Gamma-Ray Observations of the Galactic Plane at Energies E > 500 GeV

In 1998 and 1999 the Whipple Observatory 10 m telescope was used to search for diffuse γ-ray emission from the Galactic plane. At this time, the telescope was equipped with a large (4.°8) field of view camera, well suited to detect diffuse γ-ray emission. No significant evidence of emission was found. Assuming the TeV emission profile matches EGRET observations above 1 GeV with a differential spectral index of 2.4, we derive an upper limit of 3.0 × 10-8 cm-2 s-1 sr-1 for the average diffuse emission above 500 GeV in the Galactic latitude range from -2° to +2° at Galactic longitude 40°. Comparisons with EGRET observations provide a lower limit of 2.31 for the differential spectral index of the diffuse emission, assuming there is no break in the spectrum between 30 and 500 GeV. This constrains models for diffuse emission with a significant inverse Compton contribution.