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Swift Ultraviolet Light Curves of four Novae in M31

ATel #2787; M. Henze, W. Pietsch, F. Haberl (Max-Planck-Institut fuer extraterrestrische Physik)
on 12 Aug 2010; 12:57 UT
Credential Certification: Martin Henze (mhenze@mpe.mpg.de)

Subjects: Ultra-Violet, Nova

Referred to by ATel #: 3446

We present the ultraviolet light curves of four novae in the central region of M31 using nine observations with the Ultraviolet/Optical Telescope (UVOT) on board the Swift satellite during July and August 2010. All observations were carried out using the UVOT uvw1 filter (181-321nm). Below we give the observation identifications (ObsIDs) and the Modified Julian Dates (MJDs) of the observations as well as the UVOT uvw1 magnitudes (or 3-sigma upper limits) for the four novae M31N 2010-05a, M31N 2010-06a, M31N 2010-06d and M31N 2010-07a. The MJDs of the first and last observation corresponds to the following dates: 2010-07-07.07 UT and 2010-08-11.68 UT.

ObsID        MJD       2010-05a  2010-06a  2010-06d  2010-07a

00035336021 55384.07 17.2±0.1 18.5±0.1 19.8±0.2 20.6±0.3
00035336022 55386.95 17.4±0.1 17.8±0.1 19.6±0.2 > 21.0
00035336023 55391.03 17.5±0.1 17.8±0.1 19.0±0.1 19.9±0.2
00035336024 55395.05 17.5±0.1 17.7±0.1 18.9±0.1 19.6±0.2
00035336025 55399.00 17.5±0.1 17.7±0.1 19.1±0.1 20.4±0.3
00035336026 55403.69 17.7±0.1 17.8±0.1 19.2±0.2 18.3±0.1
00035336027 55407.69 17.5±0.1 17.8±0.1 19.1±0.1 17.4±0.1
00035336028 55415.19 17.6±0.1 17.8±0.1 18.4±0.1 18.8±0.1
00035336029 55419.68 17.7±0.1 18.0±0.1 18.5±0.1 19.2±0.1

The gradually declining light curve of nova M31N 2010-05a is an evidence for a slow UV evolution of this nova that was discovered on 2010-05-28.04 UT by K. Hornoch. We analyzed earlier archival observations with the Swift UVOT and found them to support this interpretation, as there the nova is detected at uvw1 magnitudes of 17.0±0.1 on 2010-06-12.04 (ObsID 00031255014, MJD = 55359.04) and 17.1±0.1 on 2010-06-24.02 (ObsID 00031255018, MJD = 55371.02). The nova M31N 2010-06a seems to have evolved to a stable source in the uvw1 filter after the first observation. Nova M31N 2010-06d was first announced as a UV source in ATel#2713 with a Swift UVOT uvw1 magnitude of 19.5±0.2 on 2010-06-24.02 UT (MJD = 55371.02). Since then the source seems to brighten gradually, with a recent increase in magnitude by 0.5 mag in observations 00035336028 and 00035336029. It also shows significant variability during the last observation 00035336029. The first detection of nova M31N 2010-07a was reported in ATel#2727 for the first observation listed in the table above. The uvw1 light curve of the source shows strong variability with a rise by 3 magnitudes in 9 days towards a pronounced maximum in observation 00035336027 and a following decline by almost 2 magnitudes within 12 days. M31N 2010-07a also exhibits variability between the different pointings of observation 00035336027. All novae are still visible as UV sources in the last observation 00035336029. According to Hachisu & Kato (2010, ApJ, 709, 680) novae are expected to be bright in UV before they become supersoft X-ray sources. Further UV observations of the central region of M31 are strongly needed to put constrains on the UV light curves and the overall photometric evolution of these four novae that can be monitored simultaneously in the same field.

All magnitudes are on the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) and have not been corrected for extinction. We wish to thank the Swift team for scheduling the ToO observations.