GCN Circular 41376
Subject
GRB 250813B: Fermi GBM Observation
Event
Date
2025-08-14T19:17:11Z (8 days ago)
From
Jacob Smith at Fermi-GBM Team <jrs0118@uah.edu>
Via
Web form
Jacob Smith (UAH) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:
At 22:51:11.24 UT on 13 August 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250813B (trigger 776818276/250813952), which was also detected by SVOM/ECLAIRs (W.J. Xie, et al. 2025, GCN 41354).
The Fermi GBM on-ground location is consistent with the SVOM/ECLAIRs position.
An optical afterglow was first reported by Ondrejov D50 (M. Jelinek, et al. 2025, GCN 41353) which also reports an extremely rapid decay (M. Jelinek, et al. 2025, GCN 41355).
A redshift of z=1.752 was report by the VLT (B. Schneider, et al. 2025, GCN 41363).
The angle from the Fermi LAT boresight is 99 degrees.
The GBM light curve consists of multiple emission episodes with a duration (T90)
of about 24.1 s (50-300 keV). The time-averaged spectrum
from T0-3.8 to T0+31.0 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.82 +/- 0.04 and the cutoff energy,
parameterized as Epeak, is 114 +/- 3 keV. A Band function fits equally well with Epeak = 112 +/- 4, alpha = -0.81 +/- 0.04, and beta = -3.67 +/- 0.79.
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+11 s in the 10-1000 keV band is 8.1 +/- 0.3 ph/s/cm^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/