Off-axis MeV and very-high-energy gamma-ray emissions from structured gamma-ray burst jets

TL;DR

This paper investigates how off-axis observations of gamma-ray burst jets produce energy-dependent emission zones and time delays, emphasizing the importance of jet structure and photon attenuation for future very-high-energy gamma-ray observations.

Contribution

It reveals that different energy photons originate from distinct regions in structured jets, challenging the one-zone approximation and highlighting the significance of off-axis emission effects.

Findings

Energy-dependent emission zones for MeV and TeV photons.

Off-axis zone-shift causes significant time delays in VHE photons.

Jet structure influences the location and attenuation of high-energy emissions.

Abstract

Very-high-energy (VHE) photons around TeV energies from a gamma-ray burst (GRB) jet will play an essential role in the multi-messenger era, with a fair fraction of the events being observed off-axis to the jet. We show that different energy photons (MeV and TeV photons in particular) arrive from different emission zones for off-axis observers even if the emission radius is the same. The location of the emission region depends on the jet structure of the surface brightness, and the structures are generally different at different energies, mainly due to the attenuation of VHE photons by electron-positron pair creation. This off-axis zone-shift effect does not justify the usual one-zone approximation and also produces a time-delay of VHE photons comparable to the GRB duration, which is crucial for future VHE observations, such as by the Cherenkov Telescope Array.