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Swift Ultraviolet Detections of five Novae in M31

ATel #2274; M. Henze, W. Pietsch, F. Haberl (Max-Planck-Institut fuer extraterrestrische Physik), M. Orio (INAF Padova and UW Madison)
on 30 Oct 2009; 09:33 UT
Credential Certification: Frank Haberl (fwh@mpe.mpg.de)

Subjects: Ultra-Violet, Nova

Referred to by ATel #: 2713, 3446, 9408

We report the detection of five novae in M31 using two observations with the Ultraviolet/Optical Telescope (UVOT) on board the Swift satellite. The observations were obtained on 2009-10-22.61 UT (MJD 55126.61) and on 2009-10-25.77 UT (MJD 55129.77), respectively, and are part of a Swift ToO UVOT monitoring of the extremely bright nova M31N 2009-10b. Below we give UVOT magnitudes and 3-sigma upper limits for M31N 2009-10b and four other novae that are within the UVOT field in the uvw1 (181-321nm, 2058 and 610 s exposure time), uvm2 (166-268nm, 1997 and 269 s), and uvw2 (112-264 nm, 2247 and 716 s) filter observations:

Nova          uvw1      uvm2      uvw2          MJD

M31N2009-02b 18.7±0.1 19.3±0.1 18.9±0.1 55126.61 18.7±0.2 19.3±0.3 19.1±0.2 55129.77
M31N2009-08a 18.5±0.2 18.7±0.2 19.1±0.3 55126.61
M31N2009-08e 18.7±0.1 18.4±0.1 18.4±0.1 55126.61 18.3±0.2 18.1±0.2 18.0±0.1 55129.77
M31N2009-10b 18.1±0.1 18.7±0.1 18.6±0.1 55126.61 18.6±0.2 19.0±0.3 19.0±0.2 55129.77
M31N2009-10c 16.9±0.1 17.1±0.1 17.4±0.1 55126.61 16.9±0.1 17.1±0.2 17.5±0.1 55129.77

Nova M31N 2009-10b does not shine out as an extremely bright object in the UVOT observations and it is fading rapidly, by about 0.4 magnitudes in only three days. This trend is confirmed for the uvw2 filter in a third 1.6 ks Swift UVOT observation on 2009-10-28.29 UT (MJD 5132.29), where M31N 2009-10b is detected at 19.5±0.1 mag. The uvw2 magnitudes of the other four novae did not change significantly in this observation. Nova M31N 2009-08a is visible right on the edge of the UVOT field of view in the second observation, therefore no reliable magnitude can be given. The novae M31N 2009-08e, M31N 2009-10b, and M31N 2009-10c are not detected in a 9.8 ks Swift UVOT uvw2 observation on 2009-06-07.95, down to a 3-sigma limiting magnitude of around 20.3 mag. Novae M31N 2009-08a and M31N 2009-02b are present in this observation. While this is expected for Nova M31N 2009-02b, which was discovered in February 2009, it is surprising that nova M31N 2009-08a is visible already two months before its optical discovery on 2009-08-04.03 UT. Therefore, it is possible that the actual optical outburst of M31N 2009-08a was missed and a re-brightening of the nova had been misidentified as the outburst. Both novae are not detected in a 1.8 ks Swift UVOT uvw1 observation on 2008-10-21.85 with a 3-sigma limiting magnitude of around 19.7 mag.

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.