[vsnet-grb-info 27991] GRB 210528B: MAXI/GSC detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30094
SUBJECT: GRB 210528B: MAXI/GSC detection
DATE:    21/05/29 00:38:27 GMT
FROM:    Motoko Serino at Aoyama Gakuin U.  <serino at phys.aoyama.ac.jp>

Y. Sugawara (JAXA), 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, 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:

The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source 
at 22:51:37 UT on 2021 May 28.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (239.539 deg, 5.983 deg) = (15 58 09, +05 58 58) (J2000)
with a statistical 90% C.L. elliptical error region
with long and short radii of 0.26 deg and 0.22 deg, respectively.
The roll angle of long axis from the north direction is 133.0 deg counterclockwise.
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
Without assumptions on the source constancy, we obtain a rectangular error
box for the transient source with the following corners:
(238.974, 6.826) deg = (15 55 53, +06 49 33) (J2000)
(238.661, 6.518) deg = (15 54 38, +06 31 04) (J2000)
(240.060, 5.113) deg = (16 00 14, +05 06 46) (J2000)
(240.373, 5.421) deg = (16 01 29, +05 25 15) (J2000)
The X-ray flux averaged over the scan was 170 +- 28 mCrab (4.0-10.0keV, 1 sigma error).
There was no significant excess flux in the previous transit at 21:18 UT
and in the next transit at 00:24 on May 29 UT with an upper limit of 20 mCrab for each.