TITLE:   GCN CIRCULAR
NUMBER:  44931
SUBJECT: GRB 260610B: Fermi GBM Observation
DATE:    26/06/13 18:21:44 GMT
FROM:    Matt Godwin <msg0028@uah.edu>

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

"At 23:46:14.25 UT on 10 June 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 260610B (trigger 802827979/260610990).
An optical counterpart is identified by GOTO (O'Neill, et al. 2026, GCN 44903). The redshift of the source is measured to be z = 0.473, ALFOSC/NOT (ONeill et al. 2026, GCN 44914).
The Fermi GBM on-ground location is consistent with the GOTO position.

The angle from the Fermi LAT boresight is 97 degrees.

The GBM light curve consists of three peaks with a duration (T90)
of about 62 s (50-300 keV). The time-averaged spectrum
from T0-2.4 to T0+55.4 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.50 +/- 0.01 and the cutoff energy,
parameterized as Epeak, is 46.8 +/- 0.4 keV.

The event fluence (10-1000 keV) in this time interval is
(1.30 +/- 0.02)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+3.8 s in the 10-1000 keV band is 7.7 +/- 0.3 ph/s/cm^2.

A Band function fits the spectrum equally well
with Epeak= 45.9 +/- 0.5 keV, alpha = -1.4800 +/- 0.0009 and beta = -3.17 +/- 0.01.

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/"