TITLE:   GCN CIRCULAR
NUMBER:  43156
SUBJECT: GRB 251220A: MAXI/GSC detection
DATE:    25/12/20 12:17:45 GMT
FROM:    Hitoshi Negoro at Nihon University/MAXI team <negoro.hitoshi@nihon-u.ac.jp>

M. Uenishi (Ehime U.), H. Negoro, M. Nakajima, K. Takagi, H. Takahashi, H. Nishio (Nihon U.), 
T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), 
T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo, A. Yoshida (AGU), 
Y. Tsuboi, H. Sugai, N. Nagashima, Y. Ishihara (Chuo U.), 
M. Shidatsu, C. Kang, T. Nakamoto, T. Usuki, S. Yatsuzuka (Ehime U.),
I. Takahashi, Y. Yatsu (Science Tokyo), S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, M. Kurihara (JAXA), 
Y. Ueda, K. Fujiwara, S. Kobayashi (Kyoto U.), M. Yamauchi, M. Nishio, C. Hiraizumi (Miyazaki U.), 
K. Yamaoka (Nagoya U.), M. Sugizaki (Kanazawa U.), W. Iwakiri (Chiba U.), T. Kawamuro (Osaka U.), 
and S. Yamada (Tohoku U)

The MAXI/GSC nova alert system triggered on a bright uncatalogued X-ray transient source 
at 10:37:16 UT on December 20, 2025.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (19.609 deg, -15.607 deg) = (01 18 26, -15 36 25) (J2000) 
with a statistical 90% C.L. elliptical error region 
with long and short radii of 0.38 deg and 0.28 deg, respectively. 
The roll angle of long axis from the north direction is 173.0 deg counterclockwise. 
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 104 +- 23 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) = (18.401, -15.400) deg = (01 13 36, -15 24 00) (J2000) 
(R.A., Dec) = (18.492, -16.164) deg = (01 13 58, -16 09 50) (J2000) 
(R.A., Dec) = (21.034, -15.868) deg = (01 24 08, -15 52 04) (J2000) 
(R.A., Dec) = (20.934, -15.105) deg = (01 23 44, -15 06 18) (J2000)

There was no significant excess flux in the previous transit at 09:04 UT 
and in the next transit at 12:10 UT with an upper limit of 20 mCrab for each.