Probing Massive Stars Around Gamma-Ray Burst Progenitors

TL;DR

This paper investigates how external photon fields around massive star progenitors of gamma-ray bursts influence observable high-energy emissions, providing a method to infer progenitor environments through gamma-ray observations.

Contribution

It introduces detailed calculations of inverse-Compton scattering in various progenitor environments, linking gamma-ray signals to progenitor properties and surroundings.

Findings

Expected gamma-ray luminosities range from 10^46 to 10^50 erg/s.

Lightcurves and spectra depend on progenitor environment parameters.

Observations can constrain progenitor types and jet dissipation sites.

Abstract

Long Gamma-Ray Bursts (GRBs) are produced by ultra-relativistic jets launched from core collapse of massive stars. Most massive stars form in binaries and/or in star clusters, which means that there may be a significant external photon field (EPF) around the GRB progenitor. We calculate the inverse-Compton scattering of EPF by the hot electrons in the GRB jet. Three possible cases of EPFs are considered: the progenitor is (I) in a massive binary system, (II) surrounded by a Wolf-Rayet-star wind, and (III) in a dense star cluster. Typical luminosities of 10^46 - 10^50 erg/s in the 1 - 100 GeV band are expected, depending on the stellar luminosity, binary separation (I), wind mass loss rate (II), stellar number density (III), etc. We calculate the lightcurve and spectrum in each case, taking fully into account the equal-arrival time surfaces and possible pair-production absorption with the prompt gamma-rays. Observations can put constraints on the existence of such EPFs (and hence on the nature of GRB progenitors) and on the radius where the jet internal dissipation process accelerates electrons.