The 1898 and 1934 outbursts of ASASSN-18ey (= MAXI J1820+070)
ATel #13066; Naoto Kojiguchi (Kyoto University), Taichi Kato (Kyoto University), Keisuke Isogai (Kyoto University), Daisaku Nogami (Kyoto University)
on 2 Sep 2019; 16:21 UT
Credential Certification: Daisaku Nogami (nogami.daisaku.5z@kyoto-u.ac.jp)
Subjects: Optical, Binary, Black Hole, Transient
We report on two past outbursts of the low-mass X-ray binary ASASSN-18ey
(= MAXI J1820+070) detected via Digital Access to a Sky Century @ Harvard
(DASCH; http://dasch.rc.fas.harvard.edu ).
The large outburst of ASASSN-18ey was discovered in March 2018 and lasted
for about 8 months (ATel #11399, #11400). Then, this object showed
two rebrightenings in March and August 2019 (ATel #12567, #12573, #12577,
#12596, #13014, #13025).
We checked the public data of the photographic plates provided by DASCH
and discovered two historical outbursts in 1898 and 1934.
ASASSN-18ey (= DASCH_N020311310706)
#yyyymmdd mag band observer
18981017.979 12.73 B DASCH
 18990928.020 14.47 B DASCH
 19340713.107 14.05 B DASCH
 19340717.102 14.20 B DASCH
*DASCH data are photographic plate data converted to APASS B-band magnitude.
By using these data, we could approximately estimate the outburst-recurrence
time of ASASSN-18ey to be 40 years.
As a similar example, it is known that the 1917 outburst of V616 Mon (= Mon X-1)
was found in Harvard photographic plates
(Eachus, Wright & Liller 1976).
