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GCN Circular 13756

GRB 120911B: Fermi-LAT detection of a burst
2012-09-12T22:29:28Z (12 years ago)
Judith Racusin at GSFC <>
J. L. Racusin (NASA/GSFC), G. Vianello (SLAC), D. Kocevski (NASA/GSFC), N. Omodei (Stanford), and M. Ohno (ISAS/JAXA), report on behalf of the Fermi-LAT team:

We report on ground analysis of the IPN detected GRB 120911B (GCN 13755) at 06:26:15 UT. The center of the IPN error box was at the edge of the LAT field of view, 67 degrees from the LAT boresight at the trigger time. The best LAT localized position is RA/Dec = 172.03, -37.51 (J2000), with a error radius of 0.3 deg (68% containment), using LAT source class events from 0-40 s after the trigger. This is within the IPN error box, 0.2 deg from the center.

The burst left the LAT field of view ~100 seconds after the trigger and returned ~4100 s later.

The LAT data show a significant increase in counts at energies >100 MeV, starting several seconds after the trigger, with a single peaked structure. The non-standard LAT Low Energy (LLE) event class shows two-peaks with a tail lasting to approximately T0+60 s.

A spectral fit of the LAT data in the 0-40 s window yields a photon index of -2.5 +/- 0.2 (68% containment, statistical only) and a flux of (2.0 +/- 0.4) x 10^-7 ph/s-cm^2 in the 100 MeV - 10 GeV energy range.

Fermi-GBM did not trigger on this GRB due to high geo-magnetic latitude, but it was clearly detectable in ground analysis. The GBM team will describe the observations in a subsequent circular.

The Fermi LAT point of contact for this burst is Masanori Ohno (

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.
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