GCN Circular 15268
Subject
Fermi LAT Upper Limits on GRB 130925A
Event
Date
2013-09-28T00:03:12Z (12 years ago)
From
Daniel Kocevski at SLAC <dankocevski@gmail.com>
D. Kocevski, J. Racusin (NASA/GSFC), G. Vianello (Stanford), M. Axelsson
(Stockholm University), Nicola Omodei (Stanford) report on behalf of the
Fermi-LAT team:
A preliminary examination of the Fermi LAT data of the field containing GRB
130925A (Lien, et al., GCN 15246) has revealed no significant emission
above 40 MeV. The GRB was located ~22 degrees away from the LAT boresight
at the time of the first GBM detected emission associated with this burst
at 03:56:23.29 UT on 25 September 2013 (GCN 15255), hereafter T0(GBM), and
the subsequent autonomous repoint ensured that the source remained in the
field-of-view for the next ~2000s until it was occulted by the Earth.
Using an unbinned likelihood analysis, we estimate the following LAT upper
limits at 95% confidence for three separate time intervals, one covering
the entire 2000s that the source stayed in the LAT field-of-view, and two
of which correspond to the first and third emission episodes detected by
INTEGRAL/SPI-ACS (GCN 15259) and Konus-Wind (GCN 15260). We note that the
source was not in the LAT field-of-view during the second interval reported
in GCNs 15259 & 15260.
Interval 1: T0(GBM) to T0(GBM)+2000 = 7.3e-07 photons cm-2 s-1 (4.8e-10
ergs cm-2 s-1)
Interval 2: T0(GBM)+900s to T0(GBM)+1100s = 5.5e-06 photons cm-2 s-1
(3.6e-09 ergs cm-2 s-1)
Interval 3: T0(GBM)+4900 to T0(GBM)+5400 = 2.5e-06 photons cm-2 s-1
(1.6e-09 ergs cm-2 s-1)
This analysis covers an energy range of 0.1 to 10 GeV with a 12 deg
extraction region centered on the best known source position (GCN 15246).
The upper limits are computed using the P7SOURCE_V6 instrument response
functions, where we assume a power-law source spectrum fixed to a photon
index of -2.1, reflecting a typical value for high-energy emission in GRBs.
For reference, T0(GBM) = T0(BAT)-900s.
The Fermi LAT point of contact for this burst is Magnus Axelsson (
magnusa@astro.su.se).
The Fermi LAT is a pair conversion telescope designed to cover the energy
band from 20 MeV to greater than 300 GeV. It is the product of an
international collaboration between NASA and DOE in the U.S. and many
scientific institutions across France, Italy, Japan and Sweden.