GCN Circular 15609

A. Kawagoe (Chuo U.), H. Negoro (Nihon U.), M. Morii (RIKEN), T. Sakamoto (AGU), 
S. Ueno, H. Tomida, S. Nakahira, M. Kimura, M. Ishikawa, Y. E. Nakagawa (JAXA),
T. Mihara, M. Sugizaki, M. Serino,  J. Sugimoto, T. Takagi, A. Yoshikawa, M. Matsuoka (RIKEN),
N. Kawai, R. Usui, T. Yoshii, Y. Tachibana (Tokyo Tech), 
A. Yoshida, Y. Nakano, Y. Kawakubo, H. Ohtsuki (AGU), 
H. Tsunemi, M. Sasaki (Osaka U.), 
M. Nakajima, H. Sakakibara, T. Onodera, K. Fukushima, K. Suzuki (Nihon U.), 
Y. Ueda, M. Shidatsu, T. Kawamuro, T. Hori (Kyoto U.), 
Y. Tsuboi, M. Higa (Chuo U.), 
M. Yamauchi, K. Yoshidome, Y. Ogawa, H. Yamada (Miyazaki U.), 
K. Yamaoka (Nagoya U.) 
report on behalf of the MAXI team:

The MAXI/GSC Nova alert system triggered a bright uncatalogued X-ray transient source 
on 2013-12-25 at 09:54:50 UT.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (95.150 deg, 5.413 deg) = (06 20 36, +05 24 46) (J2000) 
with a statistical 90% C.L. elliptical error region 
with long and short radii of 0.28 deg and 0.19 deg, respectively. 
The roll angle of the long axis from the north direction is 43.0 deg counterclockwise. 
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 273 +- 50 mCrab (4-10keV, 1 sigma error).
Without the assumption about the source constancy, we obtain a rectangular error
box for the transient source with the following corners:
(R.A., Dec) = (94.339 deg, 5.886 deg) = (06 17 21, +05 53 08) (J2000)
(R.A., Dec) = (94.737 deg, 6.296 deg) = (06 18 56, +06 17 45) (J2000)
(R.A., Dec) = (95.508 deg, 5.443 deg) = (06 22 01, +05 26 36) (J2000)
(R.A., Dec) = (95.114 deg, 5.029 deg) = (06 20 27, +05 01 44) (J2000)

There was no significant excess flux in the previous transit at 08:22 UT 
with an upper limit of 20 mCrab.
A possibility of a new X-ray transient, e.g., X-ray nova, can not be excluded
since the location is close to the Galactic plane (b = -4.5 deg). 
We also tentatively name this GRB 131225A as MAXI J0620+053.
The next transit by MAXI/GSC will be at 11:27 UT.