Spectral index-flux relation for investigating the origins of steep decay in $\gamma$-ray bursts

TL;DR

This study uncovers a spectral index-flux relation in GRB X-ray tails, challenging existing models and supporting a proton-synchrotron emission mechanism due to adiabatic cooling.

Contribution

It presents a new spectral index-flux relation in GRB tails, providing evidence for proton-synchrotron emission and challenging the high-latitude emission scenario.

Findings

Spectral index-flux relation observed in GRB X-ray tails.

Relation incompatible with high-latitude emission model.

Supports proton-synchrotron origin due to adiabatic cooling.

Abstract

$\gamma$-ray bursts (GRBs) are short-lived transients releasing a large amount of energy ($10^{51}-10^{53} $ erg) in the keV-MeV energy range. GRBs are thought to originate from internal dissipation of the energy carried by ultra-relativistic jets launched by the remnant of a massive star's death or a compact binary coalescence. While thousands of GRBs have been observed over the last thirty years, we still have an incomplete understanding of where and how the radiation is generated in the jet. Here we show a relation between the spectral index and the flux found by investigating the X-ray tails of bright GRB pulses via time-resolved spectral analysis. This relation is incompatible with the long standing scenario which invokes the delayed arrival of photons from high-latitude parts of the jet. While the alternative scenarios cannot be firmly excluded, the adiabatic cooling of the emitting particles is the most plausible explanation for the discovered relation, suggesting a proton-synchrotron origin of the GRB emission.