Possible Effects of Pair Echoes on Gamma-Ray Burst Afterglow Emission

TL;DR

This paper explores how pair echoes from high-energy gamma-ray bursts can influence observed afterglow spectra and light curves, providing insights into intergalactic magnetic fields and high-energy emission mechanisms.

Contribution

It demonstrates the potential impact of pair echoes on GRB afterglow observations and discusses how non-detections can constrain intergalactic magnetic field strength.

Findings

Pair echoes can significantly affect high-energy afterglow spectra.

Detection of pair echoes depends on the strength of the intergalactic magnetic field.

Non-detections can place constraints on the intergalactic magnetic field strength.

Abstract

High-energy emission from gamma-ray bursts (GRBs) is widely expected but had been sparsely observed until recently when the Fermi satellite was launched. If >TeV gamma rays are produced in GRBs and can escape from the emission region, they are attenuated by the cosmic infrared background photons, leading to regeneration of GeV-TeV secondary photons via inverse-Compton scattering. This secondary emission can last for a longer time than the duration of GRBs, and it is called a pair echo. We investigate how this pair echo emission affects spectra and light curves of high energy afterglows, considering not only prompt emission but also afterglow as the primary emission. Detection of pair echoes is possible as long as the intergalactic magnetic field (IGMF) in voids is weak. We find (1) that the pair echo from the primary afterglow emission can affect the observed high-energy emission in the afterglow phase after the jet break, and (2) that the pair echo from the primary prompt emission can also be relevant, but only when significant energy is emitted in the TeV range, typically E_{gamma, >0.1 TeV} > (Y/(1+Y)) epsilon_e E_k. Even non-detections of the pair echoes could place interesting constraints on the strength of IGMF. The more favorable targets to detect pair echoes may be the "naked" GRBs without conventional afterglow emission, although energetic naked GRBs would be rare. If the IGMF is weak enough, it is predicted that the GeV emission extends to >30-300 s.