Two populations of gamma-ray burst radio afterglows

TL;DR

This study reveals two distinct populations of gamma-ray burst afterglows, radio bright and radio faint, with the former showing higher energies and fluxes, challenging the notion that low radio detection is solely due to sensitivity limits.

Contribution

The paper demonstrates that the low radio detection rate of GRB afterglows is due to two intrinsically different populations, not just observational sensitivity.

Findings

Radio bright GRBs have higher gamma-ray fluence and energies.

Up to 70% of GRB afterglows are truly radio bright.

Radio bright and faint GRBs differ in physical properties.

Abstract

The detection rate of gamma-ray burst (GRB) afterglows is only ~30% at radio wavelengths, much lower than in the X-ray (~95%) or optical (~70%) bands. The cause of this low radio detection rate has previously been attributed to limited observing sensitivity. We use visibility stacking to test this idea, and conclude that the low detection rate is instead due to two intrinsically different populations of GRBs, radio bright and radio faint. We calculate that no more than 70% of GRB afterglows are truly radio bright, leaving a significant population of GRBs that lack a radio afterglow. These radio bright GRBs have higher gamma-ray fluence, isotropic energies, X-ray fluxes and optical fluxes than the radio faint GRBs, confirming the existence of two physically distinct populations. We suggest that the gamma-ray efficiency of the prompt emission is responsible for the difference between the two populations. We also discuss the implications for future radio and optical surveys.