The Evolution of Gamma-ray Burst Jet Opening Angle through Cosmic Time

TL;DR

This paper investigates how gamma-ray burst jet opening angles evolve over cosmic time, suggesting that higher redshift lGRBs are more narrowly beamed due to progenitor star density and selection effects, aligning with stellar evolution models.

Contribution

It provides a comprehensive analysis linking jet opening angle evolution with progenitor star properties and cosmic time, offering a new explanation for observed anti-correlations.

Findings

Higher redshift lGRBs have narrower jets due to denser progenitor stars.

A strong correlation exists between gamma-ray luminosity and jet opening angle.

The observed anti-correlation can be explained by progenitor evolution and selection effects.

Abstract

Jet opening angles of long gamma-ray bursts (lGRBs) appear to evolve in cosmic time, with lGRBs at higher redshifts being on average more narrowly beamed than those at lower redshifts. We examine the nature of this anti-correlation in the context of collimation by the progenitor stellar envelope. First, we show that the data indicate a strong correlation between gamma-ray luminosity and jet opening angle, and suggest this is a natural selection effect - only the most luminous GRBs are able to successfully launch jets with large opening angles. Then, by considering progenitor properties expected to evolve through cosmic time, we show that denser stars lead to more collimated jets; we argue that the apparent anti-correlation between opening angle and redshift can be accounted for if lGRB massive star progenitors at high redshifts have higher average density compared to those at lower redshifts. This may be viable for an evolving IMF - under the assumption that average density scales directly with mass, this relationship is consistent with the form of the IMF mass evolution suggested in the literature. The jet angle-redshift anti-correlation may also be explained if the lGRB progenitor population is dominated by massive stars at high redshift, while lower redshift lGRBs allow for a greater diversity of progenitor systems (that may fail to collimate the jet as acutely). Overall, however, we find both the jet angle-redshift anti-correlation and jet angle-luminosity correlation are consistent with the conditions of jet launch through, and collimation by, the envelope of a massive star progenitor.