TITLE:   GCN CIRCULAR
NUMBER:  32001
SUBJECT: GRB 220506A/MAXI J1558-266: MAXI/GSC detection
DATE:    22/05/06 12:01:55 GMT
FROM:    Motoko Serino at Aoyama Gakuin U.  <serino@phys.aoyama.ac.jp>

S. Urabe, J. Kohara (Chuo U.), H. Negoro (Nihon U.),
M. Nakajima, K. Kobayashi, K. Asakura, K. Seino (Nihon U.),
T. Mihara, T. Kawamuro, S. Yamada, T. Tamagawa, M. Matsuoka (RIKEN),
T. Sakamoto, M. Serino, S. Sugita, H. Hiramatsu, A. Yoshida (AGU),
Y. Tsuboi, W. Iwakiri, H. Kawai, Y. Okamoto, S. Kitakoga (Chuo U.),
M. Shidatsu, M. Iwasaki (Ehime U.),
N. Kawai, M. Niwano, R. Hosokawa, Y. Imai, N. Ito, Y. Takamatsu (Tokyo Tech),
S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, M. Tominaga, T. Nagatsuka, M. Kurihara (JAXA),
Y. Ueda, S. Ogawa, K. Setoguchi, T. Yoshitake, Y. Goto, R. Uematsu, K. Inaba (Kyoto U.),
H. Tsunemi (Osaka U.),
M. Yamauchi, Y. Nonaka, T. Sato, R. Hatsuda, R. Fukuoka (Miyazaki U.), K. Yamaoka (Nagoya U.),
Y. Kawakubo (LSU),
M. Sugizaki (NAOC) 
report on behalf of the MAXI team:

The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray 
transient source at 06:28:15 UT on 2022 May 6.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (239.698 deg, -26.668 deg) = (15 58 47, -26 40 04) (J2000)
with a statistical 90% C.L. elliptical error region
with long and short radii of 0.34 deg and 0.27 deg, respectively.
The roll angle of long axis from the north direction is 64.0 deg counterclockwise.
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 103 +- 22 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) = (239.079, -27.388) deg = (15 56 18, -27 23 16) (J2000)
(R.A., Dec) = (239.660, -27.603) deg = (15 58 38, -27 36 10) (J2000)
(R.A., Dec) = (240.203, -26.432) deg = (16 00 48, -26 25 55) (J2000)
(R.A., Dec) = (239.626, -26.218) deg = (15 58 30, -26 13 04) (J2000)
There was no significant excess flux in the previous transit at 00:45 UT
and in the next transit at 08:01 UT with an upper limit of 20 mCrab for each.