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Detection of nova M31N 2007-07c with Swift UVOT in the UVW2 filter

ATel #1149; W. Pietsch (Max Planck Institut f. extraterr. Physik Garching MPE, Germany), G. Sala (MPE), F. Haberl (MPE), J. Greiner (MPE)
on 20 Jul 2007; 14:36 UT
Credential Certification: Wolfgang Pietsch (wnp@mpe.mpg.de)

Subjects: Ultra-Violet, X-ray, Nova

Referred to by ATel #: 1152, 1153, 1157

An optical nova was detected in M31 (M31N 2007-7c, ATEL #1146) on 2007 July 19.00741 in the R band when its brightness was still rising. During the Swift XRT and UVOT observations of nova M31N 2006-11a performed from 2007 July 12 to 18 the new nova was also in the field of view. While during UVOT observations with the UVW2 filter on 2007 July 12 no source was visible at the position of the new nova (5 sigma upper limit of 19.6 mag or 1.08 x 10^-16 erg/s/cm^2/A), the nova was detected with a significance of more than 20 sigma in two Swift UVOT UVW2 images (1600 s and 1200 s exposure) centered on 2007 July 18.888 and 18.953. Its brightness was 17.68 +/- 0.05 mag (6.54 +/- 0.31 x 10^-16 erg/s/cm^2/A). There was no significant change in brightness between the two exposures on July 18. The nova was brighter in UVW2 by about a factor of 20 than nova M31N 2006-11a in June 2007 (ATEL #1116). The Swift UV detection of the nova is 2.8 h before the first reported detection of the nova in the optical (ATEL #1146).
No source is detected at the position of the nova in the corresponding XRT data.
The maximum in UV emission of classical novae is usually reached after the optical maximum, during the constant bolometric phase, when the ejecta becomes optically thin and the photosphere recedes to inner and hotter layers. This prompt UV emission detected in nova M31N 2007-07c before the optical maximum may correspond to the first phases of expansion of the ejecta. After that the "iron curtain stage" may follow, and the recombination of low ionized species (mainly Fe and other metals) will increase the opacity. This would move the maximum of emission into the optical band. Further monitoring in UV is urgently needed to follow the UV light curve during the constant bolometric phase where the UV brightness may rise again after a drop.
We would like to thank the Swift Team for making these observations possible, in particular N. Gehrels, the duty scientists as well as the science planners.