Serendipitous discovery of the unidentified extended TeV gamma-ray source HESS J1303-631

The serendipitous discovery of an unidentified extended TeV γ-ray source close to the galactic plane named HESS J1303-631 at a significance of 21 standard deviations is reported. The observations were performed between February and June 2004 with the HESS stereoscopic system of Cherenkov telescopes in Namibia. HESS J1303-631 was discovered roughly 0.6 ◦ north of the binary system PSR B1259-63/SS 2883, the target object of the initial observation campaign which was also detected at TeV energies in the same field of view. HESS J1303-631 is extended with a width of an assumed intrinsic Gaussian emission profile of σ = (0.16 ± 0.02) ◦ and the integral flux above 380 GeV is compatible with constant emission over the entire observational period of (17 ± 3)% of the Crab Nebula flux. The measured energy spectrum can be described by a power-law dN/dE ∼ E −Γ with a photon index of Γ= 2.44 ± 0.05stat ± 0.2syst. Up to now, no counterpart at other wavelengths is identified. Various possible TeV production scenarios are discussed.