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

Abstract

The ${\hat{I}}^3$-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 ${\hat{I}}^3$-ray emissions in the very-high-energy (VHE) domain from the five nearby GMCs with a stacking analysis based on a 4.5-year ${\hat{I}}^3$-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 ${\hat{I}}^3$-ray flux produced via CR interacting with the ISM in the energy interval 1 - 100   TeV. In addition, we investigate the presence of the CR spectral `knee' by introducing a spectral break in the ${\hat{I}}^3$-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  PeV, which is consistent with the latest measurement of the CR spectrum by ground-based experiments.