GRB 260616C

Yang-Zhao Ren, Chen-Wei Wang, Shao-Lin Xiong (IHEP) report on behalf of GECAM team:

GECAM-B was triggered on-ground by GRB 260616C, at 2026-06-16T14:47:01.950 UTC (denoted as T0), which is also detected by MAXI/GSC (Iwakiri et al., GCN #44961
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 ). 

According to the GECAM-B light curves in about 70-6000 keV, this burst mainly consists of a pulse with a duration (T90) of 29.5 +3.5/-4.5 s.

The GECAM-B light curve can be found here:
https://www.bursthub.cn//admin/static/gecambgrb260616C.png

The time-averaged spectrum from T0-4 s to T0+8 s is best fitted by a power law function. The power law index is -2.32 +0.16/-0.13. The event fluence (10-1000 keV) in this time interval is (1.99 +0.30/-0.26)E-04 erg/cm^2.

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).

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:

The MAXI/GSC detected a bright uncatalogued X-ray transient source at 14:47 UT 
on June 16 2026.
Assuming that the source flux was constant over the transit,
we obtain the source position at
(R.A., Dec) = (67.745 deg, 9.294 deg) = (04 30 58, +09 17 38) (J2000)
with a statistical 90% C.L. elliptical error region
with long and short radii of 0.39 deg and 0.3 deg, respectively.
The roll angle of long axis from the north direction is 22.0 deg counterclockwise.
There is an additional systematic uncertainty of 0.1 deg (90% containment radius).
The X-ray flux averaged over the scan was 148 +- 33 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:
(66.883, 10.254) deg = (04 27 31, +10 15 14) (J2000)
(66.471, 9.596) deg = (04 25 53, +09 35 45) (J2000)
(68.604, 8.290) deg = (04 34 24, +08 17 23) (J2000)
(69.019, 8.945) deg = (04 36 04, +08 56 42) (J2000) 

A marginal excess of about 50 mCrab may be present in the previous and next MAXI scans at 13:14 UT and 16:20 UT, respectively. However, these scans were obtained during a period of large background variations, and the significance of the excesses is currently unclear.
The possible presence of X-ray activity over multiple scans raises the possibility that this event is a new X-ray transient source rather than a GRB. We therefore tentatively name the source MAXI J0430+092. Multi-wavelength follow-up observations are encouraged.