Cascade Radiations of $e^\pm$ from $\gamma\gamma$-annihilation process as an extra component of the Early Optical/X-Ray Afterglows of Gamma-Ray Bursts

TL;DR

This paper proposes that cascade radiations from electron-positron pairs produced via gamma-gamma interactions in GRB jets significantly influence early optical and X-ray afterglows, explaining observed chromatic features.

Contribution

It introduces a model where cascade synchrotron emission accounts for chromatic breaks and plateaus in early GRB afterglows, supported by numerical calculations and application to specific GRBs.

Findings

Cascade synchrotron emission impacts early optical/X-ray afterglows.

Model explains chromatic breaks and plateaus in observed lightcurves.

Predicts reduced TeV gamma-ray flux in such GRBs.

Abstract

Chromatic break and/or plateau observed in the early optical and X-ray afterglow lightcurves challenge the conventional external shock models of gamma-ray bursts (GRBs). Detection of TeV gamma-ray afterglows indicates strong gamma-ray production within the afterglow jets. We investigate the cascade radiations of the $e^\pm$ production via the $\gamma\gamma$ interaction in the jets. Our numerical calculations show that the cascade synchrotron emission can make a significant contribution to the early optical/X-ray afterglows. The combination of the primary and cascade emission fluxes can shape a chromatic break and/or plateau in the early optical/X-ray lightcurves, depending on the jet properties. Applying our model to GRBs 050801 and 080310, we found that their optical plateaus and the late X-ray/optical lightcurves can be explained with our model in reasonable parameter values. We suggest that such a chromatic optical plateau could be a signature of strong $e^\pm$ production in GRB afterglow jets. The TeV gamma-ray flux of such kind GRBs should be significantly reduced, hence tends to be detectable for those GRBs that have a single power-law decaying optical afterglow lightcurve.