TITLE: GCN CIRCULAR NUMBER: 40612 SUBJECT: GRB 250602A: Fermi GBM Observation DATE: 25/06/03 01:47:35 GMT FROM: Jacob Smith at Fermi-GBM Team jrs0118@uah.edu

Jacob Smith (UAH) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:

At 03:04:31.31 UT on 02 June 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250602A (trigger 770526276/250602128). which was also detected by Fermi-LAT (A. Holzmann Airasca, et al. 2025, GCN 40611). The Fermi GBM on-ground location is consistent with the Fermi-LAT position.

The angle from the Fermi LAT boresight is 111 degrees.

The GBM light curve consists of a single emission episode with a duration (T90) of about 14.6 s (50-300 keV). The time-averaged spectrum from T0+0.003 to T0+8.768 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.9 +/- 0.1 and the cutoff energy, parameterized as Epeak, is 120 +/- 10 keV.

The event fluence (10-1000 keV) in this time interval is (1.7 +/- 0.1)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+0.0 s in the 10-1000 keV band is 5.8 +/- 0.3 ph/s/cm^2.

A Band function fits the spectrum equally well with Epeak= 102 +/- 22 keV, alpha = -0.7 +/- 0.3 and beta = -2.2 +/- 0.2.

The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog: https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support Page: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/

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