GCN Circular 23547

A. Sakamaki, H. Negoro, M. Nakajima, W. Maruyama, M. Aoki, K. Kobayashi (Nihon U.), 
T. Mihara, S. Nakahira, F. Yatabe, Y. Takao, M. Matsuoka (RIKEN), 
T. Sakamoto, M. Serino,  S. Sugita, Y. Kawakubo, T. Hashimoto, A. Yoshida (AGU), 
N. Kawai, M. Sugizaki, Y. Tachibana, K. Morita, T. Oeda, K. Shiraishi (Tokyo Tech), 
S. Ueno, H. Tomida, M. Ishikawa, Y. Sugawara, N. Isobe, R. Shimomukai, T. Midooka (JAXA), 
Y. Ueda, A. Tanimoto, T. Morita, S. Yamada, S. Ogawa (Kyoto U.), 
Y. Tsuboi, W. Iwakiri, R. Sasaki, H. Kawai, T. Sato (Chuo U.), 
H. Tsunemi, T. Yoneyama, K. Asakura, S. Ide (Osaka U.), 
M. Yamauchi, K. Hidaka, S. Iwahori (Miyazaki U.), T. Kawamuro (NAOJ), 
K. Yamaoka (Nagoya U.), M. Shidatsu (Ehime U.)
report on behalf of the MAXI team:

The MAXI/GSC nova alert system triggered on a bright uncatalogued X-ray 
transient source at 06:44:09 UT on December 22, 2018.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (270.731 deg, -38.848 deg) = (18 02 55, -38 50 52) (J2000) 
with a statistical 90% C.L. elliptical error region 
with long and short radii of 0.13 deg and 0.12 deg, respectively. 
The roll angle of long axis from the north direction is 132.0 deg 
counterclockwise. There is an additional systematic uncertainty of 0.1 deg
(90% containment radius).

The X-ray flux averaged over the scan was 350 +- 34 mCrab
(4.0-10.0keV, 1 sigma error).
Without assumptions on the source constancy, we obtain a rectangular error
box for the transient source with the following corners:
(R.A., Dec) = (270.981, -38.395) deg = (18 03 55, -38 23 42) (J2000)
(R.A., Dec) = (270.687, -38.351) deg = (18 02 44, -38 21 03) (J2000)
(R.A., Dec) = (270.409, -39.448) deg = (18 01 38, -39 26 52) (J2000)
(R.A., Dec) = (270.708, -39.494) deg = (18 02 49, -39 29 38) (J2000)
There was no significant excess flux in the previous transit at 05:11 UT
with an upper limit of 20 mCrab.