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

The serendipitous discovery of an unidentified extended TeVgamma-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 H.E.S.S. stereoscopic system of Cherenkov telescopes in Namibia. HESS J1303-631 was discovered roughly 0.6 deg 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 sigma = (0.16 +- 0.02) deg 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^-Gamma with a photon index of Gamma = 2.44 +- 0.05_stat +- 0.2_syst. Up to now, no counterpart at other wavelengths is identified. Various possible TeV production scenarios are discussed.