[Paper Review] Neutrinos And Cosmic Rays From Gamma Ray Bursts

Arnon Dar|arXiv (Cornell University)|May 15, 2012

Gamma-ray bursts and supernovae3 citations

TL;DR

This paper challenges the assumption that gamma-ray bursts (GRBs) are major sources of ultra-high-energy cosmic rays (UHECRs) and neutrinos, arguing that the IceCube upper limit on GRB neutrinos does not rule out GRBs as UHECR sources. Using the cannonball model instead of the fireball model, it shows that predicted neutrino fluxes are far below IceCube's limit, and that nuclear photo-disintegration and galactic magnetic fields constrain UHECR composition and escape, implying GRBs may be the primary source of Galactic cosmic rays.

ABSTRACT

The upper limit on the flux of ultra high energy neutrinos from gamma-ray bursts (GRBs) that was reported recently by the IceCube collaboration contradicts predictions based on the Fireball model of GRBs, but does not exclude GRBs as a main source of ultra-high energy cosmic rays.

Motivation & Objective

• To re-evaluate the role of gamma-ray bursts (GRBs) as sources of ultra-high-energy cosmic rays (UHECRs) in light of recent IceCube neutrino flux limits.
• To challenge the validity of the fireball model in predicting UHE neutrino fluxes from GRBs, given its inconsistencies with observational data.
• To assess whether GRBs can still be the dominant source of Galactic cosmic rays despite the IceCube upper limit on UHE neutrinos.
• To explain the observed composition and spectral features of UHECRs (e.g., the 'ankle' and 'escape-break') through galactic confinement and nuclear disintegration effects.
• To argue that the cannonball model better explains GRB properties and yields neutrino fluxes consistent with IceCube limits, unlike the fireball model.

Proposed method

• Uses the cannonball model of GRBs, which better reproduces observed GRB light curves and afterglows compared to the fireball model.
• Applies theoretical estimates of UHE neutrino fluxes based on the cannonball model, showing they are significantly lower than those predicted by the fireball model.
• Analyzes the energy dependence of UHECR acceleration, requiring protons and nuclei to reach energies ≥10×A×E for neutrino production at energy E.
• Evaluates the impact of photo-disintegration of nuclei in collisions with cosmic infrared and microwave background photons on UHECR fluxes.
• Considers the effects of galactic magnetic fields on cosmic ray propagation, particularly on isotropization and escape timescales.
• Compares spectral features (e.g., 'ankle' near 4 EeV and break near 50 EeV) with predictions from galactic confinement and nuclear disintegration, rather than the GZK cutoff.

Experimental results

Research questions

• RQ1 Can gamma-ray bursts still be the primary source of ultra-high-energy cosmic rays despite the IceCube upper limit on UHE neutrinos?
• RQ2 How do the predictions of the cannonball model for UHE neutrino fluxes from GRBs compare to the IceCube experimental upper limit?
• RQ3 Why do observed UHECR composition trends (from proton to iron-dominated) challenge the fireball model’s ability to accelerate nuclei to ultra-high energies?
• RQ4 What is the physical origin of the 'ankle' feature in the cosmic ray spectrum near 4 EeV, and how does it relate to galactic magnetic field effects?
• RQ5 Is the spectral break near 50 EeV observed by HiRes and PAO better explained by the escape of iron nuclei from the Galaxy rather than the GZK cutoff?

Key findings

• The cannonball model predicts UHE neutrino fluxes from GRBs that are significantly lower than the IceCube upper limit, making GRBs still viable as UHECR sources.
• The observed gradual transition in UHECR composition from proton-dominated to iron-dominated between 4 and 40 EeV is inconsistent with the fireball model’s ability to accelerate complex nuclei to ultra-high energies.
• UHECR nuclei with mass number A are subject to photo-disintegration in collisions with cosmic infrared and microwave background photons, drastically reducing their fluxes at ultra-high energies.
• The 'ankle' near 4 EeV in the cosmic ray spectrum is most likely due to the inability of protons and helium nuclei to isotropize or escape the Galaxy beyond this energy due to galactic magnetic fields.
• The spectral break near 50 EeV observed by HiRes and PAO is better interpreted as the escape energy of ultra-high-energy iron nuclei from the Galaxy, not the GZK cutoff, with an energy threshold Z/2 = 13 times higher than for helium.
• Galactic GRBs, especially those beamed away from Earth, remain strong candidates for the main source of Galactic cosmic rays across all energies, given the constraints from composition and propagation.