Small optical variations of MAXI J1820+070 (=ASASSN-18ey) in the faint state
ATel #12988; Keisuke Isogai (Kyoto University), Taichi Kato (Kyoto University), Kiyoe Kawauchi (University of Tokyo), Taku Nishiumi (Kyoto Sangyo University), Naoto Kojiguchi (Kyoto Univeristy), Norio Narita (Astrobiology Center), Akihiko Fukui (University of Tokyo), Nobuhiko Kusakabe (Astrobiology Center), Mariko Kimura (Kyoto University)
on 5 Aug 2019; 05:27 UT
Credential Certification: Keisuke Isogai (isogai@kusastro.kyoto-u.ac.jp)
Subjects: Optical, X-ray, Binary, Black Hole, Transient, Variables
We report on the optical observation of the low-mass X-ray binary MAXI J1820+070 (=ASASSN-18ey). We obtained r'-band optical light curve by using MuSCAT (Narita et al., JATIS, 1, 045001, 2015) mounted on the NAOJ 188cm telescope in Okayama, Japan.
The large outburst of MAXI J1820+070 was detected by the optical and X-ray observations in March 2018 (Atel #11399, #11400) and lasted for about 8 months. After the end of the main outburst, MAXI J1820+070 experienced a rebrightening in March 2019 (Atel #12567, #12573, #12577, #12596). Then, the object approached quiescence in May 2019 (ATel #12534, #12732, #12747).
According to our observation on Aug 1, 2019 (JD 2458696.97--2458697.17), the magnitude of the object was r'=17.6--17.9 which is close to the quiescent magnitude. We also detected optical flare-like variations with the amplitude of 0.2--0.3 mag (http://www.kusastro.kyoto-u.ac.jp/~isogai/atel/asassn-18ey/asassn18ey190801.png). Its timescale of several tens of minutes is much shorter than the orbital period of ~0.685 d (Torres et al. 2019, arXiv 1907.00938). Although we confirmed the 2.0-20.0 keV count rate for the last month by using MAXI On-demand process (Matsuoka et al., PASJ, 61, 999, 2019), there is no significant X-ray emission. Such the optical variations suggest that this object has not completely returned back to the quiescent state.
