Swift and MERLIN observations of Swift J194302.1+321913 during the nova outburst of V2491 Cyg

ATel #1480; E. Kuulkers, A. Ibarra (ESA/ESAC, Spain), K. L. Page, A. Beardmore, P. Evans, J. P. Osborne (U. Leicester, UK), M. Bode (Liverpool John Moores U., UK), J. J. Drake (SAO/CfA, USA), K. Mukai (NASA/GSFC & UMBC, USA), J.-U. Ness (Arizona State U., USA), M. Orio (INAF-Padova, Italy & U. of Wisconsin, USA), R. Saxton (ESA/ESAC, Spain), S. Starrfield (Arizona State U., USA), S. P.S. Eyres (U. of Central Lancashire, UK), T. J. O'Brien, T. W.B. Muxlow (U. of Manchester, UK)
on 16 Apr 2008; 20:08 UT
Credential Certification: Erik Kuulkers (ekuulker@rssd.esa.int)

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We report on Target of Opportunity observations performed with Swift on 2008 April 11 and 15, and MERLIN on April 13 of the extremely fast nova V2491 Cyg (IAUC # 8934 , # 8935 , CBET #1334, ATel #1475).

During the first Swift/XRT observation (3.76 ksec; taken near the optical peak of the nova outburst, see IAUC # 8934 ), using a 10 pixel extraction region for the source at the position of Swift J194302.1+321913, we measure 6 counts compared to an estimated background level of 1.27 counts. However, due to the optical brightness of the source we expect about 1 count to be due to optical loading. Using the Bayesian confidence limits analysis method of Kraft et al. (1991, ApJ 374, 344), this leads to a 96% confidence detection, and is thus formally consistent with a non-detection. We derive a 3-sigma upper limit on the 0.3-8 keV source count rate of about 5.5 cts/ksec.

During the second Swift observation (3.03 ksec; taken when the nova had already declined in the optical by about 1.5 magnitudes with respect to the first observation, see IAUC # 8935 ) we clearly detect a source at the position of Swift J194302.1+321913, with a background and PSF/bad-column corrected count rate of 9+/-2 cts/ksec (0.3-8 keV). The spectrum appears to be hard (but comparable to some of the pre-nova observations, see ATel #1478), given that over the whole observation sequence 1.6 (+1.9/-1.4) and 24.5 (+5.6/-5.3) counts are detected in the 0.3-1 keV and 1-8 keV energy bands, respectively. Count rate errors quoted are the 68% confidence Baysian rate errors.

These results are in contrast to the level of X-ray emission seen before the current nova outburst as detected with the Swift/XRT (ATels #1473, #1478), where the flux was about a factor of 5 and 2.5 higher, with respect to the April 11 and 15 observations, respectively. The non-detection on April 11 may be due to strong absorption by the ejected nova shell, while the appearance of the hard X-ray emission on April 15 may be attributed to shocks within the ejecta. We, note, however, that the appearance is much faster than that expected in typical classical novae (weeks to months).

The MERLIN radio observations at 5 GHz show a non-detection down to a 3-sigma upper limit of 165 microJy.

We encourage continued monitoring of the evolution of this fast changing nova at all wavelengths.