Interpreting the LHAASO Galactic diffuse emission data

TL;DR

This paper analyzes LHAASO gamma-ray data, considering realistic cosmic ray and gas models, finding that observed excesses in the inner Galaxy may be due to unresolved sources or spectral hardening, with systematic uncertainties playing a key role.

Contribution

It provides a comprehensive calculation of hadronic gamma-ray emission accounting for uncertainties, and explains the observed data in terms of known astrophysical effects and masking strategies.

Findings

LHAASO data aligns with the model in the outer Galaxy.

Inner Galaxy shows excess below 50 TeV, explained by unresolved sources or spectral hardening.

Systematic uncertainties reduce the tension between data and predictions.

Abstract

Recently, the Large High-Altitude Air Shower Observatory (LHAASO) collaboration has obtained a measurement of the gamma-ray diffuse emission in the ultra-high energy range, $10-10^3$ TeV after masking the contribution of known sources. The measurement is provided in two Galactic regions and appears to be 2-3 times higher than the gamma-ray signal expected from the hadronic interactions of diffuse cosmic rays with the interstellar medium, potentially implying that either additional emission sources exist or cosmic ray intensities have spatial variations. In this work, we calculate the hadronic gamma-ray diffuse emission outside the masks, considering a realistic gas distribution. We present a comprehensive calculation of the emission, which includes systematic uncertainties in the gas content of the Galactic disk, in the energy and spatial distribution of cosmic rays, as well as in the hadronic interaction cross-sections. Our results show that these factors mitigate the tension between data and predictions. The LHAASO data appear compatible with our baseline model in the outer Galactic region. In the inner region, the data show an excess with respect to the predictions below $\sim 50$ TeV, while at higher energies they are well described by our model. We argue that two plausible explanations for enhanced gamma-ray emission-unresolved sources and CR spectral hardening in the inner Galaxy-are likely suppressed by the LHAASO masking strategy, which excludes regions where both effects are expected to be most prominent.