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Serendipitous GALEX Detection of CSS100217:102913+404220

ATel #2554; S. Gezari (JHU), K. Forster, J. D. Neill, D. C. Martin (Caltech)
on 13 Apr 2010; 21:04 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Suvi Gezari (suvi@pha.jhu.edu)

Subjects: Ultra-Violet, AGN, Black Hole, Supernovae, Transient

Referred to by ATel #: 2629

We report serendipitous GALEX observations of the extremely luminous optical transient CSS100217:102913+404220 discovered by CRTS (ATEL #2544). The object was detected on 24 April 2004 by the All Sky Imaging Survey with FUV = 19.52 and NUV = 18.978, and on 29 January 2010 by the Medium Imaging Survey with NUV = 17.078 (the FUV detector was temporarily not operational).

The brightening of the object by 1.9 mag in the NUV corresponds to a transient source with NUV = 17.285 close to the time of the optical peak. Converting the Swift UVOT UVW2 magnitude of the object on 6 April 2010 to the AB system and correcting for Galactic extinction yields NUV ~ 18.0, and corresponds to a transient source that faded by 1.3 mag two months later to NUV = 18.6.

The NUV-r color of the transient source during its peak (NUV-r ~ +1.2), disfavors the tidal disruption event interpretation, since candidate tidal disruption events are observed to have much bluer UV-optical colors (NUV-r < -1.4 ; Gezari et al. 2009, ApJ, 698, 1367).

The GALEX magnitudes are in the AB system, with an aperture correction and a correction for Galactic extinction of E(B-V)=0.014 mag. The GALEX Team has scheduled further NUV imaging and grism observations on 17 and 29 April 2010.