[vsnet-grb-info 27179] GRB 210109A: MAXI/GSC detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29282
SUBJECT: GRB 210109A: MAXI/GSC detection
DATE:    21/01/09 12:21:51 GMT
FROM:    Hitoshi Negoro at Nihon U  <negoro.hitoshi at nihon-u.ac.jp>

R. Uematsu (Kyoto U.), H. Negoro (Nihon U.), M. Serino (AGU), 
M. Nakajima, M. Aoki, K. Kobayashi, R. Takagi, K, Asakura, K, Seino (Nihon U.),
T. Mihara, T. Tamagawa, M. Matsuoka (RIKEN),
T. Sakamoto, 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, U. Goto (Kyoto U.),
H. Tsunemi (Osaka U.), M. Yamauchi, K. Kurogi, K. Miike (Miyazaki U.), T. Kawamuro (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 on a bright uncatalogued X-ray transient source 
at UT 09:03:26 on 2021 January 9.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (62.003 deg, 4.679 deg) = (04 08 00, +04 40 44) (J2000) 
with a statistical 90% C.L. elliptical error region 
with long and short radii of 0.28 deg and 0.25 deg, respectively. 
The roll angle of the long axis from the north direction is 58.0 deg counterclockwise. 
Without assumptions on the source flux constancy, we obtain a rectangular error box
for the transient source with the following corners:
(60.844, 3.548) deg = (04 03 22, +03 32 52) (J2000) 
(61.261, 3.251) deg = (04 05 02, +03 15 03) (J2000) 
(62.919, 5.559) deg = (04 11 40, +05 33 32) (J2000) 
(62.501, 5.857) deg = (04 10 00, +05 51 25) (J2000) 
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).

The X-ray flux averaged over the scan was 108 +- 23 mCrab
(4.0-10.0keV, 1 sigma error).
There was no significant excess flux in the previous transit at UT 07:30
and in the next transit at 10:36 with an upper limit of 20 mCrab for each.