TITLE:   GCN CIRCULAR
NUMBER:  43624
SUBJECT: GRB 260206A (or XRB from 4U 0513-40): MAXI/GSC detection
DATE:    26/02/06 11:14:51 GMT
FROM:    Hitoshi Negoro at Nihon University/MAXI team <negoro.hitoshi@nihon-u.ac.jp>

H. Negoro, M. Nakajima, K. Saito (Nihon U.), T. Usuki (Ehime U.),
K. Takagi, H. Nishio (Nihon U.), T. Mihara, T. Tamagawa, N. Kawai, M. Matsuoka (RIKEN), 
T. Sakamoto, M. Serino, S. Sugita, Y. Kawakubo, H. Hiramatsu, Y. Kondo, D. Iijima, A. Yoshida (AGU), 
Y. Tsuboi, H. Sugai, N. Nagashima, Y. Ishihara (Chuo U.), M. Shidatsu, C. Kang, T. Nakamoto, 
M. Uenishi, S. Yatsuzuka (Ehime U.),I. Takahashi, Y. Yatsu (Science Tokyo), 
S. Nakahira, S. Ueno, H. Tomida, S. Ogawa, 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.), W. Iwakiri (Chiba U.), T. Kawamuro (Osaka U.), 
and S. Yamada (Tohoku U),

The MAXI/GSC nova alert system triggered on a bright uncatalogued hard X-ray transient source
at 08:47:33 UT on February 6, 2026.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (78.322 deg, -40.534 deg) = (05 13 17, -40 32 02) (J2000) 
with a statistical 90% C.L. elliptical error region 
with long and short radii of 0.33 deg and 0.23 deg, respectively. 
The roll angle of the long axis from the north direction is 38.0 deg counterclockwise. 
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:
(77.845, -39.759) deg = (05 11 22, -39 45 32) (J2000)
(77.278, -40.246) deg = (05 09 06, -40 14 45) (J2000)
(79.005, -41.397) deg = (05 16 01, -41 23 49) (J2000)
(79.569, -40.903) deg = (05 18 16, -40 54 10) (J2000)
The X-ray flux averaged over the scan was 244 +- 33 mCrab
(4.0-10.0keV, 1 sigma error).
There was no significant excess flux in the previous transit at 07:36 UT 
and in the next transit at 10:22 UT with an upper limit of 20 mCrab for each.

We note that the X-ray burster 4U 0513-40 is close to the above position, but out of the error box.
If the systematic error is large this time, the burst may come from 4U 0513-40.