GCN Circular 45816
GRB 261005B: Fermi GBM Observation
From: Matt Godwin
Issued: 2026-10-06 04:28 UTC
Type: GRB, Gamma-ray Transient
Matt Godwin (UAH) reports on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 21:39:13.77 UT on 05 October 2026, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 261005B (trigger 812929158/261005902),
which was also detected by Swift BAT (T.M. Parsotan et al. 2026, GCN 45812).
The Fermi GBM on-ground location is consistent with the Swift BAT position.
The angle from the Fermi LAT boresight is 67 degrees.
The GBM light curve consist of multiple peaks with a duration (T90)
of about 22 s (50-300 keV). The time-averaged spectrum
from T0-6.4 to T0+20.6 s is best fit by a power law function with an exponential high-energy cutoff.
The power law index is -0.56 +/- 0.1 and the cutoff energy, parameterized as Epeak, is 110 +/- 8 keV.
The event fluence (10-1000 keV) in this time interval is
(8 +/- 2)E-07 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+13 s in the 10-1000 keV band is 1.56 +/- 0.19 ph/s/cm^2.
The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
FERMIGBRST - Fermi GBM Burst Catalog (heasarc.gsfc.nasa.gov)
For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/";
Every one of the gamma-ray bursts GCN reports is posted here.
Relayed automatically from NASA's General Coordinates Network.
Transient Alerts » in reply to GRB 261005B
In plain words Routine
On the evening of 5 October 2026 (UTC), two space telescopes, NASA's Swift and Fermi, both caught a burst of gamma rays (the most energetic form of light) from a spot in the sky near the border of the constellations Ophiuchus and Hercules. The burst flickered with several peaks and lasted somewhere between about 20 and 90 seconds, depending on which instrument and energy range measured it. Swift's X-ray telescope found a fading X-ray afterglow and pinned its position to within about 2 arcseconds, but ultraviolet, optical and infrared searches hours later found no visible afterglow. No distance has been reported, so how far away it is remains unknown. It looks like an ordinary long gamma-ray burst, the kind usually linked to the death of a massive star.
How unusual is it? This is a typical gamma-ray burst: moderate brightness, an X-ray afterglow found, no visible-light afterglow and no distance measured. A redshift (distance) measurement or a supernova or other counterpart found later would make it more notable.
Written by an AI model (claude-sonnet-5-5) from the official reports in this thread; last updated . It may contain mistakes: the reports themselves are the authority.
GRB 261005B: Fermi GBM Observation