Gamma-ray pulsars in the MeV region

Most models for the magnetosphere of pulsars assume ultrarelativistic primary particles streaming along the field lines; the resulting curvature photons can initiate electro-optical showers, which provide a secondary particle population at a lower energy. Both gamma and radio radiation are thought to be explicable within this framework. We show that - because of the properties of curvature radiation - there is an intrinsic lower limit to the gamma-ray frequency which a pulsar can emit, if the power level is comparable to the rotational energy loss. Furthermore, independently of the radiative efficiency, a short duty cycle cannot be obtained at low gamma-ray frequencies, without producing at the same time certain observable spectral features. Low-energy gamma-ray experiments on suitable candidates are expected to provide far-reaching information on the pulsar mechanisms. (orig.)