Discovery of an unidentified TeV source in the field of view of PSR B1259-63 with H.E.S.S.

The detection of an unidentified extended TeV γ-ray source in the Southern Cross region close to the galactic plane being named HESS J1303-631 is reported. The observations have been performed between February and June 2004 with the new stereoscopic system of four Cherenkov telescopes operated by the H.E.S.S. collaboration in Namibia. The telescopes were initially pointed to the binary system PSR B1259-63/SS 2883 which was for the first time detected at TeV energies within this observation campaign (see parallel paper). In the same dataset the unidentified TeV source HESS J1303-631 has been discovered serendipitously roughly 0.6° north of the PSR B1259-63 position leading - for the first time in TeV γ-ray astronomy - to the detection of two sources within the same field of view. The new source is extended on the 0.2° level and - up to now - no counterpart in other wavelengths has been identified. The measured flux is compatible with constant emission on the 10% flux level of the Crab nebula and shows a hard energy spectrum which can be described by a power-law with an index of Γ = 2.2 ± 0.2 _stat. In this paper various consistency checks which confirm the celestial origin of the observed excess are presented and preliminary results on the source extension and energy spectrum of the source are reported.