Early photon-shock interaction in a stellar wind: A sub-GeV photon flash and high-energy neutrino emission from long gamma-ray bursts

Abstract

For gamma-ray bursts (GRBs) born in a stellar wind, as the reverse shock crosses the ejecta, usually the shocked regions are still precipitated by the prompt MeV γ-ray emission. Because of the tight overlapping of the MeV photon flow with the shocked regions, the optical depth for the GeV photons produced in the shocks is very large. These high-energy photons are absorbed by the MeV photon flow and generate relativistic e^± pairs. These pairs rescatter the soft X-ray photons from the forward shock as well as the prompt γ-ray photons and power detectable high-energy emission, a significant part of which is in the sub-GeV energy range. Since the total energy contained in the forward shock region and the reverse shock region are comparable, the predicted sub-GeV emission is independent of whether the GRB ejecta are magnetized (in which case the reverse shock inverse Compton and synchrotron self-Compton emission is suppressed). As a result, a sub-GeV flash is a generic signature for the GRB wind model, and it should be usually detectable by the future Gamma-Ray Large Area Space Telescope (GLAST). Overlapping also influences neutrino emission. Besides the 10 ¹⁵-10¹⁷ eV neutrino emission powered by the interaction of the shock-accelerated protons with the synchrotron photons in both the forward and reverse shock regions, there comes another 10¹⁴ eV neutrino emission component powered by protons interacting with the MeV photon flow. This last component has a similar spectrum to that generated in the internal shock phase, but the typical energy is slightly lower. © 2005. The American Astronomical Society. All rights reserved.