GCN Circular 44526

W. Iwakiri (Chiba U.),　M. Serino(AGU), 
H. Negoro, M. Nakajima, K. Saito, T. Yokoyama, M. Wada (Nihon U.), 
T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), 
T. Sakamoto, S. Sugita, Y. Kawakubo, H. Hiramatsu, A. Yoshida (AGU), 
Y. Tsuboi, Y. Ishihara, H. Ogaki (Chuo U.), 
M. Shidatsu, C. Kang, M. Uenishi, T. Usuki, S. Yatsuzuka, H. Aoe (Ehime U.),
I. Takahashi, Y. Yatsu (Science Tokyo), 
S. Nakahira, S. Ueno, H. Tomida, M. Kurihara (JAXA), 
Y. Ueda, K. Fujiwara, S. Kobayashi (Kyoto U.), 
M. Yamauchi, M. Nishio, C. Hiraizumi (Miyazaki U.), 
K. Yamaoka (Nagoya U.),
M. Sugizaki (Kanazawa U.),
T. Kawamuro (Osaka U.), 
S. Yamada (Tohoku U),
S. Ogawa (TUS)
report on behalf of the MAXI team:

At 17:13:42 UT on 2026 May 10, the MAXI/GSC nova alert system triggered 
a bright uncatalogued X-ray transient source. The trigger time and 
position are consistent with GRB 260510C reported by the Fermi team 
(GCN #44520
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 ) and the SVOM team (GCN #44521
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 ). 

The X-ray flux averaged over the scan was 338 +/- 36 mCrab (4.0-10.0 keV, 
1-sigma error).

Assuming that the source flux was constant over the transit, we obtain 
the source position at:
(R.A., Dec) = (157.881 deg, 2.824 deg) = (10 31 31, +02 49 26) (J2000)
with a statistical 90% C.L. elliptical error region with long and short 
radii of 0.17 deg and 0.14 deg, respectively. The roll angle of the long 
axis is 73.0 deg counterclockwise from the north. There is an additional 
systematic uncertainty of 0.1 deg (90% containment radius).

Without assumptions on the source constancy, we obtain a rectangular 
error box for the transient source with the following corners:
(157.267, 2.290) deg = (10 29 04, +02 17 24) (J2000)
(157.477, 2.117) deg = (10 29 54, +02 07 01) (J2000)
(158.505, 3.360) deg = (10 34 01, +03 21 35) (J2000)
(158.295, 3.533) deg = (10 33 10, +03 31 58) (J2000) 

There was no significant excess flux in the previous transit at 15:40 UT 
with an upper limit of 20 mCrab.