Observation of Multi‐TeV Diffuse Gamma Rays from the Galactic Plane with the Tibet Air Shower Array

Data from the Tibet III air shower array (with energies around 3 TeV) and from the Tibet II array (with energies around 10 TeV) have been searched for diffuse gamma rays from the Galactic plane. These arrays have an angular resolution of about 0 9. The sky regions searched are the inner Galaxy, 20° ≤ l ≤ 55°, and outer Galaxy, 140° ≤ l ≤ 225°, and | b | ≤ 2° or ≤ 5°. No significant Galactic-plane gamma-ray excess was observed. The 99% confidence level upper limits for gamma-ray intensity obtained are (for | b | ≤ 2°) 1.1 × 10 -15 cm -2 s -1 sr -1 MeV -1 at 3 TeV and 4.1 × 10 -17 cm -2 s -1 sr -1 MeV -1 at 10 TeV for the inner Galaxy, and 3.6 × 10 -16 cm -2 s -1 sr -1 MeV -1 at 3 TeV and 1.3 × 10 -17 cm -2 s -1 sr -1 MeV -1 at 10 TeV for the outer Galaxy, assuming a differential spectral index of 2.4. The upper limits are significant in the multi-TeV region when compared to those from Cerenkov telescopes in the lower energy region and other air shower arrays in the higher energy region; however, the results are not sufficient to rule out the inverse Compton model with a source electron spectral index of 2.0.