Inverse Compton Signatures of Gamma-Ray Burst Afterglows

TL;DR

This paper investigates the high-energy emission mechanisms in gamma-ray burst afterglows, focusing on inverse Compton processes, and assesses their detectability with current and future gamma-ray observatories, especially in light of recent observations of GRB 190114C.

Contribution

It provides a detailed analysis of inverse Compton scenarios for GRB afterglows, including synchrotron self-Compton and external Compton, and constrains the external infrared photon field based on observations of GRB 190114C.

Findings

SSC dominates GeV emission unless dense IR photon fields are present.

VHE emission detectability depends on ambient photon fields and EBL attenuation.

Constraints on external IR photon energy density from GRB 190114C observations.

Abstract

The afterglow emission from gamma-ray bursts (GRBs) is believed to originate from a relativistic blast wave driven into the circumburst medium. Although the afterglow emission from radio up to X-ray frequencies is thought to originate from synchrotron radiation emitted by relativistic, non-thermal electrons accelerated by the blast wave, the origin of the emission at high energies (HE; $\gtrsim$~GeV) remains uncertain. The recent detection of sub-TeV emission from GRB~190114C by MAGIC raises further debate on what powers the very high-energy (VHE; $\gtrsim 300$GeV) emission. Here, we explore the inverse Compton scenario as a candidate for the HE and VHE emissions, considering two sources of seed photons for scattering: synchrotron photons from the blast wave (synchrotron self-Compton or SSC) and isotropic photon fields external to the blast wave (external Compton). For each case, we compute the multi-wavelength afterglow spectra and light curves. We find that SSC will dominate particle cooling and the GeV emission, unless a dense ambient infrared photon field, typical of star-forming regions, is present. Additionally, considering the extragalactic background light attenuation, we discuss the detectability of VHE afterglows by existing and future gamma-ray instruments for a wide range of model parameters. Studying GRB~190114C, we find that its afterglow emission in the \fermi-LAT band is synchrotron-dominated.The late-time \fermi-LAT measurement (i.e., $t\sim 10^4$~s), and the MAGIC observation also set an upper limit on the energy density of a putative external infrared photon field (i.e. $\lesssim 3\times 10^{-9}\,{\rm erg\,cm^{-3}}$), making the inverse Compton dominant in the sub-TeV energies.