Constraining Magnetization of Gamma-Ray Bursts Outflows using Prompt Emission Fluence

TL;DR

This paper analyzes how magnetic energy dissipation in gamma-ray burst outflows limits thermal photon production, suggesting that afterglow calorimetry can constrain the outflow's magnetization.

Contribution

It provides a theoretical upper limit on thermal photon energy fraction in Poynting flux dominated outflows and discusses how afterglow observations can constrain GRB magnetization.

Findings

Thermal photon energy fraction cannot exceed 30% of kinetic energy.

Non-thermal photon energy during prompt emission is at most equal to the kinetic energy.

Calorimetry during afterglow can constrain outflow magnetization.

Abstract

I consider here acceleration and heating of relativistic outflow by local magnetic energy dissipation process in Poynting flux dominated outflow. Adopting the standard assumption that the reconnection rate scales with the Alfven speed, I show here that the fraction of energy dissipated as thermal photons cannot exceed (13 $\hat \gamma -14)^{-1} = 30$% (for adiabatic index $\hat \gamma = 4/3$) of the kinetic energy at the photosphere. Even in the most radiatively efficient scenario, the energy released as non-thermal photons during the prompt phase is at most equal to the kinetic energy of the outflow. These results imply that calorimetry of the kinetic energy that can be done during the afterglow phase, could be used to constrain the magnetization of gamma-ray bursts (GRB) outflows. I discuss the recent observational status, and its implications on constraining the magnetization in GRB outflows.