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GCN Circular 14101

GRB 121225B: Fermi LAT Detection
2012-12-25T15:38:34Z (11 years ago)
Daniel Kocevski at SLAC <>
D. Kocevski (NASA/GSFC U.), J. L. Racusin (NASA/GSFC) and E. Sonbas (Adiyaman Univ.), report on behalf of the Fermi-LAT team:

The Fermi Large Area Telescope (LAT) detected emission from GRB 121225B (GBM  trigger 378122456/121225417) at approximately 10:00:54 UT on December 25th, 2012.

The burst location was at the edge of the the LAT field of view at an angle of ~65 degrees to the LAT boresight, and had a zenith angle of 92 degrees. No significant excess is seen using standard analysis procedures.  The burst exited the nominal LAT field of view roughly ~100 s after the GBM trigger and remained above an angle of 70 degrees to the LAT boresight for at least the next 5000 s.

Using the non-standard LAT Low Energy (LLE) data selection, over 100 counts above background were detected in a single FRED like pulse, peaking 20s after the GBM trigger and coinciding with the time of the GBM emission, with a significance of 10 sigma. This detection is due to low energy gamma-rays, below 75 MeV, and therefore has insufficient spatial resolution to provide a reliable LAT localization. Indeed, no events were observed above 75 MeV using the standard analysis classes.

A GBM circular on GRB 121225B is forthcoming.

The Fermi LAT point of contact for this burst is Daniel Kocevski (daniel.kocevski at

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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