The formation of hard VHE spectra from GRB afterglow via Two-Zone Synchrotron Self-Compton Emission

TL;DR

This paper proposes a two-zone synchrotron self-Compton model to explain the unexpectedly hard very high energy spectra observed in GRB afterglows, challenging traditional one-zone models and matching multi-wavelength observations.

Contribution

The study introduces a two-zone configuration that accounts for magnetic field differences, producing hard IC spectra consistent with observations, unlike previous one-zone models.

Findings

Two-zone model reproduces the observed VHE and X-ray spectra.

Hard electron distributions lead to harder IC emission.

Model explains the spectral slope similarity across bands.

Abstract

Electron Compton scattering of target photons into the gamma-ray energy band (inverse Compton scattering --IC--) is commonly expected to dominate the very high energy spectra in gamma-ray bursts especially during the afterglow phase. For sufficiently large center-of-mass energies in these collisions, the effect of the electron recoil starts reducing the scattering cross section (the Klein-Nishina regime). The IC spectra generated in the Klein-Nishina regime is softer and has a smaller flux level compared to the synchrotron spectra produced by the same electrons. The detection of afterglow emission from nearby GRB 190819A in the very high energy (VHE) domain with H.E.S.S. has revealed an unexpected feature: the slope of the VHE spectrum matches well the slope of the X-ray spectra, despite expectations that for the IC production process, the impact of the Klein-Nishina effect should be strong. The multi-wavelength spectral energy distribution appears to be inconsistent with predictions of one-zone synchrotron-self-Compton models. We study the possible impact of two-zone configuration on the properties of IC emission when the magnetic field strength differs considerably between the two zones. Synchrotron photons from the strong magnetic field zone provide the dominant target for cooling of the electrons in the weak magnetic field zone, which results in a formation of hard electron distribution and consequently of a hard IC emission. We show that the two-zone model can provide a good description of the X-ray XRT and VHE H.E.S.S. data.