First Light on GRBs with Fermi

TL;DR

This paper reviews Fermi observations of gamma-ray bursts, compares leptonic and hadronic models based on gamma-ray constraints, and discusses implications for cosmic ray acceleration.

Contribution

It provides a comparative analysis of leptonic and hadronic models for GRB emissions using Fermi data, highlighting energetic favorability and cosmic ray acceleration potential.

Findings

Leptonic models are energetically favored over hadronic models.

Fermi data constrains outflow Lorentz factors in GRBs.

GRBs may accelerate cosmic rays up to 10^{20} eV.

Abstract

Fermi LAT (Large Area Telescope) and GBM (Gamma ray Burst Monitor) observations of GRBs are briefly reviewed, keeping in mind EGRET expectations. Using gamma\gamma constraints on outflow Lorentz factors, leptonic models are pitted against hadronic models, and found to be energetically favored. Interpretation of the Fermi data on GRBs helps establish whether GRBs accelerate cosmic rays, including those reaching $\approx 10^{20}$ eV.