LHAASO Detection of Ultra-High-Energy Gamma-Ray Emission toward the Giant Molecular Clouds

TL;DR

This paper reports the first detection of very-high-energy gamma-ray emissions from five nearby giant molecular clouds using LHAASO, providing insights into cosmic ray interactions and the potential location of the cosmic ray knee.

Contribution

It presents the first VHE gamma-ray detection from GMCs with LHAASO and investigates the cosmic ray spectral knee using gamma-ray data, aligning with recent cosmic ray spectrum measurements.

Findings

Detection of gamma-ray emissions from GMCs in the 1-100 TeV range.

Gamma-ray spectra consistent with cosmic ray interactions in the ISM.

No significant evidence for the cosmic ray knee, but data favors a knee above 0.9 PeV.

Abstract

The $\gamma$-ray from Giant molecular clouds (GMCs) is regarded as the most ideal tool to perform in-situ measurement of cosmic ray (CR) density and spectra in our Galaxy. We report the first detection of $\gamma$-ray emissions in the very-high-energy (VHE) domain from the five nearby GMCs with a stacking analysis based on a 4.5-year $\gamma$-ray observation with the Large High Altitude Air Shower Observatory (LHAASO) experiment. The spectral energy distributions derived from the GMCs are consistent with the expected $\gamma$-ray flux produced via CR interacting with the ISM in the energy interval 1 - 100 $~\rm$ TeV. In addition, we investigate the presence of the CR spectral `knee' by introducing a spectral break in the $\gamma$-ray data. While no significant evidence for the CR knee is found, the current KM2A measurements from GMCs strongly favor a proton CR knee located above 0.9$~\rm$ PeV, which is consistent with the latest measurement of the CR spectrum by ground-based experiments.