TITLE:   GCN CIRCULAR
NUMBER:  30067
SUBJECT: GRB 210521A: MAXI/GSC detection
DATE:    21/05/21 15:04:39 GMT
FROM:    Motoko Serino at Aoyama Gakuin U.  <serino@phys.aoyama.ac.jp>

S. Yamada (Kyoto U.), H. Negoro, M. Nakajima, K. 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, W. Iwakiri, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.),
M. Shidatsu (Ehime U.),
N. Kawai, M. Niwano, R. Hosokawa (Tokyo Tech),
S. Nakahira, Y. Sugawara, S. Ueno, H. Tomida, M. Ishikawa, M. Tominaga, T. Nagatsuka (JAXA),
Y. Ueda, 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:

The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source 
at 13:04:08 UT on 2021 May 21.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (264.674 deg, -61.752 deg) = (17 38 41, -61 45 07) (J2000)
with a 90% C.L. statistical error of 0.12 deg and an additional systematic uncertainty of
0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 508 +- 51 mCrab
(4.0-10.0keV, 1 sigma error).
Without assumptions on the source constancy, we obtain a rectangular error
box for the transient source with the following corners:
(R.A., Dec) = (264.912, -61.246) deg = (17 39 38, -61 14 45) (J2000)
(R.A., Dec) = (264.426, -61.253) deg = (17 37 42, -61 15 10) (J2000)
(R.A., Dec) = (264.502, -62.494) deg = (17 38 00, -62 29 38) (J2000)
(R.A., Dec) = (265.008, -62.486) deg = (17 40 01, -62 29 09) (J2000)
There was no significant excess flux in the previous transit at 12:43 UT
and in the next transit at 14:16 UT with an upper limit of 20 mCrab for each.