ASASSN-21ml: Discovery of an Extreme (Delta g > 10 mag) L-Dwarf Flare
ATel #14778; M. Rizzo Smith, K. Z. Stanek, C. S. Kochanek, C. Christy, D. Desai, Z. Way, T. A. Thompson (OSU), B. J. Shappee (Univ. of Hawaii), T. W.-S. Holoien (Carnegie Observatories), J. L. Prieto (Diego Portales; MAS), Subo Dong (KIAA-PKU), M. Stritzinger (Aarhus)
on 12 Jul 2021; 21:45 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Krzysztof Stanek (stanek.32@osu.edu)
Subjects: Optical, Transient
During the ongoing All-Sky Automated Survey for SuperNovae (ASAS-SN, Shappee et al. 2014, Kochanek et al. 2017) we identified a bright (g~13.8) transient source, named ASASSN-21ml (RA=02:57:06.31, Dec=-08:06:44.5) on 2021 UT July 10.106, using data from the quadruple 14-cm ``Payne-Gaposchkin'' telescope. There was no previous activity at this position in the 3,900+ archival ASAS-SN images of this position, going back to July 2013. In five exposures taken with two cameras over approximately 4 minutes, the source brightened slightly from the initial magnitude g = 13.87 on UT 2021-07-10.1058 to g=13.76 on UT 2021-07-10.1070, before fading to g = 14.13 on
UT 2021-07-10.1083 (see the most recent ASAS-SN Sky Patrol light curve).
ASASSN-21ml has a faint (g = 24.3 +/- 0.5), extremely red (g-i ~ 4.5) counterpart SDSS J025706.27-080645.4 approximately ~1" away from the ASAS-SN position, which is within the ASAS-SN astrometric uncertainties. That counterpart is classified as an L-dwarf, using an archival SDSS/BOSS spectrum. Given the observed peak magnitude of g~13.9, the g-band amplitude of the flare was ~10.4 mag, making it one of the largest amplitude dwarf star flares ever observed.
We thank Las Cumbres Observatory and its staff for their continued support of ASAS-SN. ASAS-SN is funded in part by the Gordon and Betty Moore Foundation through grant GBMF5490 to the Ohio State University, NSF grant AST-1908570, the Mt. Cuba Astronomical Foundation, the Center for Cosmology and AstroParticle Physics (CCAPP) at OSU, and the Chinese Academy of Sciences South America Center for Astronomy (CASSACA).
