Galactic diffuse gamma-ray emission from GeV to PeV energies in light of up-to-date cosmic ray measurements

TL;DR

This study compares recent gamma-ray observations from Fermi-LAT and LHAASO with model predictions based on cosmic ray data, revealing excess emissions that suggest additional sources or complex propagation effects in the Galaxy.

Contribution

It provides a combined analysis of gamma-ray data from 1 GeV to 1 PeV, constrains cosmic ray propagation models, and proposes an exponential cutoff component to explain observed excesses.

Findings

Detected gamma-ray excesses between a few GeV and 60 TeV.

Proposed an exponential cutoff component with index -2.40 and cutoff at ~30 TeV.

Identified potential unresolved sources or complex propagation as explanations.

Abstract

The diffuse gamma-ray emission between 10 and 1000 TeV from the Galactic plane was recently measured by the Large High Altitude Air Shower Observatory (LHAASO). These observations will help tremendously in constraining the propagation and interaction of cosmic rays in the Milky Way. Additionally, new measurements of CR spectra reach a very high precision up to 100 TeV energies, revealing multiple spectral structures of various species. In this work, we confront the model prediction of the diffuse gamma-ray emission, based on up-to-date measurements of the local cosmic ray spectra and simplified propagation setup, with the measurements of diffuse gamma-rays. To better constrain the low-energy part of the model, we analyze the 14.6 years of Fermi-LAT data to extract the Galactic diffuse emission between 1 and 500 GeV from the same sky regions of LHAASO, after subtracting the contribution from known sources and the isotropic diffuse gamma-ray background. The joint Fermi-LAT and LHAASO spectra thus cover a very wide energy range from 1 GeV to 1 PeV with small gaps from 0.5 to 10 TeV. Compared with the prediction, we find that clear excesses between several GeV and ~60 TeV of the diffuse emission exist. Possible reasons to explain the excesses may include unresolved sources or more complicated propagation models. We illustrate that an exponential-cutoff-power-law component with an index of -2.40 and cutoff energy of ~30 TeV is able to account for such excesses.