[Paper Review] Galactic cosmic ray propagation: sub-PeV diffuse gamma-ray and neutrino emission

Bing-Qiang Qiao, Wei Liu+3 more|arXiv (Cornell University)|Apr 8, 2021

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This paper investigates the origin of sub-PeV diffuse gamma rays detected by the Tibet AS³ experiment, using cosmic ray propagation models to explain their flux. It finds a tension between observed gamma rays and local cosmic ray measurements, concluding that Galactic cosmic rays contribute at most 15% to the observed high-energy neutrino flux, with most neutrinos originating from extragalactic sources.

ABSTRACT

The Tibet ASgamma experiment just reported their measurement of sub-PeV diffuse gamma ray emission from the Galactic disk, with the highest energy up to 957 TeV. These gamma-rays are most likely the hadronic origin by cosmic ray interaction with interstellar gas in the Galaxy. This measurement provides direct evidence to the hypothesis that the Galactic cosmic rays can be accelerated beyond PeV energies. In this work, we try to explain the sub-PeV diffuse gamma-ray spectrum within cosmic rays diffusive propagation model. We find there is a tension between the sub-PeV diffuse gamma rays and the local cosmic ray spectrum. To describe the sub-PeV diffuse gamma-ray flux, it generally requires larger local cosmic-ray flux than measurement in the knee region. We further calculate the PeV neutrino flux from the cosmic ray propagation model. Even all of these sub-PeV diffuse gamma rays originate from the propagation, the Galactic neutrinos only account for less than ~15% of observed flux, most of which are still from extragalactic sources.

Motivation & Objective

• To explain the recently observed sub-PeV diffuse gamma-ray emission from the Galactic disk using cosmic ray propagation models.
• To assess the consistency between the observed sub-PeV gamma-ray flux and local cosmic ray measurements in the knee region (~4 PeV).
• To evaluate the contribution of Galactic cosmic ray propagation to the diffuse high-energy neutrino flux observed by IceCube.
• To test whether current propagation models (homogeneous diffusion and SDP models) can simultaneously reproduce the sub-PeV gamma-ray data and local cosmic ray spectra.
• To determine the maximum possible fraction of the observed astrophysical neutrino flux that can be attributed to Galactic sources via hadronic interactions.

Proposed method

• Utilizes three cosmic ray propagation models: homogeneous diffusion and two versions of the Spatially Dependent Propagation (SDP) model.
• Applies the GALPROP software to simulate cosmic ray diffusion and calculate resulting gamma-ray and neutrino fluxes from pion decay in hadronic interactions.
• Employs the DRAGON code to compute neutrino spectra from the same hadronic interactions, accounting for the energy-dependent ratio between gamma rays and neutrinos.
• Compares model-predicted gamma-ray fluxes with the Tibet AS³ experiment's measurement of diffuse emission between 100 TeV and 1 PeV.
• Uses IceCube 7.5-year neutrino data (power-law index γ = -2.87) as a benchmark to evaluate the Galactic contribution to the observed neutrino flux.
• Adjusts local cosmic ray source normalization to match the sub-PeV gamma-ray data, then evaluates the resulting neutrino flux relative to observations.

Fig. 1: Calculation of B/C ratio to obtain the propagation parameters in HD, SDP-A and SDP-B models, with B/C data taken from the AMS-02 measurement (Aguilar et al., 2016 ) .

Experimental results

Research questions

• RQ1 Can current cosmic ray propagation models explain the observed sub-PeV diffuse gamma-ray flux from the Galactic disk?
• RQ2 What is the required local cosmic ray flux level to reproduce the sub-PeV gamma-ray data, and how does it compare to direct measurements near the knee energy?
• RQ3 To what extent can Galactic cosmic ray propagation account for the observed high-energy neutrino flux detected by IceCube?
• RQ4 How does the choice of propagation model (homogeneous vs. SDP) affect the predicted gamma-ray and neutrino spectra?
• RQ5 Are there inconsistencies between the sub-PeV gamma-ray observations and the local cosmic ray spectrum that suggest a need for modified propagation or additional sources?

Key findings

• The SDP-B propagation model best reproduces the observed sub-PeV diffuse gamma-ray flux, but only if the local cosmic ray flux exceeds the observed all-particle spectrum at ~10 PeV.
• Even under the most favorable SDP-B model, the predicted gamma-ray flux falls short of observations at 1 TeV (25° < l < 100°) and above 100 TeV (50° < l < 200°).
• The calculated diffuse neutrino flux from Galactic cosmic ray propagation is at most ~15% of the observed IceCube flux, indicating that most high-energy neutrinos are extragalactic.
• The SDP-B model requires a significantly enhanced local cosmic ray flux that conflicts with direct measurements and fails to reproduce the softening of CR nuclei at ~20 TeV.
• The observed sub-PeV gamma rays may not be fully explained by propagation alone, suggesting a contribution from unresolved or nearby PeVatrons like the Cygnus Cocoon.
• The tension between sub-PeV gamma-ray observations and local cosmic ray data implies that propagation models may need modification, such as a harder spectrum near the Galactic center while preserving local propagation.

Fig. 2: Calculated proton and helium energy spectra in propagation models of HD, SDP-A and SDP-B. The proton and helium data are taken from AMS-02 (Aguilar et al., 2015 , 2017 ) , CREAM-II (Yoon et al., 2017 ) , NUCLEON (Atkin et al., 2017 ) , DAMPE (An et al., 2019 ) and KASCADE (Apel et al., 2013