Very high energy gamma-ray observations of the Galactic Plane with the CANGAROO-III telescopes

In 2004, we searched for very high energy (VHE) gamma-ray emission from the Galactic Plane using the CANGAROO-III stereoscopic observation system. Two different longitude regions (ℓ=−19°.5 and ℓ=+13°.0) on the Galactic Plane were observed during July and August 2004. We analyzed events that triggered three telescopes aiming to measure the diffuse emission component. No significant signal associated with the Galactic Plane was found from either of the regions. Assuming that the gamma-ray spectrum is described by a single power-law for energies ranging between a few GeV and TeV, lower limits of the power-law spectral indices were found to be 2.2 for both of the regions with a 99.9% confidence level. This result is consistent with the other VHE measurements and constrains a hypothesis in which a very hard (∼2.0) cosmic ray electron spectrum was introduced to explain the EGRET GeV anomaly.