Unresolved sources naturally contribute to PeV $\gamma$-ray diffuse emission observed by Tibet AS$\gamma$

TL;DR

This paper demonstrates that unresolved gamma-ray sources significantly contribute to the diffuse PeV gamma-ray emission observed by Tibet ASγ, reducing the need for assumptions of cosmic-ray spectrum hardening in the Galactic center.

Contribution

It predicts the contribution of unresolved sources to diffuse gamma-ray emission and shows this accounts for Tibet ASγ observations without spectrum hardening assumptions.

Findings

Unresolved sources account for a significant part of the diffuse gamma-ray emission.

The combined unresolved sources and diffuse emission match Tibet ASγ data.

No need for cosmic-ray spectrum hardening to explain observations.

Abstract

The Tibet AS$\gamma$ experiment provided the first measurement of the total diffuse gamma-ray emission from the Galactic disk in the sub-PeV energy range. Based on the analysis of TeV sources included in the HGPS catalog, we predict the expected contribution of unresolved sources in the two angular windows of the Galactic plane observed by Tibet AS$\gamma$. We show that the sum of this additional diffuse component due to unresolved sources and the truly diffuse emission, due to cosmic ray interaction with the interstellar medium, well saturates the Tibet data, without the need to introduce a progressive hardening of the cosmic-ray spectrum toward the Galactic center.