Very-high-energy Emission and Cascade Radiation of Gamma-Ray Burst Afterglows: Homogeneous versus Wind External Media

Recent detection of sub-TeV emission from gamma-ray bursts (GRBs) represents a breakthrough in the GRB study. The multiwavelength data of the afterglows of GRB 190114C support the synchrotron self-Compton (SSC) origin for its sub-TeV emission. We present a comparative analysis on the SSC emission of GRB afterglows in the homogeneous and wind environments in the framework of the forward shock model. The γ γ absorption of very-high-energy photons due to pair production within the source and the Klein–Nishina effect on the inverse Compton scattering are considered. Generally a higher SSC flux is expected for a larger circum-burst density due to a larger Compton parameter, but meanwhile the internal γ γ absorption is more severe for sub-TeV emission. The flux ratio between the SSC component and the synchrotron component decreases more quickly with time in the wind medium case than in the homogeneous density medium case. The light curves of the SSC emission are also different for the two types of media. We also calculate the cascade emission resulting from the absorbed high-energy photons. In the ISM environment with n ≳ 1 cm⁻³, the cascade synchrotron emission could be comparable to the synchrotron emission of the primary electrons in the optical band, which may flatten the optical afterglow light curve at an early time (t < 1 hr). In the wind medium with A _* ≳ 0.1, the cascade emission in the eV–GeV band is comparable or even larger than the emission of the primary electrons at the early time.