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

The serendipitous discovery of an unidentified extended TeV $\gamma$-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 $0.6\degr$ 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 $\sigma = (0.16 \pm 0.02)\degr$ and the integral flux above $380\,\mathrm{GeV}$ is compatible with constant emission over the entire observational period of $(17 \pm 3)$% of the Crab Nebula flux. The measured energy spectrum can be described by a power-law $\mathrm{d}N/\mathrm{d}E \sim E^{-\Gamma}$ with a photon index of $\Gamma = 2.44 \pm 0.05_{\mathrm{stat}} \pm 0.2_{\mathrm{syst}}$. Up to now, no counterpart at other wavelengths is identified. Various possible TeV production scenarios are discussed.