Measurement of Ultra-High-Energy Diffuse Gamma-Ray Emission of the Galactic Plane from 10 TeV to 1 PeV with LHAASO-KM2A

The diffuse Galactic gamma-ray emission, mainly produced via interactions between cosmic rays and the interstellar medium and/or radiation field, is a very important probe of the distribution, propagation, and interaction of cosmic rays in the Milky Way. In this Letter, we report the measurements of diffuse gamma rays from the Galactic plane between 10 TeV and 1 PeV energies, with the square kilometer array of the Large High Altitude Air Shower Observatory (LHAASO). Diffuse emissions from the inner (15 degrees< l < 125 degrees, |b| < 5 degrees) and outer (125 degrees < l < 235 degrees, |b| < 5 degrees) Galactic plane are detected with 29.1 sigma and 12.7 sigma significance, respectively. The outer Galactic plane diffuse emission is detected for the first time in the very- to ultra-high-energy domain (E > 10 TeV). The energy spectrum in the inner Galaxy regions can be described by a power-law function with an index of -2.99 +/- 0.04, which is different from the curved spectrum as expected from hadronic interactions between locally measured cosmic rays and the line-of-sight integrated gas content. Furthermore, the measured flux is higher by a factor of similar to 3 than the prediction. A similar spectrum with an index of -2.99 +/- 0.07 is found in the outer Galaxy region, and the absolute flux for 10 less than or similar to E less than or similar to 60 TeV is again higher than the prediction for hadronic cosmic ray interactions. The latitude distributions of the diffuse emission are consistent with the gas distribution, while the longitude distributions show clear deviation from the gas distribution. The LHAASO measurements imply that either additional emission sources exist or cosmic ray intensities have spatial variations.

Automated summary

The diffuse Galactic gamma-ray emission, measured by LHAASO, provides important information about the distribution and interaction of cosmic rays in our Milky Way galaxy. The measurements show detections of diffuse emissions from both the inner and outer Galactic plane, with energy spectra described by power-law functions. These findings contribute to our understanding of cosmic ray propagation and the existence of additional emission sources.