TITLE:   GCN CIRCULAR
NUMBER:  17772
SUBJECT: GRB 150428C/MAXI J0400+678: MAXI/GSC detection
DATE:    15/04/28 09:17:41 GMT
FROM:    H. Negoro at Nihon U.  <negoro@phys.cst.nihon-u.ac.jp>

F. Honda, M. Fujita, H. Negoro (Nihon U.), M. Serino (RIKEN), S. Nakahira, 
S. Ueno, H. Tomida,  M. Kimura, M. Ishikawa, Y. E. Nakagawa (JAXA)
T. Mihara, M. Sugizaki, M. Shidatsu, J. Sugimoto, T. Takagi, M. Matsuoka (RIKEN),
N. Kawai, T. Yoshii, Y. Tachibana, Y. Ono (Tokyo Tech),
A. Yoshida, T. Sakamoto, Y. Kawakubo, H. Ohtsuki (AGU),
H. Tsunemi, R. Imatani (Osaka U.),
M. Nakajima, T. Namba, T. Masumitsu, K. Tanaka (Nihon U.),
Y. Ueda, T. Kawamuro, T. Hori (Kyoto U.),
Y. Tsuboi, S. Kanetou (Chuo U.),
M. Yamauchi, D. Itoh (Miyazaki U.),
K. Yamaoka (Nagoya U.),
M. Morii (ISM)
report on behalf of the MAXI team:

The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray transient source at 08:24:15 UT.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (60.247 deg, 67.818 deg) = (04 00 59, +67 49 03) (J2000)
with a 90% C.L. statistical error of 0.29 deg and an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 161 +- 27 mCrab
(4-10keV, 1 sigma error).

Without assumptions on the source constancy, we obtain a rectangular error
box for the transient source with the following corners:
(59.061 deg, 67.515 deg) = (03 56 14, +67 30 54) (J2000)
(58.970 deg, 68.084 deg) = (03 55 52, +68 05 00) (J2000)
(61.826 deg, 67.549 deg) = (04 07 18, +67 32 57) (J2000)
(61.794 deg, 68.119 deg) = (04 07 10, +68 07 09) (J2000)

There was no significant excess flux in the previous transit at 06:51 UT
with an upper limit of 20 mCrab.

We note that no significant X-ray flux above 10 keV was detected,
and the spectrum was represented by a blackbody model with
the temperature of approximately 1 keV. Thus, the burst might be an X-ray burst
from an unknown source. We tentatively name the source MAXI J0401+678.