TITLE:   GCN CIRCULAR
NUMBER:  45993
SUBJECT: GRB 261020A: MAXI GSC detection
DATE:    26/10/20 09:49:16 GMT
FROM:    Motoko Serino at Aoyama Gakuin U. <serino@phys.aoyama.ac.jp>

T. Kato, M. Serino(AGU), W. Iwakiri (Chiba U.),
H. Negoro, M. Nakajima, K. Saito, T. Yokoyama, M. Wada (Nihon U.), 
T. Mihara, A. Aoyama, M. Ota, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), 
T. Sakamoto, S. Sugita, Y. Kawakubo, H. Hiramatsu,  A. Yoshida (AGU), 
Y. Tsuboi, Y. Ishihara, H. Ogaki (Chuo U.), 
M. Shidatsu, C. Kang, M. Uenishi, T. Usuki, S. Yatsuzuka, H. Aoe (Ehime U.),
I. Takahashi, Y. Yatsu (Science Tokyo), 
S. Nakahira, S. Ueno, H. Tomida, 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.),
T. Kawamuro (Osaka U.), 
S. Yamada (Tohoku U),
S. Ogawa (TUS)
report on behalf of the MAXI team:

The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient 
source at 05:42:38 UT on 20 October 2026.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (290.658 deg, -35.016 deg) = (19 22 37, -35 00 57) (J2000)
with a statistical 90% C.L. elliptical error region
with long and short radii of 0.53 deg and 0.45 deg, respectively.
The roll angle of long axis from the north direction is 61.0 deg counterclockwise.
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 71 +- 18 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:
(289.257, -34.240) deg = (19 17 01, -34 14 24) (J2000)
(288.926, -35.140) deg = (19 15 42, -35 08 24) (J2000)
(292.410, -35.950) deg = (19 29 38, -35 57 00) (J2000)
(292.706, -35.040) deg = (19 30 49, -35 02 23) (J2000)
There was no significant excess flux in the previous transit at 04:09 UT
and in the next transit at 07:15 UT with an upper limit of 20 mCrab for each.

Fermi/GBM triggered GRB 261020A at 05:37:08 UT. MAXI/GSC detected X-ray emission approximately 330 s 
later, during its first scan of the region after the trigger.