GCN Circular 14100

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

At 2012-12-25T09:50:24 UT, the MAXI/GSC nova alert system triggered on a bright uncatalogued X-ray transient source.
The transient emission lasted at least 33 seconds
within the 55 second long triangular transit response of
MAXI/GSC. We identify this event as GRB 121225A.

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

 (R.A., Dec) = (264.86 deg, -66.07 deg) = (17 39 27, -66 04 16)(J2000)

with a 90% C.L. statistical error of 0.2 deg and an additional systematic 
uncertainty of 0.2 deg (90% containment radius). The 4-10 keV flux was  500 +- 50 mCrab.

There was no significant excess flux in the previous transit at 08:17 UT 
and in the next transit at 11:23 UT$B!!(Bwith an upper limit of 20 mCrab for each.