MAXI/GSC detection of a possible X-ray flare from the RS CVn binary system EI Eri
ATel #14561; S. Kitakoga, W. Iwakiri (Chuo U.) H. Negoro, M. Nakajima, Kobayashi, K. Asakura, K. Seino (Nihon U.), T. Mihara, T. Tamagawa, M. Matsuoka (RIKEN), T. Sakamoto, M. Serino, S. Sugita, K. Komachi, A. Yoshida (AGU), Y. Tsuboi, H. Kawai, Y. Okamoto (Chuo U.), M. Shidatsu (Ehime U.), N. Kawai, R. Adachi, M. Niwano, R. Hosokawa (Tokyo Tech), S. Nakahira, Y. Sugawara, S. Ueno, H. Tomida, M. Ishikawa, M. Tominaga, T. Nagatsuka (JAXA), Y. Ueda, S. Yamada, S. Ogawa, K. Setoguchi, T. Yoshitake, Y. Goto, R. Uematsu (Kyoto U.), H. Tsunemi (Osaka U.), M. Yamauchi, Y. Nonaka, T. Sato, R. Hatsuda, R. Fukuoka (Miyazaki U.), T. Kawamuro (UDP/NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC) report on behalf of the MAXI team:
on 19 Apr 2021; 13:46 UT
Credential Certification: Wataru Iwakiri (wataru.iwakiri@riken.jp)
Subjects: X-ray, Binary, Star, Transient
The MAXI/GSC nova alert system triggered an X-ray flare-like event at 2021-04-18 13:32 (UT). The estimated X-ray flux was about 79 +/- 19 mCrab (1 sigma error) in the 4 - 10 keV band. Assuming that the source flux was constant over the transit, we obtain the source position at
(R.A., Dec) = (62.116 deg, -7.697 deg) = (04 08 27, -07 41 49) (J2000)
with a statistical 90% C.L. elliptical error region with long and short radii of 0.32 deg and 0.29 deg, respectively. The roll angle of the long axis from the north direction is 162.0 deg counterclockwise. Considering an additional systematic uncertainty of 0.1 deg (90% containment radius), this position is consistent with that of the RS-CVn type binary EI Eri. During the next transit at 15:06 UT, any source was not detected significantly (< 45 mCrab at 4 - 10 keV) with MAXI/GSC.
The upper limit of the e-folding decay time is less than about 2.7 h because the X-ray flux decreased to less than 45 mCrab at the next scan transit of MAXI (92 minutes after).
Although we cannot exclude the possibility that this event is a GRB, it is likely to be associated with a stellar flare from EI Eri. If it is correct, it is the first detection of a stellar flare from EI Eri in the 11 years of MAXI observations.
