[vsnet-grb-info 11960] GRB 120528C: MAXI/GSC detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  13349
SUBJECT: GRB 120528C: MAXI/GSC detection
DATE:    12/05/30 03:54:42 GMT
FROM:    Motoko Suzuki at RIKEN  <motoko at crab.riken.jp>

M. Morii (Tokyo Tech), H. Negoro (Nihon U.), M. Serino (RIKEN),
S. Ueno, H. Tomida, S. Nakahira, M. Ishikawa (JAXA),
T. Mihara, M. Sugizaki, T. Yamamoto, M. Matsuoka (RIKEN), 
N. Kawai, R. Usui, K. Ishikawa (Tokyo Tech),
A. Yoshida (AGU),
H. Tsunemi, M. Kimura (Osaka U.), 
M. Nakajima, M. Asada, H. Sakakibara, N. Serita (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.), 
K. Yamaoka (Waseda U.)
report on behalf of the MAXI team:

MAXI/GSC detected an uncatalogued X-ray transient source at the 4.4
sigma level in the 2-20 keV band. The maximum of the raw count rate in
the 2-20 keV band, before the correction of the triangular transit
response of MAXI/GSC, was observed around 2012-05-28T21:21:58. The
enhancement lasted at least 30 seconds before and after the maximum
within the about 40 seconds transit response.

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

(R.A., Dec) = (+12.93 deg, -0.95 deg) = (00 51 44, -00 57 18)(J2000) 

with a statistical 90% C.L. elliptical error region
with long and short radii of 0.8 deg and 0.6 deg, respectively.
The position angle of long axis is 0 deg.
There is an additional systematic uncertainty of 0.1 deg (90%
containment radius).

The preliminary flux of the transient source was 37 +- 16 mCrab (4-10
keV). There was no significant detection at the transit location in the
previous and the next orbit (92 min before and after the detection)
with an upper limit of 20 mCrab. There is no known bright X-ray source
at the detected position. A hard spectrum of the source suggests the
source was a GRB, but another possibility can not be excluded.

Follow-up observations are encouraged.