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GCN Circular 35238

GRB 231129C: Fermi-LAT detection
2023-11-30T18:39:08Z (6 months ago)
Nicola Omodei at Stanford University <>
M. Arimoto (Kanazawa University), N. Omodei (Stanford University), D. Kocevski (NASA/MSFC) report on behalf of the Fermi-LAT Collaboration:

On November 29, 2023 Fermi-LAT detected high-energy emission from GRB 231129C, which was also detected by Fermi-GBM (trigger 722977823 / 231129799, GCN 35221),  MAXI-GSC (GCN 35223), CALET (GCN 35228), AstroSat (GCN 35230),  GECAM-B (GCN 3523), Glowbug (GCN 35235), and GRBAlpha (GCN 35236).
The best LAT on-ground location is found to be R.A., Dec. = 9.1, -81.9 (degrees, J2000) with an error radius of 0.7 deg (90% containment, statistical error only). 
This was 49 deg from the LAT boresight at the time of the GBM trigger:
T0 = 19:10:18.11 UT.
The data from the Fermi-LAT show a significant increase in the event rate after the GBM trigger that is spatially correlated with the GBM emission with high significance. The photon flux above 100 MeV in the time interval 0-200 s after the GBM trigger is (3.2 +/- 0.9)E-4 ph/cm2/s.
The estimated photon index above 100 MeV is -2.4 +/- 0.4. The highest-energy photon is a 0.7 GeV event which is observed 4 seconds after the GBM trigger.

The Fermi-LAT point of contact for this burst is Feraol Fana Dirirsa (
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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