Constraints on the bulk Lorentz factor of Gamma-Ray Burst jets from Fermi/LAT upper limits

TL;DR

This paper introduces a new method to constrain the maximum Lorentz factor of Gamma-Ray Burst jets using Fermi/LAT upper limits on high-energy emission, suggesting typical values around a few hundred.

Contribution

The study presents a novel approach to estimate upper limits on the Lorentz factor of GRB jets from non-detections in Fermi/LAT data, aligning with previous lower limits.

Findings

Upper limits on Gamma_0 are around 200 for homogeneous media.

For wind-like media, upper limits range from 100 to 400.

Results support that typical GRB jet Lorentz factors are a few hundred.

Abstract

It is largely recognized that Gamma-Ray Burst (GRB) jets involve ultra-relativistic motion. However, the value of the Lorentz factor Gamma_0 is still not clear and only lower limits are known for most bursts. We suggest here a new method to obtain upper limits on Gamma_0. The early high-energy synchrotron afterglow flux depends strongly on Gamma_0. Upper limits on GeV emission therefore provide uppers limit on Gamma_0. Applying this method to 190 Fermi GRBs that have not been detected by the Fermi-LAT we place upper limits on the high-energy afterglow flux, and in turn on Gamma_0. For bursts at a typical redshift z=2, we find values of the order of 200 (and above) for a homogeneous density medium, and in the range 100-400 for a wind-like medium. These upper limits are consistent with (and are very close to) lower limits and direct estimates inferred using other methods, suggesting that the typical Lorentz factors of GRB jets are of order a few hundred.