ASAS-SN Archival Discoveries of a 2016 Galactic Nova and a Bright Microlensing Event Toward the Galactic Bulge

ATel #10740; T. Jayasinghe, C. S. Kochanek, K. Z. Stanek (OSU), Subo Dong (KIAA-PKU), J. L. Prieto (Diego Portales; MAS), J. V. Shields, T. A. Thompson (OSU), T. W.-S. Holoien, B. J. Shappee (Carnegie Observatories), L. Chomiuk, J. Strader (MSU)
on 12 Sep 2017; 20:10 UT
Credential Certification: Krzysztof Stanek (stanek.32@osu.edu)

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As part of an ongoing effort by ASAS-SN project (Shappee et al. 2014; Kochanek et al. 2017) to characterize and catalog all bright variable stars (e.g., Jayasinghe et al. 2017, ATel #10677, #10710), we report the discovery of a previously unrecognized Galactic Nova candidate ASASSN-16ra and a very bright gravitational microlensing event toward the Galactic bulge.

Nova candidate ASASSN-16ra appears in ASAS-SN data starting on UT 2016-03-08.35 at V~13.3. This field was not observed by ASAS-SN prior to that date.The light curve for ASASSN-16ra shows a peak brightness of V~12.4 on UT 2016-04-02.38, eventually falling below the ASAS-SN detection limit (V>17.2) on UT 2016-10-02.005. ASASSN-16ra also shows significant variations of >1 mag near the peak over the span of a few days. ASASSN-16ra is located in a region of the Galactic disk with significant visual extinction (A_V=8.3, Schlafly & Finkbeiner 2011).

 
Object       RA (J2000)    DEC (J2000)    Gal l (deg)   Gal b (deg)   Peak UT Date    Peak V mag 
ASASSN-16ra  15:41:45.24  -53:08:05.9     326.89516      1.63144     2016-04-02.382    12.43 

The likely microlensed source ASASSN-V J182456.34-305816.7 (position from Gaia, g~14.3) is located toward the Galactic bulge (l,b=2.362,-8.43). ASAS-SN data show a well defined and symmetric event with a clear peak at V ~ 11.9 near UT 2017-04-06.346. We fit the standard single-lens microlensing models to the ASAS-SN data and find a best-fit model with a peak magnification of 16. The best-fit parameters are the time of closest approach t_0=2457848.59 +/- 0.15 days (HJD), the impact parameter u_0=0.062 +/- 0.009, and the Einstein crossing time t_E = 70.8 +/- 7.1 days.

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).