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Swift/UVOT constrains outburst of Nova M31 2015-02a to well before optical detection

ATel #7104; Corentin Delvaux, Jochen Greiner, Wolfgang Pietsch (all MPE Garching)
on 18 Feb 2015; 20:24 UT
Credential Certification: J. Greiner (jcg@mpe.mpg.de)

Subjects: Ultra-Violet, Nova

Referred to by ATel #: 7158, 7818

Based on monitoring observations of the central region of the Andromeda Galaxy M31 with the Ultraviolet/Optical Telescope (UVOT) on board the Swift satellite from 2015-01-17 to 2015-02-18, we derived the UV light curve (UVW1 filter, 181-321 nm) for the recently discovered nova M31 2015-02a (ATel #7065, and #7076). The following table gives the observation ID, the exposure time, the date and start time of the observations, UVW1 magnitudes (Vega system) and 2-sigma statistical uncertainties for M31 2015-02a.

ObsID        Exp      Date           Time             Magnitude 
             [ks]                    (MJD)            (Vega) 
00031283028  4.0      2015-01-17     57039.46          <21.01 
00031283029  3.9      2015-01-25     57047.31        20.19±0.52 
00031283030  4.1      2015-02-02     57055.24        18.90±0.16 
00031283031  3.2      2015-02-10     57063.55        18.87±0.17 
00031283032  3.7      2015-02-14     57067.48        19.88±0.40 
00031283033  1.1      2015-02-18     57071.28          >19.50 

The systematic uncertainty of these magnitudes is estimated to be 0.03 mag.

M31 2015-02a has been detected by Swift/UVOT already 20 days before the first optical detection at Feb 13.75625 UT as reported in ATel #7065. The upper limit on 2015-01-17 constrains the outburst time to Jan 18-24. The nova seems to have reached its maximum in the UVW1 filter around 2015-02-10 and appears to have faded since then.

We encourage spectroscopic observations of M31 2015-02a.

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.