Optical Flashes from Internal Pairs Formed in Gamma-Ray Burst Afterglows

TL;DR

This paper presents a numerical method to calculate the energy distribution of pairs formed in gamma-ray burst afterglows, linking pair formation to observable gamma-ray data and predicting optical counterparts based on source parameters.

Contribution

It introduces a formalism for internal pair production in GRB afterglows, connecting pair formation to gamma-ray observations and source Lorentz factor, with implications for optical counterpart predictions.

Findings

Bright optical counterparts (R < 10) can last up to 100 seconds.

The number of pairs decreases sharply with higher Lorentz factors.

Pairs from internal processes are unlikely if Lorentz factor exceeds several hundreds.

Abstract

We develop a numerical formalism for calculating the distribution with energy of the (internal) pairs formed in a relativistic source from unscattered MeV--TeV photons. For GRB afterglows, this formalism is more suitable if the relativistic reverse-shock that energizes the ejecta is the source of the GeV photons. The number of pairs formed is set by the source GeV output (calculated from the Fermi-LAT fluence), the unknown source Lorentz factor, and the unmeasured peak energy of the LAT spectral component. We show synchrotron and inverse-Compton light-curves expected from pairs formed in the shocked medium and identify some criteria for testing a pair origin of GRB optical counterparts. Pairs formed in bright LAT afterglows with a Lorentz factor in the few hundreds may produce bright optical counterparts (R < 10) lasting for up to one hundred seconds. The number of internal pairs formed from unscattered seed photons decreases very strongly with the source Lorentz factor, thus bright GRB optical counterparts cannot arise from internal pairs if the afterglow Lorentz factor is above several hundreds.