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ASAS-SN Discovery of a Bright Optical Transient Near M31

ATel #5267; B. J. Shappee (Ohio State), J. L. Prieto (Princeton), K. Z. Stanek, C. S. Kochanek, T. W-S. Holoien, J. Jencson, U. Basu, J. F. Beacom (Ohio State), D. Szczygiel, G. Pojmanski (Warsaw University Observatory), M. Dubberley, M. Elphick, S. Foale, E. Hawkins, D. Mullens, W. Rosing, R. Ross, Z. Walker (Las Cumbres Observatory), J. Brimacombe (Coral Towers Observatory)
on 12 Aug 2013; 20:22 UT
Credential Certification: Benjamin Shappee (shappee@astronomy.ohio-state.edu)

Subjects: Optical, Cataclysmic Variable, Transient

Referred to by ATel #: 5273, 5274

During the ongoing All Sky Automated Survey for SuperNovae ( ASAS-SN or "Assassin"), using data from the double 14-cm "Brutus" telescope in Haleakala, Hawaii, we discovered a new transient source in our field containing M31:

 
Object       RA (J2000)    DEC (J2000)     Disc. UT Date   Approx. Disc. V mag   
ASASSN-13bz  0:44:27.672 +43:42:42.12     2013 Aug 12.39     16.1  
This source is most likely a CV in outburst, but it is close enough in projection to M31 to be a very young supernova.

No source is detected on 2013 UT Aug 9.60 or in earlier images (V>17 mag), with initial strong detection in two ASAS-SN images on 2013 UT Aug 12.39, followed by a pair of confirmation image taken 4.7 hours later (2013 UT Aug 12.5855). There was no detectable change in the source flux between these two epochs. See the ASAS-SN discovery image, where the top left panel shows the reference image, the top right shows the DSS image on the same angular scale, the lower left shows the 90-sec V-band image obtained on Aug. 9.60, and the lower right shows the first detection image obtained on Aug. 12.39. The circle with a 15" radius has the same position in each image. The new source is approximately 2.44 degrees North and 0.31 degrees East of the center of M31 as measured from our reference image. This corresponds to a projected separation of 34 kpc for a source in M31 given a distance of 780 kpc (Stanek & Garnavich 1998).

We searched archival data at the position of the transient and find that CRTS doesn't show any previous outbursts at this position. SDSS does not cover this region and the nearest source in Vizier is a B=19.5, R=18.8 mag USNO-A2.0 star 9.3" from the estimated position. The CFHT MegaCam archive contains several deep images obtained around this area. There is a cataloged u-band point source and candidate progenitor of this transient detected at RA = 11.1160454, DEC = +43.71147 (2.1" from its position) in images obtained on 2013 Sept. 09 (PI: J. Sick). The source has u=22.55 +/- 0.02 mag. This source is also detected in a g-band image obtained on 10/21/2008 (PI: A. McConnachie) with g=22.8 +/- 0.05 mag.

For more information about the ASAS-SN project, see this link and also ASAS-SN Transients page.