TITLE:   GCN CIRCULAR
NUMBER:  44382
SUBJECT: GRB 260421B: Fermi GBM Observation
DATE:    26/04/21 19:36:16 GMT
FROM:    oindabimukherjee@gmail.com

O. Mukherjee (USRA) reports on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 04:13:21.17 UT on 21 April 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260421B (trigger 798437606/260421176),
which was also detected by SVOM/ECLAIRs (Zhao et al. 2026, GCN 44362),
and VLT/X-shooter with spectroscopic redshift z = 2.115 (Schneider et al. 2026, GCN 44368).
The Fermi GBM on-ground location (GCN 44360) is consistent with the SVOM/ECLAIRs position.

The angle from the Fermi LAT boresight is 58 degrees.

The GBM light curve consists of a single emission episode with a duration (T90)
of about 18 s (50-300 keV). The time-averaged spectrum
from T0-1.6 to T0+24.9 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.11 +/- 0.01 and the cutoff energy,
parameterized as Epeak, is 159 +/- 1 keV.

The event fluence (10-1000 keV) in this time interval is
(1.06 +/- 0.01)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+15 s in the 10-1000 keV band is 15.4 +/- 0.3 ph/s/cm^2.

A Band function fits the spectrum equally well
with Epeak= 130 +/- 1 keV, alpha = -1 +/- 0.005 and beta = -2.31 +/- 0.02.

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/"