GCN Circular 14118

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

At 2012-12-29T05:01:08 UT, the MAXI/GSC detected an uncatalogued X-ray
transient source. Assuming that the source flux was constant over the
transit, we obtain the source position at
(R.A., Dec.) = (189.70deg, -50.84deg) = (12:38:48.98, -50:50:36.0)(J2000)
with a 90% C.L. statistical error of 0.4 deg and an additional systematic
uncertainty of 0.1 deg (90% containment radius).

The position and time are consistent with those of GRB 121229A (Sonbas
et al, GCN #14115). The transit by MAXI occurred from 2012/12/29
05:00:47 (=T0+26 s, where T0 is the Swift/BAT trigger time) to T0+70s.
The averaged X-ray flux was 103 +/- 23 mCrab (2-20 keV).
The spectrum is fitted by a power-law model with a photon index of
1.85 (-0.58/+0.68). There was no significant excess flux at the previous
transit at 2012/12/28 18:12 UT and at the next transit at 2012/12/29
06:43 UT with an upper limit of 20 mCrab for each.