Very High-energy Afterglow Emission of GRB 190829A: Evidence for Its Hadronic Origin?

Abstract

The detection of multi-TeV gamma rays from the afterglow phase of GRB 190829A by the High Energy Stereoscopic System telescope is an addition to the already existing list of two GRBs observed in very high-energy (VHE) gamma rays in recent years. Jets of blazars and GRBs have many similarities and the photohadronic model is very successful in explaining the VHE gamma-ray spectra from the high-energy blazars. Recently, the photohadronic model has been successfully applied to study the sub-TeV gamma rays from the afterglow phases of GRB 180720B and GRB 190114C. We employed this model again to explain the VHE spectra observed for the two consecutive nights from GRB 190829A. We show that the spectra of GRB 190829A can be due to the interactions of high-energy protons with the synchrotron self-Compton photons in the forward shock region of the GRB jet, similar to the low emission state of the VHE flaring events of high-energy blazars. We speculate that, if in the future, it is possible to observe the VHE gamma-ray spectra from nearby GRBs in their afterglow phases, then some of them could only be explained by employing two different spectral indices. If confirmed, such VHE spectra could be interpreted as a result of the interactions of the high-energy protons with the photons, both from the synchrotron background and the synchrotron self-Compton background in the forward shock region.