"Self-absorbed" GeV light-curves of Gamma-Ray Burst afterglows

TL;DR

This paper studies how photon-photon absorption affects the high-energy light-curves and spectra of gamma-ray burst afterglows, revealing different behaviors depending on the surrounding medium and energy injection.

Contribution

It provides a detailed analysis of absorption effects on GRB afterglows, highlighting how medium type and energy injection influence observable light-curve features and spectral evolution.

Findings

Optical thickness decreases before peak in wind-like media, causing spectral hardening.

Optical thickness increases in homogeneous media, leading to a peak and spectral softening.

Observed LAT afterglows rarely show the predicted spectral softening.

Abstract

We investigate the effect that the absorption of high-energy (above 100 MeV) photons produced in GRB afterglow shocks has on the light-curves and spectra of Fermi-LAT afterglows. Afterglows produced by the interaction of a relativistic outflow with a wind-like medium peak when the blast-wave deceleration sets in, and the afterglow spectrum could be hardening before that peak, as the optical thickness to pair-formation is decreasing. In contrast, in afterglows produced in the interaction with a homogeneous medium, the optical thickness to pair-formation should increase and yield a light-curve peak when it reaches unity, followed by a fast light-curve decay, accompanied by a spectral softening. If energy is injected in the blast-wave, then the accelerated increase of the optical thickness yields a convex afterglow light-curve. Other features, such as a double-peak light-curve or a broad hump, can arise from the evolution of the optical thickness to photon-photon absorption. Fast decays and convex light-curves are seen in a few LAT afterglows, but the expected spectral softening is rarely seen in (and difficult to measure with) LAT observations. Furthermore, for the effects of photon-photon attenuation to shape the high-energy afterglow light-curve without attenuating it too much, the ejecta initial Lorentz factor must be in a relatively narrow range (50-200), which reduces the chance of observing those effects.