Discovery of the binary pulsar PSRB1259-63 in very-high-energy gamma rays around periastron with HESS

We report the discovery of very-high-energy (VHE) gamma-ray emission of the binary system PSRB1259-63/SS2883 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 gamma-ray signal from the binary system was detected with a total significance above 13 sigma. The flux was found to vary significantly on timescales of days which makes PSRB1259- 63 the first variable galactic source of VHE gamma-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/1TeV)(-Gamma) with a photon index Gamma = 2.7 +/- 0.2(stat) +/- 0.2(sys) and flux normalisation F-0 = ( 1.3 +/- 0.1stat +/- 0.3(sys)) x 10(-12) TeV-1 cm(-2) s(-1). This detection of VHE gamma-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 gamma-ray emission to be produced by the inverse Compton mechanism, the magnetic field strength can be directly estimated to be of the order of 1G.