INVERSE COMPTON-SCATTERING - GAP PARAMETERS, ENERGY-LOSS OF THE PARTICLES, AND POSSIBLE IMPLICATIONS FOR PULSAR RADIO-EMISSION

The process of inverse Compton scattering (ICS) of high-energy particl es off the thermal photons above the pulsar polar cap is the dominant mechanism causing polar gap breakdown. In this paper, an approximate a nalytic description of the parameters of the two modes of the ICS gap, namely, the ''resonant ICS gap'' and the ''thermal ICS gap,'' are der ived under the assumption of a multipolar magnetic field configuration in the neutron star's vicinity. For typical values of P, B, and T, th e thermal ICS gap usually has a lower height and smaller potential dro p but results in larger Lorentz factors for the secondary particles ej ected from the gap. Above the polar gap, the secondary particles do no t keep the energy gained from the primary gamma-rays, as expected. The y will lose much of their energy via different mechanisms, among which the ICS process is dominant, as pointed out previously by different a uthors. We calculate the height-dependent Lorentz factor decrease of t he secondary particles via ICS above the gap. It is found that, for a certain polar cap temperature and a temperature of the whole neutron s tar surface, the particles will lose most of their energy as a result of the ''resonant'' scattering within a not very high altitude (usuall y similar to 0.1-0.01 neutron star radii) after they escape from the g ap, then keep nearly unchanged energies for a long distance of several to a dozen stellar radii (the so-called Lorentz platform), and then e ndure severe energy loss again, due to the ''nonresonant'' scattering dominant at higher altitudes. This effect has a direct implication for pulsar radiation theories, if pulsar radio emission comes from tens o r hundreds of kilometers above the neutron star surface, which is just the height range of the ''Lorentz platform'' that we discuss. We caut ion that, if the polar cap temperature of a pulsar is relatively high, but not high enough to switch its gap to the ''thermal mode,'' the se condary particles will receive lower energies from the resonant ICS ga p but endure severe energy loss above the gap, so that their Lorentz f actors may be below the possible threshold for pulsar radio emission. In this case, the pulsar will ''null,'' or be radio-quiet. Some pulsar s' ''nulling'' feature and Geminga's radio-quiet behavior may be a con sequence of this effect.