Probing the Efficiency of Electron-Proton Coupling in Relativistic Collisionless Shocks through the Radio Polarimetry of Gamma-Ray Burst Afterglows

TL;DR

This paper proposes a method to constrain the electron-proton coupling fraction in gamma-ray burst afterglows by analyzing Faraday rotation effects on radio polarization, which can inform shock physics and energy estimates.

Contribution

It introduces a novel approach using late-time radio polarimetry to measure the electron-proton coupling fraction in GRB shocks, addressing a key uncertainty in GRB energetics.

Findings

Faraday rotation can suppress linear polarization at late times.

ALMA observations can detect these effects for nearby GRBs.

Constraints on the electron-proton coupling fraction are feasible with current technology.

Abstract

The late-time optical/radio afterglows of $\gamma$-ray bursts (GRBs) are believed to be synchrotron emission of electrons accelerated in relativistic collisionless shocks propagating in the ambient medium of the sources. However, the fraction $f$ of electrons that are coupled to protons and accelerated remains unclear and a large number of thermal electrons that are not coupled to protons may be left behind. If $f<1$, the true explosion energies of GRBs are $f^{-1}$ times larger than those commonly estimated with $f=1$. Thus the value of $f$ gives an important constraint on the nature of the central engine of GRBs and the physics of collisionless shocks. Although early-time radio observations can probe the thermal electrons, they are difficult at present. We show that the Faraday rotation effects of the thermal electrons may suppress the linear polarization of the afterglow at frequencies higher than the absorption frequency in the late time, if the magnetic field is ordered at least in parts, and that $f$ can be constrained through the observation of the effects. We find that those effects may be detected with late-time, > 1 day, polarimetry with ALMA for a burst occurring within 1 Gpc (i.e., z ~ 0.2), if $f \sim 10^{-1}$.