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New Swift UVOT transient SWIFT J004255.6+411412 in M 31

ATel #5091; F. Hofmann, R. Sturm, J. Greiner, G. Vasilopoulos, P. Maggi (Max-Planck-Institut fuer extraterrestrische Physik, MPE)
on 28 May 2013; 16:36 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Richard Sturm (rsturm@mpe.mpg.de)

Subjects: Ultra-Violet, Nova, Star, Transient

Referred to by ATel #: 5092, 5099, 5145, 5271, 5910

In a monitoring observation of the central region of the Andromeda Galaxy (M 31) with the Ultra-violet/Optical Telescope (UVOT) on board the Swift satellite (ObsID 00035336102, starting 2013-05-27.15 UT), we detected a new UV transient (UVW1 filter, 181-321 nm). The source is located at RA 00h 42m 55.62s, Dec +41d 14' 12.3" (± 0.5", J2000, 90% confidence level). The following table lists the Swift ObsID, the MJD at the beginning of the exposure, UVW1 magnitudes (Vega system), and 1σ statistical uncertainties. Multiple exposures of the same ObsID are numbered by []. A 3σ upper limit ('>') is given for the last M 31 Swift monitoring observation before the detection.

 ID              MJD        Exposure[s]   UVW1 magnitude    
 00035336101     56359.01   3906         >20.44 
 00035336102[1]  56439.15    851          15.16±0.06 
 00035336102[2]  56439.87   1511          -- attitude problem 
 00035336102[3]  56439.94   1562          16.96±0.07 
The simultaneous X-ray observation with the Swift XRT showed no X-ray emission at the position of the transient (3σ upper limit of 6.6x10^-3 counts/s).
The source is a M 31 nova candidate. Its position is consistent with two previous M 31 novae (M31N 1960-12a and M31N 1962-11b, see M 31 (apparent) optical nova catalogue and references therein), both being candidate recurrent novae. However, the UV luminosity is high and the decay is fast (by 1.8 mag within 19 hours) compared to other novae in M 31 (see ATel #3446). Alternatively, the transient might be a relatively long flare of a Galactic star, in which case the source LGGS J004255.66+411412.8 (Massey et al. 2006, AJ, 131, 2478; V = 22.12 mag, B-V = 0.87 mag) is the likely counterpart.
In order to investigate the nature of the newly detected UV transient, spectroscopic observations in the optical are encouraged.
All magnitudes are on the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) and have not been corrected for extinction. 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.