TITLE:   GCN CIRCULAR
NUMBER:  44831
SUBJECT: GRB 260604C: Fermi GBM Observation
DATE:    26/06/05 02:49:32 GMT
FROM:    eliza.neights@gmail.com

E. Neights (GWU, NASA GSFC) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 20:18:44.41 UT on 04 June 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260604C (trigger 802297129/260604846),
which was also detected by SVOM ECLAIRs (Gotz et al. 2026, GCN 44823), 
MASTER (Lipunov et al. 2026, GCN 44827), and LAST (Konno et al. 2026, GCN 44828).
The Fermi GBM on-ground location is consistent with the SVOM ECLAIRs position.

The angle from the Fermi LAT boresight is 102 degrees.

The GBM light curve consists of a single emission episode with a duration (T90)
of about 6.4 s (50-300 keV). The time-averaged spectrum
from T0-2.2 to T0+12.0 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.03 +/- 0.01 and the cutoff energy,
parameterized as Epeak, is 551 +/- 2 keV.

The event fluence (10-1000 keV) in this time interval is
(2.55 +/- 0.05)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+4.5 s in the 10-1000 keV band is 44.1 +/- 0.6 ph/s/cm^2.

A Band function fits the spectrum equally well
with Epeak= 394 +/- 3 keV, alpha = -0.95 +/- 0.01 and beta = -2 +/- 0.03.

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/"