ACCELERATION BY PULSAR WINDS IN BINARY-SYSTEMS

In the absence of accretion torques, a pulsar in a binary system will spin down due to electromagnetic dipole radiation, and the spin-down power will drive a wind of relativistic electron-positron pairs. Winds from pulsars with short periods will prevent any subsequent accretion but may be confined by the companion star atmosphere, wind, or magnetosphere to form a standing shock. We investigate the possibility of particle acceleration at such a pulsar wind shock and the production of VHE and UHE γ-rays from interactions of accelerated protons in the companion star's wind or atmosphere. We find that in close binaries containing active pulsars, protons will be shock accelerated to a maximum energy dependent on the pulsar spin-down luminosity. If a significant fraction of the spin-down power goes into particle acceleration, these systems should be sources of VHE and possibly UHE γ-rays. We discuss the application of the pulsar wind model to binary sources such as Cygnus X-3, as well as the possibility of observing VHE γ-rays from known binary radio pulsar systems.