On the Scattering and Absorption of Electromagnetic Radiation within Pulsar Magnetospheres

Abstract

The scattering and absorption of coherent radio emission by ultra-relativistic particles streaming outwards along the open field lines of a pulsar magnetosphere are investigated for incident frequencies in the guiding-centre frame less than or comparable with the particle gyrofrequency, ω_G. Two scattering regimes are isolated in which the dominant particle oscillations are parallel and perpendicular to the magnetic field, and both spontaneous and induced scattering processes are considered. For the former (longitudinal) regime, a quantum-electrodynamical calculation of the cross-section correct to first order in (ħω/mc^2) is also presented. In addition, cyclotron absorption of radio photons and their subsequent re-emission are investigated. On the basis of these calculations, it is concluded that: (i) induced scattering processes can influence the spectrum and polarization of the radio pulses if they are emitted well within the pulsar magnetosphere, (ii) neither spontaneous scattering nor small-pitch-angle synchrotron emission seems to provide a satisfactory explanation for the optical pulses from NP 0532, (iii) the avoidance of cyclotron absorption imposes important constraints on models of the radio emission process, and (iv) resonant scattering of thermal radiation from the neutron star surface is probably not an important effect.

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