Fermi Gamma-ray Space Telescope

Detection of pair-opacity effects in the prompt emission of the peculiar GRB 100724B

G. Vianello
J. Granot, N. Omodei, J. Cohen-tanugi

Abstract:

The Gamma-Ray Burst 100724B is among the brightest GRBs detected by Fermi/GBM in its 7 years of observations, with a low-energy fluence similar to the very bright GRB 090902B and GRB 090926A. However, its prompt emission is surprisingly faint above 100 MeV. A careful analysis of the data from the Fermi Large Area Telescope indeed reveals a clear spectral cutoff around 30 MeV. When compared to the already published case of GRB 090926A, which was very bright at LAT energies, this GRB differs significantly because the cutoff is at much lower energies and is clearly visible during the whole prompt emission. In a detailed time-resolved analysis of the prompt emission this feature appears to be consistent with theoretical expectations from intrinsic opacity to pair production. We will present Fermi GBM and LAT observations, and the results of our modeling with both phenomenological functions and theoretical models for pair-production opacity. We will also present the results of our time-domain analysis and theoretical implications.