Inverse compton scattering: Gap parameters, energy loss of the particles, and possible implications for pulsar radio emission

Abstract

The process of inverse Compton scattering (ICS) of high-energy particles off the thermal photons above the pulsar polar cap is the dominant mechanism causing polar gap breakdown. In this paper, an approximate analytic description of the parameters of the two modes of the ICS gap, namely, the "resonant ICS gap" and the "thermal ICS gap, " are derived under the assumption of a multipolar magnetic field configuration in the neutron star's vicinity. For typical values of P, B, and T, the thermal ICS gap usually has a lower height and smaller potential drop but results in larger Lorentz factors for the secondary particles ejected from the gap. Above the polar gap, the secondary particles do not keep the energy gained from the primary γ-rays, as expected. They will lose much of their energy via different mechanisms, among which the ICS process is dominant, as pointed out previously by different authors. We calculate the height-dependent Lorentz factor decrease of the secondary particles via ICS above the gap. It is found that, for a certain polar cap temperature and a temperature of the whole neutron star surface, the particles will lose most of their energy as a result of the "resonant" scattering within a not very high altitude (usually ∼0.1-0.01 neutron star radii) after they escape from the gap, then keep nearly unchanged energies for a long distance of several to a dozen stellar radii (the so-called Lorentz platform), and then endure 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 or hundreds of kilometers above the neutron star surface, which is just the height range of the "Lorentz platform "that we discuss. We caution 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 secondary particles will receive lower energies from the resonant ICS gap but endure severe energy loss above the gap, so that their Lorentz factors may be below the possible threshold for pulsar radio emission. In this case, the pulsar will "null, " or be radio-quiet. Some pulsars' "nulling" feature and Geminga's radio-quiet behavior may be a consequence of this effect. © 1997. The American Astronomical Society. All rights reserved.