The 2009 Fermi Symposium

The Collimation and Energetics of Fermi-LAT Gamma-Ray Bursts

Stephen Cenko, University of California, Berkeley

Abstract:

We present broadband (X-ray, optical, and radio) observations of the afterglows of three gamma-ray bursts (GRBs 090323, 090328, and 090902B) detected by the Large Area Telescope on-board Fermi. The tremendous isotropic energy release from these events (∼ 10⁵⁴,erg) make them ideal for testing various GRB progenitor models. Our late-time observations, particularly in the radio, allow us to constrain the collimation of these ultra-relativistic outflows, and hence to calculate the beaming-corrected energy release (both from the prompt emission and powering the afterglow) for each event. Interestingly, all three events appear to have exploded in relatively low density environments. We speculate this may be caused by the low angular momentum loss from the progenitor required for the large initial Lorentz factors implied by the detection of GeV photons.