The High Energy Budget Allocations in Shocks and GRB

TL;DR

This paper analyzes the energy distribution in long Gamma Ray Bursts using Fermi Observatory data, finding that GRBs contribute minimally to ultrahigh-energy cosmic rays and discussing implications for neutrino detection.

Contribution

It provides new estimates of GRB energy input rates, compares high-energy components with UHECR requirements, and discusses neutrino constraints and detection prospects.

Findings

GRB flux in 3-30 MeV is small despite wide energy range

High-energy GRB component contains about 0.3% of UHECR energy

AMANDA II limits challenge GRB origin for UHECR if neutrino energy fraction is high

Abstract

The statistical distribution of energies among particles responsible for long Gamma Ray Burst (GRB) emission is analyzed in light of recent results of the Fermi Observatory. The allsky flux, $F_{\gamma}$, recorded by the Gamma Ray Burst Monitor (GBM) is shown, despite its larger energy range, to be not significantly larger than that reported by the Burst and Transient Explorer (BATSE), suggesting a relatively small flux in the 3 - 30 MeV energy range. The present-day energy input rate in $\gamma$-rays recorded by the GBM from long GRB is found, assuming star-formation rates in the literature, to be $\dot W(0)=0.5 F_{\gamma} H/c = 5 \times 10^{42}\ \rm{erg/Mpc^3 yr}$. The Large Area Telescope (LAT) fluence, when observed, is about 5-10\% per decade of the total, in good agreement with the predictions of saturated, non-linear shock acceleration. The high-energy component of long GRBs, as measured by Fermi, is found to contain only $\sim 10^{-2.5}$ of the energy needed to produce ultrahigh-energy cosmic rays (UHECR) above 4 Eev, assuming the latter to be extragalactic, when various numerical factors are carefully included, if the cosmic ray source spectrum has a spectral index of -2. The observed $\gamma$-ray fraction of the required UHECR energy is even smaller if the source spectrum is softer than $E^{-2}$. The AMANDA II limits rule out such a GRB origin for UHECR if much more than $10^{-2}$ of the cosmic ray energy goes into neutrinos that are within, and simultaneous with, the $\gamma$-ray beam. It is suggested that "orphan" neutrinos out of the $\gamma$-ray beam might be identifiable via orphan afterglow { or other wide angle signatures of GRB in lieu of coincidence with prompt $\gamma$-rays}, and it is recommended that feasible single neutrino trigger criteria be established to search for such coincidences.