ASAS-SN Discovery of an Extreme (delta V~9 mag) M-dwarf Flare

ATel #5276; K. Z. Stanek, B. J. Shappee, C. S. Kochanek, T. W-S. Holoien, 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), J. Brimacombe (Coral Towers Observatory)
on 14 Aug 2013; 16:13 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"), using data from the double 14-cm "Brutus" telescope in Haleakala, Hawaii, we discovered an interesting transient:

  
Object       RA (J2000)   DEC (J2000)   Disc. UT Date    Disc. V mag  
ASASSN-13cb  02:21:16.92  19:40:19.9 	2013-08-14.52 	  13.0 

ASASSN-13cb was detected in images obtained 2013 UT August 14.52 as a very bright V=13.0 new source. There was no previous activity seen at this position, including ASAS-SN data taken on 2013 August 12 and August 8. Between our two exposures separated by ~120 sec the transient faded significantly, by about 25%. Following the initial detection of ASASSN-13cb, we obtained a pair of ASAS-SN confirmation images taken 2.4 hours later (2013 UT Aug 14.62). ASASSN-13cb is only marginally detected in these images, with V~17.5.

There is a faint (g=22.7) and red (g-r=1.57) SDSS star 1.2" away from our position. There is also a 2MASS star 2.2" away with K= 13.9 and J-K=1.1. We note that at this position the line-of-sight Galactic extinction is fairly significant, A_V=0.81 (Schlafly & Finkbeiner 2011) and E(g-r)=0.3, so it is not immediately clear if the likely counterpart is a nearby, intrinsically red M-dwarf or a more distant, intrinsically bluer but reddened source. However, given the 2MASS color of J-K=1.1 and the maximum expected E(J-K)=0.12, the intrinsic color of the source is at least (J-K)_0=1.0, so it is most likely a nearby M-dwarf. In fact the star is reported to have a relatively large proper motion of 0.11"/year, again making it most likely a nearby M-dwarf. Such a very red (J-K)_0 color makes the star an M8 or M9 spectral type M-dwarf (West et al. 2011).

In this case the ASASSN-13cb transient would be due to delta V~9 mag flare from an M-dwarf star, and given very short duration of such events likely even stronger. Such an extreme M-dwarf flare would be very unusual (e.g., Kowalski et al. 2009), if possibly not unprecedented (e.g., ATel #4586, #5186; IBVS 1160: http://www.konkoly.hu/cgi-bin/IBVS?1160 ).

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