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Swift UVOT detection of PNV J00422413+4111549: a very fast nova in M 31 or a flaring foreground star?

ATel #3446; W. Pietsch, M. Henze (MPE, Germany), K. Hornoch (Ondrejov Observatory, Czech Republic)
on 22 Jun 2011; 12:52 UT
Credential Certification: Wolfgang Pietsch (wnp@mpe.mpg.de)

Subjects: Optical, Ultra-Violet, X-ray, Nova, Star

Referred to by ATel #: 5091

The optical light curve of the nova candidate PNV J00422413+4111549 (candidate three in ATel #3409) can be constrained using two deep M 31 observations obtained with the 0.65-m telescope at Ondrejov (K. Hornoch) where the source is not detected 1.4 days before and 5.6 days after the discovery (see Table).

PNV J00422413+4111549 was in the Swift UVOT field of view during some exposures of three UVW1 filter (181-321 nm) ToO observations of M 31 in June 2011. During Swift observation 00032009001 a bright variable source was detected at position RA=00h42m24.14s, Dec=41d11'54.8" (J2000, position error 0.2") coincident with PNV J00422413+4111549. Between the first and second exposure the UV brightness increased by 0.5 mag and then dropped exponentially in the following exposures by 1.8 mag within 1/3 of a day. The optical detection was at the end of this outburst UV light curve. In observations 00032010001 and 00032011001 the source had faded beyond the detection threshold (see Table).

In the corresponding XRT observations no source was detected at the position of the nova candidate with a 3 sigma upper limit of 8.5x10-3 ct/s.

Telescope/      ObsID/Exposure  Time(2011-06)   Filter    Mag 
Instrument                               [UT] 
Ondrejov 65cm                      	6.049	    R          >20.5 
Swift UVOT      00032009001/1           7.040    UVW1   16.37+- 0.07 
Swift UVOT      00032009001/2           7.107    UVW1   15.72+- 0.06 
Swift UVOT      00032009001/3           7.174    UVW1   16.11+- 0.07 
Swift UVOT      00032009001/5           7.307    UVW1   17.00+- 0.09 
SLOTIS 60cm                       	7.418	    R	 18.3+- 0.2 
Swift UVOT      00032009001/7           7.441    UVW1   17.53+- 0.12 
Swift UVOT      00032010001            11.324    UVW1          >19.4 
Ondrejov 65cm                          13.053	    R          >20.8 
Swift UVOT      00032011001            15.896    UVW1          >20.6 
All UVW1 magnitudes are on the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627). Magnitudes have not been corrected for extinction.

The UVW1 luminosity of the transient during maximum was about a magnitude brighter than the brightest Swift UVW1 detection reported for novae in M 31 so far (see e.g. ATels #2787, #2274, and #1673).The optical data seem consistent with the outburst of a (very) fast nova in M 31. However, one would have expected from theoretical models (e.g. Hachisu & Kato 2010, ApJ, 709, 680) that the UV brightness might not decay that fast. An alternative explanation for the event may be a large and long outburst on a Galactic flare star. From the lack of an optical counterpart brighter than R=21 mag in the Local Galaxy Group Survey M31 catalog (Massey et al. 2006, AJ, 131, 2478) such a red star would have to be very faint. Detection of a red counterpart in deep multi-color photometry of the field might help to strengthen a flare star interpretation.

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 planers.