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ASASSN-16ae: Discovery of an Extreme (Delta V ~ 10 mag) Flare on an Ultracool Dwarf Star

ATel #8553; B. J. Shappee (Hubble Fellow, Carnegie Observatories), K. Z. Stanek (Ohio State), S. Schmidt (Leibniz Institute for Astrophysics - Potsdam), T. W.-S. Holoien, J. S. Brown, C. S. Kochanek, D. Godoy-Rivera, U. Basu (Ohio State), J. L. Prieto (Diego Portales; MAS), D. Bersier (LJMU), Subo Dong, Ping Chen (KIAA-PKU), J. Brimacombe (Coral Towers Observatory)
on 15 Jan 2016; 23:14 UT
Credential Certification: Benjamin Shappee (bshappee@obs.carnegiescience.edu)

Subjects: Optical, Transient

Referred to by ATel #: 8803

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

Object       RA (J2000)   DEC (J2000)   Disc. UT Date    Disc. V mag    
ASASSN-16ae  05:33:41.40  +00:14:32.7   2016-01-10.32     13.85 

ASASSN-16ae was detected in images obtained on 2016 UT January 10.32 as a bright (V = 13.85) new source. There was no previous activity seen at this position in the 1000+ images taken at this location since January 2012. This includes ASAS-SN data taken on 2016 January 8 and 9. Due to built-in overlap between different ASAS-SN fields, there is also a weak limit from the edge of another CCD of V>16.2 on 2016 UT January 10.20 2014-11-28.5937 (~2.8 hours before the flare detection). Between our three exposures, separated by ~100 seconds, the transient faded significantly, from V = 13.51 on UT 2016-01-10.32247 to V = 13.89 on 2016-01-10.32373 to V = 14.16 on UT 2016-01-10.32498.

A counterpart has been identified in SDSS data: a faint (r = 22.45 +/- 0.15), extremely red (r-z ~ 3.57) source ~1" away from the ASAS-SN detection. The i-z, i-J, i-K, and z-J colors are consistent with those of L0 and L1 dwarfs from Schmidt et al. (2015, AJ, 149, 158). The 2MASS J-H, H-K, and J-K colors are slightly peculiar for an early-L dwarf, but the JHK photometry is near the faint limit for 2MASS and flagged as a lower S/N detection. Applying the distance relation of Schmidt et al. (2010, AJ, 139, 1808) gives a distance of 66 pc. Based on spectrophotometry of early-L dwarfs from Schmidt et al. (2015, AJ, 149, 158), we estimate a V-r color of 1.09 +/- 0.15 for L0 dwarfs. Using this color, we estimate a quiescent V magnitude of 23.54 +/- 0.21 which indicates total flare magnitude of delta V ~ 10.

ASASSN-16ae is most likely a delta V~10 mag flare from an L-dwarf star, and given very short duration of such events possibly even stronger. Flares nearly as large have been observed from late-M dwarfs (e.g., Stanek et al. 2013, ATel #5276; Schmidt et al. 2015, ApJ, 781, 24; Simonian et al. 2014, ATel #6771), but this is the reddest object with a dramatic flare.

We thank LCOGT and its staff for their continued support of ASAS-SN. ASAS-SN is supported by NSF grant AST-1515927, the Mt. Cuba Astronomical Foundation, the Center for Cosmology and AstroParticle Physics (CCAPP) at OSU, and the Robert Martin Ayers Sciences Fund. For more information about the ASAS-SN project, see the ASAS-SN Homepage and the list of all ASAS-SN transients.