GCN Circular 21422

H. Kawai (Chuo U.), S. Nakahira, M. Serino, T. Mihara, M. Sugizaki,
W. Iwakiri, M. Shidatsu, M. Matsuoka (RIKEN), 
H. Negoro, M. Nakajima, T. Kawase, A. Sakamaki (Nihon U.),
S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai (JAXA),
N. Kawai, S. Sugita, T. Yoshii, Y. Tachibana, S. Harita, Y. Muraki, K. Morita (Tokyo Tech), 
A. Yoshida, T. Sakamoto, Y. Kawakubo, Y. Kitaoka, T. Hashimoto (AGU), 
H. Tsunemi, T. Yoneyama (Osaka U.), 
Y. Ueda, T. Hori, A. Tanimoto, S. Oda (Kyoto U.), 
Y. Tsuboi, Y. Nakamura, R. Sasaki (Chuo U.), 
M. Yamauchi, C. Hanyu, K, Hidaka (Miyazaki U.), 
T. Kawamuro (NAOJ), 
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 at UT 02:47:57 UT on 2017 August 8. 
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (157.564 deg, -28.277 deg) = (10 30 15, -28 16 37) (J2000) 
with a statistical 90% C.L. elliptical error region 
with long and short radii of 0.34 deg and 0.33 deg, respectively. 
The roll angle of long axis from the north direction is 64.0 deg counterclockwise. 
We estimated a rectangular error box for the transient source with the following corners:
(157.227, -28.921) deg = (10 28 54, -28 55 16) (J2000) 
(157.014, -28.758) deg = (10 28 03, -28 45 29) (J2000) 
(158.133, -27.077) deg = (10 32 32, -27 04 37) (J2000) 
(158.343, -27.238) deg = (10 33 22, -27 14 16) (J2000).
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).

The X-ray flux averaged over the scan was 77 +- 18 mCrab
(2.0-4.0keV, 1 sigma error).
There was no significant excess flux in the previous transit at UT 01:15
and in the next transit at UT 04:20 with an upper limit of 20 mCrab for each.