1995A&A...301..749R

For over twenty years numerous claims have been made about positive detections of high energy radiation from several X-ray binaries by means of both Cherenkov light detectors and particle detectors. If real, those findings require incorporation of episodic relativistic processes into the conventional `thermal' picture of these objects. We calculate the transfer of high energy photons through the magnetic field of an accreting neutron star taking as an example Her X-1 - a prime candidate for a high-energy gamma-ray source. In some models such photons originate episodically in the π<sup>0</sup>-decay process in outer parts of the object's magnetosphere. They may, however, be subject to substantial absorption due to presence of a strong magnetic field. With the help of Monte Carlo simulations we follow the development and propagation of electron-positron pair cascades initiated by one-photon pair creation. In the case of a dipolar magnetic field the π<sup>0</sup>-decay photons can escape freely but only within a very narrow cone with an opening angle of ∼1deg. Otherwise the resulting cascade spectra have high energy cut-offs ranging from ∼1TeV to ∼10TeV. We conclude that Her X-1 is an unlikely source of PeV radiation unless very special conditions are met. At energies below ∼1TeV we expect an enhancement of the photon flux in comparison with the `optically thin' case (i.e. when one-photon absorption is neglected). Therefore the object may be occasionally seen with the atmospheric Cherenkov technique. We also address the question of theoretical photon spectra in the MeV and GeV bands in the context of the recent experiments like COMPTEL and EGRET.