Implications of a PeV neutrino spectral cutoff in GRB models

TL;DR

This paper explores how PeV neutrino observations support the idea that gamma-ray bursts are sources of ultra-high energy cosmic rays, predicting a spectral cutoff at 10 PeV and detailing jet dissipation properties.

Contribution

It provides analytical and numerical evidence linking PeV neutrino fluxes to GRB jet dissipation regions, predicting spectral cutoffs and jet parameters based on the GRB-UHECR connection.

Findings

Neutrino spectra have a cutoff at ≤10 PeV.

Dissipation occurs at 3×10^11 to 3×10^13 cm from the engine.

Jets have Lorentz factors of 100-500 and carry significant Poynting flux.

Abstract

The recent discovery of extragalactic PeV neutrinos opens a new window to the exploration of cosmic-ray accelerators. The observed PeV neutrino flux is close to the Waxman-Bahcall upper bound implying that gamma-ray bursts (GRBs) may be the source of ultra-high energy cosmic rays (UHECRs). Starting with the assumption of the GRB-UHECR connection, we show using both analytical estimates and numerical simulations that the observed neutrinos can originate at the jet as a result of photopion interactions with the following implications: the neutrino spectra are predicted to have a cutoff at energy $\le 10$ PeV; the dissipation responsible for the GRB emission and cosmic-ray acceleration takes place at distances $r_{\rm diss} \approx 3 \times 10^{11}-3 \times 10^{13} \ {\rm cm}$ from the central engine; the Thomson optical depth at the dissipation region is $\tau_{\rm T} \sim 1$; the jet carries a substantial fraction of its energy in the form of Poynting flux at the dissipation region, and has a Lorentz factor $\Gamma \simeq 100-500$. The non-detection of $\sim$PeV neutrinos coincident with GRBs will indicate that GRBs are either poor cosmic accelerators or the dissipation takes place at small optical depths in the jet.