ASAS-SN Discovery of a Bright Candidate Microlensing Event ASASSN-19cq

ATel #12495; K. V. Sokolovsky, J. Strader, K. Dage, L. Shishkovsky, E. Aydi, L. Chomiuk, A. Kawash (MSU), K. Z. Stanek, C. S. Kochanek, 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)
on 12 Feb 2019; 02:01 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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During the ongoing All Sky Automated Survey for SuperNovae (ASAS-SN, Shappee et al. 2014 ApJ, 788, 48, Kochanek et al. PASP, 129, 4502), using data from the quadruple 14-cm "Cassius" and "Paczynski" telescopes in CTIO, Chile we detected a brightening of an otherwise unremarkable star located towards the Galactic Bulge (l=13.417, b=7.639). The star, which had median magnitude g=15.7 as observed before the seasonal gap, brightened to g=14.25 +/-0.04 on 2019-02-11.388 UT (latest observation). Its ASAS-SN Sky Patrol lightcurve shows gradual brightening starting at least on 2019-02-04.658 UT, and possibly as early as 2018.

We obtained a low resolution spectrum of the source with the Goodman spectrograph at the 4.1-m SOAR telescope (CTIO, Chile) on 2019-02-11.363 UT. The spectrum shows no emission lines and the Balmer H_alpha to H_gamma lines as well as the Na I D (5892A) in absorption. The spectrum is consistent with that of a K-type star with the Na I D absorption being of possible interstellar origin. The source position derived from the SOAR acquisition image with respect to the UCAC3 (Zacharias et al. 2010 AJ, 139, 2184) stars in the field is: RA,Dec=17:47:05.88, -13:31:42.0 +/-0.15 (J2000).

The stellar spectrum together with the flat ASAS-SN lightcurve followed by a gradual rise suggest that ASASSN-19cq is a microlensing event detected before the maximum magnification. Follow-up observations are strongly encouraged.

We would like to 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, the Chinese Academy of Sciences South America Center for Astronomy (CASSACA), and the Villum Fonden (Denmark).

SOAR spectrum and finding chart