GRB 230520A

Y. Umeki (Miyazaki U.), M. Serino (AGU), Y. Kawakubo (LSU),
H. Negoro, M. Nakajima, K. Kobayashi, M. Tanaka, Y. Soejima, Y. Kudo (Nihon U.),
T. Mihara, T. Kawamuro, S. Yamada, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN),
T. Sakamoto, S. Sugita, H. Hiramatsu, H. Nishikawa, A. Yoshida (AGU),
Y. Tsuboi, S. Urabe, S. Nawa, N. Nemoto (Chuo U.),
M. Shidatsu (Ehime U.),
I. Takahashi, M. Niwano, S. Sato, N. Higuchi, Y. Yatsu (Tokyo Tech),
S. Nakahira, S. Ueno, H. Tomida, M. Ishikawa, S. Ogawa, T. Kurihara (JAXA),
Y. Ueda, K. Setoguchi, T. Yoshitake, Y. Nakatani (Kyoto U.),
M. Yamauchi, Y. Hagiwara, Y. Otsuki (Miyazaki U.),
K. Yamaoka (Nagoya U.),
M. Sugizaki (NAOC),
W. Iwakiri (Chiba U.) 
report on behalf of the MAXI team:

The MAXI/GSC nova alert system triggered a bright uncatalogued X-ray 
transient source at 05:03:24 UT on 20 May 2023.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (142.422 deg, 1.524 deg) = (09 29 41, +01 31 26) (J2000)
with a statistical 90% C.L. elliptical error region
with long and short radii of 0.04 deg and 0.03 deg, respectively.
The roll angle of long axis from the north direction is 90.0 deg counterclockwise.
Without assumptions on the source constancy, we obtain a rectangular error
box for the transient source with the following corners:
(142.239, 1.362) deg = (09 28 57, +01 21 43) (J2000)
(142.281, 1.327) deg = (09 29 07, +01 19 37) (J2000)
(142.576, 1.677) deg = (09 30 18, +01 40 37) (J2000)
(142.535, 1.712) deg = (09 30 08, +01 42 43) (J2000) 
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 1405 +- 69 mCrab
(4.0-10.0keV, 1 sigma error).
There was no significant excess flux in the previous transit at 03:30 UT
with an upper limit of 20 mCrab.

T. Sakamoto, A. Yoshida, S. Sugita (AGU), Y. Kawakubo (LSU), 
K. Yamaoka (Nagoya U), S. Nakahira (RIKEN), Y. Asaoka (ICRR),
S. Torii, Y. Akaike, K. Kobayashi (Waseda U),
Y. Shimizu, T. Tamura (Kanagawa U), N. Cannady (GSFC/UMBC),
M. L. Cherry (LSU), S. Ricciarini (U of Florence),
P. S. Marrocchesi (U of Siena),
and the CALET collaboration:

The long GRB 230520A (MAXI/GSC detection: Umeki et al., 
GCN Circ. 33831
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 ) was detected in the ground analysis of the CALET 
Gamma-ray Burst Monitor (CGBM) data around 05:03:24 UTC on 20 
May 2023 (referenced to GCN Circ 33831
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 ).
The burst signal was seen by all CGBM detectors.

The burst light curve shows a multi-peaked structure that starts
at T-82.9 sec, peaks at T+9.0 sec, and ends at T+34.9 sec.
The T90 and T50 durations measured by the SGM data are 98.1 +/- 4.3 sec
and 67.5 +/- 3.7 sec (40-1000 keV), respectively.

The ground-processed light curve is available at

http://cgbm.calet.jp/cgbm_trigger/ground/1368594148/

The CALET data used in this analysis are provided by
the Waseda CALET Operation Center located at Waseda University.