Spectral index-flux relation for investigating the origins of steep decay in \u03b3-ray bursts

Samuele Ronchini,Francesco Brighenti,Marica Branchesi,Maria Edvige Ravasio,Om Sharan Salafia,Giancarlo Ghirlanda,Gabriele Ghisellini,Paolo D’Avanzo,Stefano Ascenzi,Maria Grazia Bernardini,Simone Dall’Osso,Gor Oganesyan

doi:10.1038/s41467-021-24246-x

Abstract

γ-ray bursts (GRBs) are short-lived transients releasing a large amount of energy (1051 − 1053 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.

Highlights

Considering that the emitting surface of the jet is curved, an on-axis observer first receives photons from the line of sight and later photons from higher latitudes[13,14,15], which are less Doppler boosted
In order to investigate the spectral evolution during the steep decay (SD) phase, we select a sample of GRBs from the archive of the X-ray Telescope (XRT, 0.3–10 keV) on-board the Neil Gehrels Swift Observatory (Swift)[19]
In order to further test the solidity of the α − F relation, we extend our analysis to a second sample of GRBs, which present directly a SD at the beginning of the XRT light curve, instead of an X-ray pulse, often observed in early X-ray afterglows[8,9]

Summary

Introduction

Considering that the emitting surface of the jet is curved, an on-axis observer first receives photons from the line of sight and later photons from higher latitudes[13,14,15], which are less Doppler boosted. This gives rise to the so called high-latitude emission (HLE). We systematically analyze the X-ray spectral evolution during the SD phase as motivated by fact that temporal and spectral evolution during the tail of prompt pulses can provide clues about emission and cooling processes in GRB jets. We conclude discussing the implications for the physics of GRB jets, their composition, and radiation mechanisms

Methods

Results

Conclusion