Discovery of the binary pulsar PSR B1259-63 in very-high-energy gamma rays around periastron with HESS

We report the discovery of very-high-energy (VHE) -ray emission of the binary system PSR B1259-63/SS 2883 of a radio pulsar orbiting a massive, luminous Be star in a highly eccentric orbit. The observations around the 2004 periastron passage of the pulsar were performed with the four 13 m Cherenkov telescopes of the HESS experiment, recently installed in Namibia and in full operation since December 2003. Between February and June 2004, a -ray signal from the binary system was detected with a total significance above 13. The flux was found to vary significantly on timescales of days which makes PSR B1259-63 the first variable galactic source of VHE -rays observed so far. Strong emission signals were observed in pre-and post-periastron phases with a flux minimum around periastron, followed by a gradual flux decrease in the months after. The measured time-averaged energy spectrum above a mean threshold energy of 380 GeV can be fitted by a simple power law F 0 (E/1 TeV) - with a photon index = 2.7 0.2 stat 0.2 sys and flux normalisation F 0 = (1.3 0.1 stat 0.3 sys ) 10 -12 TeV -1 cm -2 s -1 . This detection of VHE -rays provides unambiguous evidence for particle acceleration to multi-TeV energies in the binary system. In combination with coeval observations of the X-ray synchrotron emission by the RXTE and INTEGRAL instruments, and assuming the VHE -ray emission to be produced by the inverse Compton mechanism, the magnetic field strength can be directly estimated to be of the order of 1 G.