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Identification of TCP J00423987+4114355 and TCP J00423876+4110453 with known optical objects in M 31

ATel #3755; K. Hornoch (Astronomical Institute, Ondrejov, Czech Republic), S. Fabrika, T. Fatkhullin, O. Sholukhova (Special Astrophysical Observatory, Russia), M. Henze, W. Pietsch (Max-Planck-Institut fuer extraterrestrische Physik, Germany), J. Gorosabel (IAA-CSIC, Spain), and P. Kubanek (Institute of Physics ASCR, Czech republic)
on 10 Nov 2011; 15:48 UT
Credential Certification: Martin Henze (mhenze@mpe.mpg.de)

Subjects: Optical, Ultra-Violet, Nova, Transient

Referred to by ATel #: 4281

The discovery of two UV transients in observations of the M 31 central region with the Ultraviolet/Optical Telescope (UVOT) on board the Swift satellite was reported recently by M. Henze et al. (see ATel #3712).

An optical transient coincident with TCP J00423987+4114355 was independently detected and observed by K. Hornoch since Oct. 4.871 UT with the 0.65-m telescope at Ondrejov. The source brightened and reached peak magnitude R = 18.3 +/- 0.2 on Oct. 10 UT (measured on a co-added 3060-s R-band CCD frame taken on Oct. 9.989 UT with the 1.23-m telescope at Calar Alto). After the peak, the object continuously faded. A precise position and magnitudes were measured on CCD B-band (120-s, B = 20.8 +/- 0.15) and V-band (180-s, V = 20.0 +/- 0.15) images taken around Oct. 29.641 UT with the Russian 6-m BTA telescope equipped with the SCORPIO spectral camera. The object is located at R.A. = 0h42m39s.95, Decl. = +41o14'35".3 (equinox 2000.0, 1 sigma accuracy of 0.25") which is only 0.30" away from the position of LGGS J004239.97+411435.5 (Massey et al. 2007, AJ, 134, 2474) and 0.69" away from the position of LGGS J004239.90+411435.7. This strongly suggest a connection between the transient and one of the two LGGS objects. Visual comparison of the 6-m BTA frame with a high quality archival R-band Subaru telescope image from 2001 Nov. 22.321 UT (obtained from the SMOKA science archive) and with an LGGS R-band image suggests an identification of TCP J00423987+4114355 with LGGS J004239.90+411435.7 to be more probable.

In a similar way we can confirm the identification of TCP J00423876+4110453 with the H-alpha emission line star LGGS J004238.77+411044.6 proposed by A.W. Shafter et al. (see ATel #3744). We measured the magnitudes and the precise position of TCP J00423876+4110453 on our B-band (180-s, B = 21.0 +/- 0.1) and V-band (180-s, V = 20.5 +/- 0.1) images from the 6-m BTA telescope taken around Oct. 29.647 UT. The object is located at R.A. = 0h42m38s.80, Decl. = +41o10'44".0 (equinox 2000.0, 1 sigma accuracy of 0.25") which is 0.69" away from the position of LGGS J004238.77+411044.6. Visual comparison of the 6-m BTA co-added frame with the Subaru telescope image and the LGGS image confirms the connection between the two objects.

If we consider systematic shifts between the positions from the LGGS and the UCAC3 catalogues, the internal accuracy of the LGGS positions of objects as faint as 20-21 mag, and the uncertainty of our astrometry, an identification of both UV transients with known objects is very probable. This result was confirmed by direct comparison of the optical images with the Swift discovery data of the two UV sources. Additionally, the photometric properties of TCP J00423876+4110453 i.e. very low maximum brightness and low amplitude of brightness changes visible on tens of Ondrejov and Calar Alto images (although with the object mostly close to their limit) during the last four months are not consistent with the behaviour of most novae. Thus, it is very unlikely that the objects could be novae.