Structure of Gamma-Ray Burst jets: intrinsic versus apparent properties

TL;DR

This paper introduces the concept of apparent jet structure in gamma-ray bursts, linking it to intrinsic structure, and demonstrates how a Gaussian intrinsic profile results in a power-law apparent structure, impacting GRB observations and models.

Contribution

It defines the apparent structure of GRB jets, shows how intrinsic Gaussian structures produce power-law apparent structures, and explores implications for GRB observations and models.

Findings

Gaussian intrinsic structure yields power-law apparent structure

Model consistent with GRB luminosity function constraints

Predicts fewer orphan afterglows than standard models

Abstract

With this paper we introduce the concept of apparent structure of a GRB jet, as opposed to its intrinsic structure. The latter is customarily defined specifying the functions epsilon(theta) (the energy emitted per jet unit solid angle) and Gamma(theta) (the Lorentz factor of the emitting material); the apparent structure is instead defined by us as the isotropic equivalent energy E_iso(theta_v) as a function of the viewing angle theta_v. We show how to predict the apparent structure of a jet given its intrinsic structure. We find that a Gaussian intrinsic structure yields a power law apparent structure: this opens a new viewpoint on the Gaussian (which can be understood as a proxy for a realistic narrow, well collimated jet structure) as a possible candidate for a quasi-universal GRB jet structure. We show that such a model (a) is consistent with recent constraints on the observed luminosity function of GRBs; (b) implies fewer orphan afterglows with respect to the standard uniform model; (c) can break out the progenitor star (in the collapsar scenario) without wasting an unreasonable amount of energy; (d) is compatible with the explanation of the Amati correlation as a viewing angle effect; (e) can be very standard in energy content, and still yield a very wide range of observed isotropic equivalent energies.