Gamma Ray Burst reverse shock emission in early radio afterglows

TL;DR

This paper models the radio reverse shock emission in gamma-ray burst afterglows, emphasizing the effects of synchrotron self-absorption and magnetization on detectability with current and future radio telescopes.

Contribution

It provides detailed calculations of self-absorbed radio reverse shock emission under various conditions, highlighting the impact of magnetization and medium density on observability.

Findings

Radio RS emission is suppressed by self-absorption at low frequencies.

Detectability of RS emission depends on medium density and magnetization.

Most RS emissions are undetectable below 1 GHz unless the environment is very low density.

Abstract

Reverse shock (RS) emission from Gamma Ray Bursts is an important tool in investigating the nature of the ejecta from the central engine. If the ejecta magnetization is not high enough to suppress the RS, a strong RS emission component, usually peaking in the optical/IR band early on, would give important contribution to early afterglow light curves. In the radio band, synchrotron self-absorption may suppress early RS emission, and also delay the RS peak time. In this paper, we calculate the self-absorbed RS emission in the radio band for different dynamical conditions. In particular, we stress that the RS radio emission is subject to self-absorption in both reverse and forward shocks. We calculate the ratio between the reverse to forward shock flux at the RS peak time for different frequencies, which is a measure of the detectability of the RS emission component. We then constrain the range of physical parameters for a detectable RS, in particular the role of magnetization. We notice that unlike optical RS emission which is enhanced by moderate magnetization, a moderately magnetized ejecta does not necessarily produce a brighter radio RS due to the self-absorption effect. For typical parameters, the RS emission component would not be detectable below 1 GHz unless the medium density is very low (e.g. $n < 10^{-3} ~{\rm cm^{-3}}$ for ISM and $A_* < 5\times 10^{-4}$ for wind). These predictions can be tested with the afterglow observations with current and upcoming radio facilities such as JVLA, LOFAR, FAST, and SKA.