The unusual radio afterglow of the ultra-long gamma-ray burst GRB 130925A

TL;DR

This paper reports the discovery of an unusual radio afterglow from the ultra-long gamma-ray burst GRB 130925A, revealing unique spectral features and potential implications for electron acceleration mechanisms and progenitor models.

Contribution

It provides the first comprehensive radio dataset for an ultra-long GRB, identifying a steep spectral cut-off and a secondary component, suggesting complex shock interactions and electron distributions.

Findings

Radio emission peaked at 7 GHz 2.2 days post-burst

Radio spectrum above 10 GHz shows a steep cut-off

Evidence of both reverse and forward shock contributions

Abstract

GRB 130925A is one of the recent additions to the growing family of ultra-long GRBs (T90$ \gtrsim 1000$ s). While the X-ray emission of ultra-long GRBs have been studied extensively in the past, no comprehensive radio dataset has been obtained so far. We report here the early discovery of an unusual radio afterglow associated with the ultra-long GRB 130925A. The radio emission peaks at low-frequencies ($\sim 7$ GHz) at early times, only $2.2$ days after the burst occurred. More notably, the radio spectrum at frequencies above $10$ GHz exhibits a rather steep cut-off, compared to other long GRB radio afterglows. This cut-off can be explained if the emitting electrons are either mono-energetic or originate from a rather steep, $dN/dE \propto E^{-4}$, power-law energy distribution. An alternative electron acceleration mechanism may be required to produce such an electron energy distribution. Furthermore, the radio spectrum exhibits a secondary underlying and slowly varying component. This may hint that the radio emission we observed is comprised of emission from both a reverse and a forward shock. We discuss our results in comparison with previous works that studied the unusual X-ray spectrum of this event and discuss the implications of our findings on progenitor scenarios.