Prompt X-ray and Optical Excess Emission due to Hadronic Cascades in Gamma-Ray Bursts

TL;DR

This paper models low-energy spectral excesses in gamma-ray bursts as hadronic cascade emissions initiated by ultra-high-energy protons, explaining observed X-ray and optical features and linking them to high-energy gamma-ray and cosmic ray production.

Contribution

The study introduces a Monte Carlo simulation model for hadronic cascade emissions in GRBs, explaining spectral excesses and connecting them to high-energy gamma rays and cosmic rays.

Findings

Synchrotron radiation from secondary pairs reproduces observed soft spectra.

Model explains excess X-ray and optical emissions in specific GRBs.

Spectral excesses are potential signatures of ultra-high-energy cosmic ray production.

Abstract

A fraction of gamma-ray bursts exhibit distinct spectral features in their prompt emission below few 10s of keV that exceed simple extrapolations of the low-energy power-law portion of the Band spectral model. This is also true for the prompt optical emission observed in several bursts. Through Monte Carlo simulations, we model such low-energy spectral excess components as hadronic cascade emission initiated by photomeson interactions of ultra-high-energy protons accelerated within GRB outflows. Synchrotron radiation from the cascading, secondary electron-positron pairs can naturally reproduce the observed soft spectra in the X-ray band, and in some cases the optical spectra as well. These components can be directly related to the higher energy radiation at GeV energies due to the hadronic cascades. Depending on the spectral shape, the total energy in protons is required to be comparable to or appreciably larger than the observed total photon energy. In particular, we apply our model to the excess X-ray and GeV emission of GRB 090902B, and the bright optical emission of the "naked-eye" GRB 080319B. Besides the hard GeV components detected by {\it Fermi}, such X-ray or optical spectral excesses are further potential signatures of ultra-high-energy cosmic ray production in gamma-ray bursts.