ASAS-SN Discovery of Two Very Bright Transients

ATel #5186; K. Z. Stanek, B. J. Shappee, C. S. Kochanek, J. Jencson, U. Basu, J. F. Beacom (Ohio State), J. L. Prieto (Princeton), 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)
on 2 Jul 2013; 19:33 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Krzysztof Stanek (stanek.32@osu.edu)

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During the ongoing All Sky Automated Survey for SuperNovae (ASAS-SN or "Assassin"; e.g., ATel #5168, #5181, #5183), using data from the double 14-cm "Brutus" telescope in Haleakala, Hawaii, we discovered two new interesting transients:

  
Object       RA (J2000)   DEC (J2000)   Disc. UT Date    Disc. V mag  
ASASSN-13ba  01:17:00.56 -05:06:04.6    2013 July 2.55      12.3 
ASASSN-13ax  18:00:05.78 +52:56:35.3    2013 July 1.41      13.5 

ASASSN-13ax was present in images obtained 2013 UT July 1.41 but undetected (V > 17) on 2013 UT June 28. Vizier reveals an optical source with B = 20.5 detected 2.9" from our ASAS-SN position in the USNO-B1 catalog and also a 2.8" SDSS match to a g=21.2 blue star. ASASSN-13ax is most likely a CV in a very strong ~7 mag outburst.

ASASSN-13ba was detected in images obtained 2013 UT July 2.55 as a very bright V=12.3 new source. There was no previous activity seen in this position before, including data taken on 2013 June 28 (see our discovery image, the left panel shows the reference image, the two other panels show both 2013 UT July 2 ASAS-SN images). There is a fairly bright (g=18.3) and red (g-r=1.5) SDSS star 1.2" away from our position. We note that at this position, with Galactic latitude b=-7.5887, the maximum line-of-sight Galactic extinction is quite significant, A_V=1.63 (Schlafly & Finkbeiner 2011) and E(g-r)=0.6, so it is not immediately clear if the likely counterpart is a nearby, intrinsically red M-dwarf or a further away, intrinsically bluer but dust-reddened source. However, given the 2MASS observed J-K=0.9 and maximum expected E(J-K)=0.24, the intrinsic color of that source is at least (J-K)_0=0.67, likely making it a nearby M-dwarf. In this case ASASSN-13ba transient would be due to delta V~5 mag flare from a M-dwarf star, and given short duration of such events (~hours), likely even stronger at maximum. Such a strong M-dwarf flare would be very unusual (e.g., Kowalski et al. 2009). Continued observations to reveal the nature of this source are encouraged.

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