H. Negoro, M. Nakajima, M. Aoki, K. Kobayashi, R. Takagi, K. Asakura, K. Seino (Nihon U.),
T. Mihara, T. Tamagawa, M. Matsuoka (RIKEN),
T. Sakamoto, M. Serino, S. Sugita, H. Nishida, K. Komachi, A. Yoshida (AGU),
Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, Y. Okamoto, S. Kitakoga (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, K. Kurogi, K. Miike (Miyazaki U.),
T. Kawamuro (UDP/NAOJ), K. Yamaoka (Nagoya U.), Y. Kawakubo (LSU), M. Sugizaki (NAOC)
report on behalf of the MAXI team:
The MAXI/GSC detected a bright uncatalogued soft X-ray transient source at 22:15 UT on 2021 March 20.
Assuming that the source flux was constant over the transit,
at 2-4 keV we obtain the source position at
(R.A., Dec) = (4.731 deg, -47.476 deg) = (00 18 55, -47 28 34) (J2000)
with a 90% C.L. statistical error of 0.45 deg and an additional systematic uncertainty of 0.1 deg
(90% containment radius), and, at 4-10 keV we obtained at
(R.A., Dec) = (4.630 deg, -47.423 deg) = (00 18 31, -47 25 22) (J2000)
with a statistical 90% C.L. elliptical error region
with long and short radii of 0.64 deg and 0.61 deg, respectively.
The roll angle of long axis from the north direction is 104.0 deg counterclockwise.
The 2-4 keV and 4-10 keV X-ray fluxes averaged over the scan were 144 +- 28 mCrab
and 59 +- 20 mCrab, respectively.
There was no significant excess flux in the previous transit at 20:42 UT
and in the next transit at 01:21 UT on March 21 with an upper limit of 20 mCrab for each.
(Currently, there are no data at 23:47.)
There is no known bright X-ray source within the error regions,
and no significant short-term variations during the ~45 sec scan.
From the above nature, the source is likely a part of a long gamma-ray burst,
but we tentatively name the source MAXI J0018-474.