[Paper Review] Multi-messenger Implications of Sub-PeV Diffuse Galactic Gamma-Ray Emission

Ke Fang, Kohta Murase|arXiv (Cornell University)|Apr 19, 2021

Astrophysics and Cosmic PhenomenaPhysics and Astronomy88 references47 citations

TL;DR

This paper derives the diffuse Galactic neutrino flux from sub-PeV gamma-ray data measured by the Tibet ASγ experiment, accounting for gamma-ray attenuation and cosmic-ray uncertainties. It finds that Galactic neutrinos contribute ≤5–10% to the all-sky neutrino flux at 100 TeV, with future telescopes like IceCube-Gen2 and KM3Net expected to detect this component, offering key insights into the origin of sub-PeV cosmic rays and multi-messenger connections in the Milky Way.

ABSTRACT

The diffuse Galactic gamma-ray flux between 0.1 and 1 PeV has recently been measured by the Tibet AS$\gamma$ Collaboration. The flux and spectrum are consistent with the decay of neutral pions from hadronuclear interactions between Galactic cosmic rays and the interstellar medium (ISM). We derive the flux of the Galactic diffuse neutrino emission from the same interaction process that produces the gamma rays. Our calculation accounts for the effect of gamma-ray attenuation inside the Milky Way and uncertainties due to the spectrum and distribution of cosmic rays, gas density, and infrared emission of the ISM. We find that the contribution from the Galactic plane to the all-sky neutrino flux is $\lesssim5-10\%$ around 100 TeV. The Galactic and extragalactic neutrino intensities are comparable in the Galactic plane region. Our results are consistent with the upper limit reported by the IceCube and ANTARES Collaborations, and predict that next-generation neutrino experiments may observe the Galactic component. We also show that the Tibet AS$\gamma$ data imply either an additional component in the cosmic-ray nucleon spectrum or contribution from discrete sources, including Pevatrons such as superbubbles and hypernova remnants, and PeV electron accelerators. Future multi-messenger observations between 1 TeV and 1 PeV are crucial to decomposing the origin of sub-PeV gamma rays.

Motivation & Objective

• To determine the diffuse Galactic neutrino flux implied by recent sub-PeV gamma-ray measurements from the Tibet ASγ experiment.
• To assess the contribution of the Galactic plane to the all-sky neutrino flux, particularly around 100 TeV.
• To evaluate the detectability of Galactic neutrinos with next-generation neutrino telescopes such as IceCube-Gen2 and KM3Net.
• To investigate whether the observed gamma-ray emission implies additional components in the cosmic-ray spectrum or contributions from discrete sources like Pevatrons or PeV electron accelerators.

Proposed method

• Uses the hadronic interaction model where pion decay produces both gamma rays and neutrinos, applying the relation E²ν dNν/dEν ≈ (3/2) × (E²γ dNγ/dEγ)|Eν=Eγ/2 to link gamma-ray and neutrino spectra.
• Incorporates detailed gamma-ray attenuation along the line of sight due to pair production with interstellar radiation fields, including CMB and infrared dust emission.
• Models the cosmic-ray and gas density distributions in cylindrical symmetry around the Galactic center, using the GC frame for calculations.
• Applies the optical depth formalism τγγ(Eγ, x₀, xₒb) = ∫ ds λ⁻¹γγ(Eγ, ŝ, x₀ + sŝ) to compute survival probability of high-energy gamma rays.
• Solves for the diffuse neutrino intensity by integrating over source regions, including attenuation effects and cosmic-ray spectrum uncertainties.
• Considers multiple cosmic-ray models and source distributions to assess systematic uncertainties in the neutrino flux prediction.

Experimental results

Research questions

• RQ1 What is the expected diffuse Galactic neutrino flux corresponding to the observed sub-PeV diffuse gamma-ray emission from the Tibet ASγ experiment?
• RQ2 How does gamma-ray attenuation within the Milky Way affect the inferred neutrino flux from hadronic interactions?
• RQ3 To what extent does the Galactic plane contribute to the total all-sky neutrino flux at 100 TeV, and how does it compare to the extragalactic component?
• RQ4 Can next-generation neutrino telescopes like IceCube-Gen2 and KM3Net detect the predicted Galactic neutrino component?
• RQ5 What do the Tibet ASγ data imply about the presence of unresolved Pevatrons or PeV electron accelerators in the Galaxy?

Key findings

• The diffuse Galactic neutrino flux toward the Galactic plane is estimated to be ≤(3–6)×10⁻⁹ GeV cm⁻² s⁻¹ sr⁻¹ at 100 TeV, consistent with current IceCube and ANTARES upper limits.
• The Galactic contribution to the all-sky neutrino flux is ≤5–10% at 100 TeV, with Galactic and extragalactic intensities being comparable in the Galactic plane region.
• Next-generation neutrino telescopes such as IceCube-Gen2 and KM3Net are expected to achieve sensitivities of ∼3×10⁻⁹ GeV cm⁻² s⁻¹ sr⁻¹ and ∼(3–6)×10⁻⁹ GeV cm⁻² s⁻¹ sr⁻¹, respectively, enabling detection of the predicted Galactic component.
• The Tibet ASγ data suggest the presence of an additional component in the cosmic-ray nucleon spectrum or contributions from discrete sources such as Pevatrons (e.g., superbubbles, hypernova remnants) or PeV electron accelerators.
• The observed spectral index of ∼2.53 in the sub-PeV range supports a Galactic origin for neutrinos below 100 TeV, though extragalactic contributions remain dominant.
• The data are consistent with a scenario where unresolved Pevatrons or leptonic sources (e.g., inverse Compton from 3 PeV electrons) can explain the high-energy gamma-ray emission, highlighting the need for future multi-messenger observations.