Origin of Galactic sub-PeV diffuse gamma-ray emission: Constraints from high-energy neutrino observations

TL;DR

This paper investigates the origin of sub-PeV diffuse gamma-ray emission in the Galaxy, highlighting potential tensions with neutrino observations and proposing an extra cosmic ray component related to PeV sources like Cygnus Cocoon.

Contribution

It introduces the idea that an additional hard-spectrum cosmic ray component can explain gamma-ray observations while remaining consistent with neutrino non-detections.

Findings

Gamma-ray flux is compatible with hadronic models but tensions exist with neutrino non-detections.

An extra cosmic ray component from sources like Cygnus Cocoon can resolve these tensions.

Predicted gamma-ray spectrum extends up to 1 PeV with possible spectral hardening.

Abstract

Very recently, diffuse gamma rays with $0.1\,{\rm PeV}<E_\gamma <1\,\rm PeV$ have been discovered from the Galactic disk by the Tibet air shower array and muon detector array (Tibet AS+MD array). While the measured sub-PeV flux may be compatible with the hadronic origin in the conventional Galactic cosmic ray propagation model, we find that it is in possible tension with the non-detection of Galactic neutrino emissions by the IceCube neutrino telescope. We further find that the presence of an extra cosmic ray component of relatively hard spectrum, which is probably related to the Cygnus Cocoon region and other PeV cosmic-ray sources in the Galactic disk, would alleviate the tension. This scenario implies the existence of an extreme accelerator of either protons or electrons beyond PeV in the Cygnus region, and predicts the continuation of the gamma-ray spectrum of Cygnus Cocoon up to 1 PeV with a possible hardening beyond $\sim 30-100\,$TeV.