GCN 44773: GRB 260601B: CALET Gamma-Ray Burst Monitor detection

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

The CALET Gamma-ray Burst Monitor (CGBM) detected GRB 260601B
at 19:12:46.66 UTC on 1 June 2026 (trigger #1464376270;
https://cgbm.calet.jp/cgbm_trigger/flight/1464376270/index.html).
A CGBM Notice was distributed in near real time.

This event was also reported by GECAM-B (Yu et al., GCN #44767;
Wang et al., GCN #44768) and NuSTAR (Waratkar et al., GCN #44771).

The burst signal was seen by all CGBM detectors.
The burst light curve shows a multi-peaked structure starting
at T+0.1 s, peaking at T+4.9 s, and ending at T+101.4 s.
The T90 and T50 durations measured with the SGM data are 70.0 +/- 4.7 s
and 21.9 +/- 1.0 s in the 40-1000 keV band, respectively.

Einstein Probe/WXT detected an X-ray transient, EP 260601a
(Yang et al., GCN #44766), about 35 s after the CGBM trigger.
The position of EP 260601a was above the Earth horizon as seen from
CALET, and was observable by CGBM at the trigger time, with an incident
angle of 83 degrees.

The ground-processed light curve is available at:
https://cgbm.calet.jp/cgbm_trigger/ground/1464376270/

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