Observation of Very High Energy Gamma Rays from HESS J1804-216 with CANGAROO-III Telescopes

We observed the unidentified TeV gamma-ray source HESS J1804-216 with the CANGAROO-III atmospheric Cerenkov telescopes from May to July in 2006. We detected very high energy gamma rays above 600 GeV at the 10 sigma level in an effective exposure of 76 hr. We obtained a differential flux of (5.0+/-1.5_{stat}+/-1.6_{sys})\times 10^{-12}(E/1 TeV)^{-\alpha} cm^{-2}s^{-1}TeV^{-1} with a photon index \alpha of 2.69 +/- 0.30_{stat} +/- 0.34_{sys}, which is consistent with that of the H.E.S.S. observation in 2004. We also confirm the extended morphology of the source. By combining our result with multi-wavelength observations, we discuss the possible counterparts of HESS J1804-216 and the radiation mechanism based on leptonic and hadronic processes for a supernova remnant and a pulsar wind nebula.