ASASSN-17oz: Discovery of an Unusual, Ongoing Stellar Outburst

ATel #10991; T. Jayasinghe, K. Z. Stanek, C. S. Kochanek (OSU), J. L. Prieto (Diego Portales; MAS), J. V. Shields, T. A. Thompson (OSU), Subo Dong (KIAA-PKU), B. J. Shappee (Univ. of Hawaii), T. W.-S. Holoien (Carnegie Observatories), E. Falco (CfA), R. Leadbeater (Three Hills Observatory), J. Rupert (MDM Observatory), J. Brimacombe (Coral Towers Observatory), R. A. Koff (Antelope Hills Observatory), R. S. Post (Post Astronomy),
on 21 Nov 2017; 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, Shappee et al. 2014, Kochanek et al. 2017), using data from the quadruple 14-cm "Brutus" (Haleakala, Hawaii) and "Leavitt" (Fort Davis, Texas) telescopes, we discovered an unusual transient source that is indicative of a stellar outburst.

The source ASASSN-17oz matches to a g~16.2 Gaia star (2MASS colors J-H=0.52, H-Ks=0.25) and was first observed by ASAS-SN on UT 2014-01-04. ASASSN-17oz has >1400 epochs of ASAS-SN photometry with a mean V mag of ~16.3 and showed no signs of significant intrinsic variability over this baseline in our variability analysis pipeline (Jayasinghe et al. 2018, in prep). The latest ASAS-SN photometry on UT 2017-11-19 suggests that ASASSN-17oz brightened by ~2.6 mag in both the V (blue points) and g (yellow points) bands over a duration of ~25 days, and is now beginning to fade.

We obtained multiple optical spectra of ASASSN-17oz on UT Nov. 12-18 with MDM 2.4m (+OSMOS), FTN 2m (+FLOYDS), FLWO 1.5m (+FAST), and Three Hills Observatory (+ALPY). The spectra show a blue continuum with relatively broad Balmer lines in absorption with a narrow core in emission (Hbeta, Hgamma, Hdelta), Halpha in emission (EW ~ 4 Angstroms), He 5875 in emission, and Na I D doublet in absorption. These spectroscopic characteristics are similar to spectra of some cataclysmic variables.

 
Object         RA (J2000)     DEC (J2000)    Peak. V/g mag   
 
ASASSN-17oz   06:07:54.964   +45:12:18.237        ~13.7        

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.