GCN Circular 13895

M. Asada, H. Negoro (Nihon U.), M. Serino (RIKEN), M. Morii  (Tokyo Tech), 
Nakahira (JAXA), K. Yamaoka (Waseda U.), S. Ueno, H. Tomida, M. Ishikawa (JAXA),
T. Mihara, M. Sugizaki, T. Yamamoto, J. Sugimoto, M. Matsuoka (RIKEN), 
N. Kawai, R. Usui, K. Ishikawa, T. Yoshii  (Tokyo Tech),
A. Yoshida (AGU), H. Tsunemi, M. Kimura (Osaka U.), 
M. Nakajima (Nihon U.), 
Y. Ueda, K. Hiroi, M. Shidatsu, R. Sato (Kyoto U.),
Y. Tsuboi, M. Higa (Chuo U.)
M. Yamauchi, Y. Nishimura, T. Hanayama, K. Yoshidome (Miyazaki U.)
report on behalf of the MAXI team:

At 2012-10-25T07:46:30 UT, the MAXI/GSC nova alert system triggered
on a bright uncatalogued X-ray transient source.
The transient emission lasted at least 20 seconds
within the 44 second long triangular transit response of
MAXI/GSC. We identify this event as GRB 121025A.

Assuming that the source flux was constant over the transit,
we obtain the source position at

 (R.A., Dec) = (248.75 deg, +27.73 deg) = (16 35 00, +27 44 05)(J2000)

with a 90% C.L. statistical error of 0.27 deg and an additional systematic uncertainty
of 0.1 deg (90% containment radius). The 4-10 keV flux was  129 (+26/-22) mCrab.

Within the scan transit, the variability of the source was marginally detected with a chance 
probability of 0.084 for 4 - 10 keV band.

There was no significant excess flux in the previous transit at 06:13 UT and 
in the next transit at 09:18 UT with an upper limit of 20 mCrab for each.

Follow-up observations are encouraged.