[Paper Review] Ultrahigh energy cosmic rays and high energy astrophysical neutrinos

Marco Stein Muzio, Glennys R. Farrar+1 more|arXiv (Cornell University)|Aug 13, 2021

Astrophysics and Cosmic Phenomena1 references4 citations

TL;DR

The paper investigates the joint constraints of ultrahigh energy cosmic rays (UHECRs) and high-energy astrophysical neutrinos, showing that photon-dominated source environments better explain UHECR data than gas-dominated ones, while gas-dominated sources conflict with current neutrino limits. Accurate PeV-range neutrino measurements could validate diffusive shock acceleration and distinguish source environments.

ABSTRACT

We explore the joint implications of ultrahigh energy cosmic ray (UHECR) source environments -- constrained by the spectrum and composition of UHECRs -- and the observed high energy astrophysical neutrino spectrum. Acceleration mechanisms producing power-law CR spectra $\propto E^{-2}$ are compatible with UHECR data, if CRs at high rigidities are in the quasi-ballistic diffusion regime as they escape their source environment. Both gas- and photon-dominated source environments are able to account for UHECR observations, however photon-dominated sources do so with a higher degree of accuracy. However, gas-dominated sources are in tension with current neutrino constraints. Accurate measurement of the neutrino flux at $\sim 10$ PeV will provide crucial information on the viability of gas-dominated sources, as well as whether diffusive shock acceleration is consistent with UHECR observations. We also show that UHECR sources are able to give a good fit to the high energy portion of the astrophysical neutrino spectrum, above $\sim$ PeV. This common origin of UHECRs and high energy astrophysical neutrinos is natural if air shower data is interpreted with the extsc{Sibyll2.3c} hadronic interaction model, which gives the best-fit to UHECRs and astrophysical neutrinos in the same part of parameter space, but not for EPOS-LHC.

Motivation & Objective

• To determine the viability of gas- and photon-dominated source environments in explaining ultrahigh energy cosmic ray (UHECR) observations.
• To assess the consistency of UHECR acceleration mechanisms with the observed high-energy astrophysical neutrino spectrum.
• To evaluate whether diffusive shock acceleration can simultaneously explain UHECR spectra and neutrino fluxes.
• To identify which hadronic interaction models—Sibyll2.3c or EPOS-LHC—best reconcile UHECR and neutrino data in a common parameter space.
• To determine the role of future PeV neutrino measurements in constraining UHECR source physics.

Proposed method

• Modeling UHECR acceleration in source environments with power-law spectra ∝ E⁻², assuming quasi-ballistic diffusion at high rigidities.
• Using the Sibyll2.3c and EPOS-LHC hadronic interaction models to simulate air shower development and compare predictions with UHECR data.
• Applying constraints from the observed UHECR spectrum and composition to evaluate source environment compatibility.
• Comparing predicted neutrino fluxes from UHECR sources against the observed high-energy astrophysical neutrino spectrum above ~1 PeV.
• Assessing tension between gas-dominated source models and current neutrino flux limits from IceCube and other experiments.
• Evaluating the consistency of diffusive shock acceleration with both UHECR and neutrino data across different source environments.

Experimental results

Research questions

• RQ1 Can gas-dominated source environments simultaneously explain UHECR data and current high-energy neutrino constraints?
• RQ2 Do photon-dominated source environments provide a better fit to UHECR observations than gas-dominated ones?
• RQ3 Is diffusive shock acceleration consistent with both the UHECR spectrum and the observed astrophysical neutrino flux above 1 PeV?
• RQ4 Which hadronic interaction model—Sibyll2.3c or EPOS-LHC—best reconciles UHECR and high-energy neutrino data in the same parameter space?
• RQ5 How will future PeV-range neutrino measurements help distinguish between gas- and photon-dominated UHECR source environments?

Key findings

• Power-law cosmic ray spectra ∝ E⁻² are compatible with UHECR data when CRs at high rigidities undergo quasi-ballistic diffusion during escape.
• Photon-dominated source environments provide a better fit to UHECR observations than gas-dominated environments.
• Gas-dominated sources are in tension with current high-energy neutrino constraints, suggesting they may be disfavored.
• Accurate measurement of the neutrino flux at ∼10 PeV could resolve the viability of gas-dominated sources and test the consistency of diffusive shock acceleration.
• UHECR sources can simultaneously account for the high-energy portion of the astrophysical neutrino spectrum above ∼1 PeV.
• The Sibyll2.3c hadronic interaction model provides the best-fit to both UHECR and high-energy neutrino data in the same parameter space, unlike EPOS-LHC.