Swift Ultraviolet detection of three Novae and two new transients in M 31

ATel #3712; M. Henze, W. Pietsch, J. Greiner (Max-Planck-Institut fuer extraterrestrische Physik)
on 26 Oct 2011; 12:38 UT
Credential Certification: Martin Henze (mhenze@mpe.mpg.de)

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We report the detection of three novae and the discovery of two UV transients in recent observations of the M 31 central area with the Ultraviolet/Optical Telescope (UVOT) on board the Swift satellite. Three observations have been obtained so far as part of an on-going Swift monitoring of the M 31 center: ObsID 00035336038 (exposure time 4.0 ks, started at 2011-10-05.83 UT), 00035336039 (2.1 ks, 2011-10-16.48 UT) and 00035336040 (3.7 ks, 2011-10-21.21 UT). All observations used the UVOT uvw1 filter (181-321nm).

We detected UV emission from the M 31 novae M31N 2011-05a (= PNV J00424359+4116378; see ATel #3388), PNV J00425701+4117095 (discovered 2011-10-02.163; see ATels #3701,#3702) and PNV J00425571+4117524 (discovered 2011-10-19.715; see ATels #3693,#3699). Furthermore, we discovered two previously unknown UV sources, hereafter designated by TCP names according to the guidelines of the CBAT transient objects confirmation page (TOCP, www.cbat.eps.harvard.edu/unconf/tocp.html), at the following positions: TCP J00423987+4114355: RA = 00h42m39.87s, Dec = +41d14'35.5"; TCP J00423876+4110453: RA = 00h42m38.76s, Dec = +41d10'45.3" (J2000, 1 sigma accuracy of 0.3"). Below, we give the UVOT uvw1 light curves of the objects.

 
Date [UT]                         Novae                                                  UV transients 
(2011-10-)  M31N 2011-05a  PNV J00425701+4117095  PNV J00425571+4117524   TCP J00423987+4114355  TCP J00423876+4110453 
 05.83       18.7 +/- 0.3       20.5 +/- 0.5                                   18.3 +/- 0.1           19.6 +/- 0.2 
 16.48       18.7 +/- 0.4       18.5 +/- 0.1                 > 19.9            18.2 +/- 0.1           19.4 +/- 0.2 
 21.21       18.7 +/- 0.3       18.2 +/- 0.1           16.8 +/- 0.1            19.0 +/- 0.2           20.3 +/- 0.2 

The evolution of the UV brightness is clearly different for the three novae. While M31N 2011-05a appears to be a slowly evolving nova still detected 5 months after optical discovery without variability over 15 days, PNV J00425701+4117095 brightens considerably within those 15 days. PNV J00425571+4117524 shows even stronger variability in UV and fades by about 0.5 mag within the five hours of observation 00035336040. Such a behavior is in agreement with optical observations that report this nova as having been remarkably bright at maximum (15.0 mag, see CBAT TOCP).

Despite a quasi-continuous optical monitoring of the M 31 central area by various observers there are no optical counterparts known for the two new UV sources as of today (see e.g. CBAT TOCP). However, many UV transients in M 31 have been identified with optical novae in M 31. For some of them the UV emission has even been detected preceding the optical detection (see e.g. ATels #2713 and #2727). Specifically, source TCP J00423987+4114355 may well have been missed in the optical due to the position close to the M 31 center. On the other hand, the time of outburst for both sources could have happened before the detection in UV as there was no UV coverage of the area since June 15th this year that would constrain the time of outburst. Therefore, optical observers are encouraged to check their archives for possible counterparts of these two sources.

All magnitudes are on the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627). Magnitudes 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.