Gamma-ray burst jets: uniform or structured?

TL;DR

This paper investigates how different jet structures in Gamma-Ray Bursts influence their luminosity functions, revealing that both uniform and steep structured jets can fit observed data and introducing the concept of apparent jet structure.

Contribution

It demonstrates that various jet structures, including uniform and steeply structured jets, can reproduce observed luminosity functions and introduces the idea of apparent structure differing from intrinsic structure.

Findings

Both low and high luminosity GRBs can be described by a single luminosity function.

Uniform and steep structured jets can fit current luminosity function data.

Apparent jet structure can differ significantly from intrinsic structure, especially for Gaussian jets.

Abstract

The structure of Gamma-Ray Burst (GRB) jets impacts on their prompt and afterglow emission properties. Insights into the still unknown structure of GRBs can be achieved by studying how different structures impact on the luminosity function (LF): i) we show that low ($10^{46} < L_{\rm iso} < 10^{48}$ erg/s) and high (i.e. with $L_{\rm iso} > 10^{50}$ erg/s) luminosity GRBs can be described by a unique LF; ii) we find that a uniform jet (seen on- and off-axis) as well as a very steep structured jet (i.e. $\epsilon(\theta) \propto \theta^{-s}$ with $s > 4$) can reproduce the current LF data; iii) taking into account the emission from the whole jet (i.e. including contributions from mildly relativistic, off-axis jet elements) we find that $E_{\rm iso}(\theta_{\rm v})$ (we dub this quantity "apparent structure") can be very different from the intrinsic structure $\epsilon(\theta)$: in particular, a jet with a Gaussian intrinsic structure has an apparent structure which is more similar to a power law. This opens a new viewpoint on the quasi-universal structured jet hypothesis.