Systematic parameter space study for the UHECR origin from GRBs in models with multiple internal shocks

TL;DR

This study systematically explores whether long-duration Gamma-Ray Bursts with multiple internal shocks can account for ultra-high energy cosmic rays, analyzing spectrum, composition, and neutrino fluxes under various model assumptions.

Contribution

It introduces a comprehensive model for GRBs with multiple internal shocks, assessing their viability as UHECR sources and predicting neutrino fluxes within observational constraints.

Findings

GRBs can explain UHECRs under various model assumptions.

Predicted neutrino fluxes are below current detection but accessible to future experiments.

Heavy mass fraction at injection exceeds 70% (95% CL).

Abstract

We scrutinize the paradigm that conventional long-duration Gamma-Ray Bursts (GRBs) are the dominant source of the ultra-high energy cosmic rays (UHECRs) within the internal shock scenario by describing UHECR spectrum and composition and by studying the predicted (source and cosmogenic) neutrino fluxes. Since it has been demonstrated that the stacking searches for astrophysical GRB neutrinos strongly constrain the parameter space in single-zone models, we focus on the dynamics of multiple collisions for which different messengers are expected to come from different regions of the same object. We propose a model which can describe both stochastic and deterministic engines, which we study in a systematic way. We find that GRBs can indeed describe the UHECRs for a wide range of different model assumptions with comparable quality albeit with the previously known problematic energy requirements; the heavy mass fraction at injection is found to be larger than 70% (95% CL). We demonstrate that the post-dicted (from UHECR data) neutrino fluxes from sources and UHECR propagation are indeed below the current sensitivities but will be reached by the next generation of experiments. We finally critically review the required source energetics with the specific examples found in this study.