TITLE:   GCN CIRCULAR
NUMBER:  31261
SUBJECT: GRB 211218A, MAXI J0909-041: MAXI/GSC detection
DATE:    21/12/18 05:17:50 GMT
FROM:    Hitoshi Negoro at Nihon U  <negoro.hitoshi@nihon-u.ac.jp>

R. Uematsu (Kyoto U.), H. Negoro (Nihon U.), M. Serino (AGU), 
M. Nakajima, K. Kobayashi, K. Asakura, K. Seino (Nihon U.), 
T. Mihara, T. Tamagawa, J. Li, M. Matsuoka (RIKEN), 
T. Sakamoto, S. Sugita, K. Komachi, H. Hiramatsu, A. Yoshida (AGU), 
Y. Tsuboi, W. Iwakiri, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.), 
M. Shidatsu, M. Iwasaki (Ehime U.),
N. Kawai, M. Niwano, R. Hosokawa, Y. Imai, N. Ito, Y. Takamatsu (Tokyo Tech), 
S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, M. Tominaga, T. Nagatsuka, M. Kurihara (JAXA), 
Y. Ueda, S. Yamada, S. Ogawa, K. Setoguchi, T. Yoshitake, Y. Goto, K. Inaba (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 01:01:29 UT 2021 December 18. Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (137.278 deg, -4.110 deg) = (09 09 06, -04 06 36) (J2000) 
with a statistical 90% C.L. elliptical error region with long and short radii of 0.42 deg and 0.38 deg, 
respectively. 
The roll angle of long axis from the north direction is 62.0 deg counterclockwise. 
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 118 +- 31 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) = (135.984, -5.236) deg = (09 03 56, -05 14 09) (J2000)
(R.A., Dec) = (136.566, -5.656) deg = (09 06 15, -05 39 21) (J2000)
(R.A., Dec) = (138.302, -3.262) deg = (09 13 12, -03 15 43) (J2000)
(R.A., Dec) = (137.722, -2.843) deg = (09 10 53, -02 50 34) (J2000)
There was no significant excess flux in the previous transit at 12/17 23:28 UT 
and in the next transit at 12/18 02:34 UT with an upper limit of 20 mCrab for each.

 Finally, we note that due to the poor statistics, we can not confirm that this event is a GRB.
 Therefore, we also name the source MAXI J0909-041.