Measurement of Very-High-Energy Diffuse Gamma-Ray Emissions from the Galactic Plane with LHAASO-WCDA

Area of Science:

• Astrophysics and Particle Physics
• Cosmic Ray Physics
• Galactic Gamma-Ray Astronomy

Background:

• Diffuse Galactic gamma-ray emission is crucial for understanding cosmic ray propagation and interactions within the Milky Way.
• Previous studies have utilized various detectors, but a gap exists between low-energy space-based measurements and ultra-high-energy observations.

Purpose of the Study:

• To measure the diffuse gamma-ray emission from the Galactic plane (15° to 235° longitude, -5° to +5° latitude) in the 1 to 25 TeV energy range.
• To investigate the spectral properties and spatial distribution of this emission and compare it with existing models and higher-energy observations.

Main Methods:

• Utilized the Water Cherenkov Detector Array (WCDA) at the Large High Altitude Air Shower Observatory for measurements.
• Masked known source regions to isolate diffuse emission.
• Analyzed spectral characteristics using power-law functions and compared with Square Kilometer Array (KM2A) data.

Main Results:

• Detected diffuse emission with high statistical significance (24.6σ in the inner, 9.1σ in the outer Galactic plane).
• Determined spectral indices of -2.67±0.05 in the inner and -2.83±0.19 in the outer regions.
• Observed a spectral softening around 30 TeV in the inner region and fluxes 1.5-2.7 times higher than model predictions.

Conclusions:

• The WCDA measurements provide crucial data bridging energy gaps in cosmic ray studies.
• Discrepancies between observed fluxes and model predictions, along with spatial distribution deviations, highlight the need for improved wideband diffuse emission models.
• The spectral shape may vary across different Galactic longitude regions, indicating complex cosmic ray propagation.