-0.10 keV; kT_2 = 5.9+3.5/-1.6 keV), together with a blackbody (kT = 60+8/-6 eV). Other models may fit the data equally well. The required absorbing column is consistent with E(B-V)=0.3 (IAUC # 8935 ; see also ATel #1478). The Baysian hardness ratio for this spectrum was 0.57+/-0.04.

We have searched the longest XRT observations (May 8 and 10) for evidence of the optical period reported in ATel #1514. We find no significant modulation, with 90% confidence upper limits on the sine amplitude of <12% on May 10 for the total XRT band over both candidate periods. A spectral band-limited analysis also did not yield a modulation detection. The Swift UVOT filter wheel was in the blocked position during the first two post-outburst observations because of the ~8th magnitude brightness of the nova. From April 17 the UVW2 filter (1928+/-657 Angstrom) was used, although the data were saturated until May 2. On this date the UVW2 magnitude was ~11.1. In subsequent observations the object faded, reaching a UVW2 magnitude of ~12.0 on May 14. We do not detect modulation at the periods given in ATel #1514 due to the scatter in our photometric values, which have a standard deviation of 0.07 magnitudes. The rise and fall of a hard X-ray component, followed very shortly after by a rising soft X-ray component, resembles the behaviour seen by Swift of the recurrent nova RS Oph 2006 (eg ATel #764). The less well observed classical novae may show similar evolution, although typically on much longer timescales (eg Ness et al 2007, ApJ 663, 505). We thank the Swift PI, science team, and mission operations team for their support of these observations.