ASAS-SN Discovery of an Unusual, Deep Dimming Episode of a Previously Non-Variable Star
T. Jayasinghe, K. Z. Stanek, C. S. Kochanek, P. Vallely, J. V. Shields, 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)
During the ongoing All Sky Automated Survey for SuperNovae (ASAS-SN, Shappee et al. 2014, Kochanek et al. 2017), using data from the quadruple 14-cm "Cassius" and "Paczynski" (CTIO) telescopes, we identified a stellar source undergoing an unusual dimming episode in the southern hemisphere. The source ASASSN-V J213939.3-702817.4 (position from Gaia DR2, source_id=6396259296582909440 , G~12.5, d=1.1 kpc, M_V~2.5) was first observed by ASAS-SN on UT 2014-05-13. Presently, ASASSN-V J213939.3-702817.4 has more than ~1580 data points.

The source ASASSN-V J213939.3-702817.4 has a quiescent mean magnitude of g~12.95 (V~12.65). The latest ASAS-SN photometry shows that ASASSN-V J213939.3-702817.4 gradually faded from g~12.96 at HJD 2458635.78, to g~14.22 at 2458637.95. This source appears to be returning to its quiescent state and has brightened to g~13.29 at HJD 2458638.89. We do not identify similar dimming episodes or other variability in the previous ASAS-SN V/g-band photometry with a baseline of ~1800 d. Since RCB variables typically are brighter than M_V~-2 (Tisserand et al. 2009) and given that this source has a relatively faint absolute magnitude of M_V~2.5, this is likely not an RCB variable. In addition, ALLWISE data is not indicative of a possible infrared excess (W1-W2= -0.06, W2-W3= -0.12). Additional follow-up observations are strongly encouraged.

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-1515927, 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). For more information about the ASAS-SN project, see the ASAS-SN Homepage and the list of all ASAS-SN transients.
optical,star,supernovae,transient,variables