The nature of low-frequency cutoffs in the radio emission spectra of pulsars

Abstract

Low-frequency cutoffs were first observed in the radiation spectra of several pulsars about a decade ago¹. The detailed study of the low-frequency (ω≲100 MHz) radio emission spectra of more than 50 pulsars^2–7 has shown that practically all pulsars observed have low-frequency cutoffs in their spectra. In most pulsars the spectral cutoffs are an inherent feature of pulsar emission and are obviously associated with the emission mechanism⁸. The most favoured hypothesis at present is that the radio emission of pulsars is a curvature radiation of charged plasma bunches that move, with a Lorentz factor of Γ ≅ 10³, practically along curved magnetic field lines^9,10. The kinetic energy of plasma bunches is comparable with the brightness temperature of radio emission measured in energy units. Hence, the reabsorption of the curvature radiation by the radiating plasma bunches themselves may be essential for the formation of the radio emission spectra of pulsars. Here we show that this radiation reabsorption may be responsible for low-frequency cutoffs in the pulsar radio emission spectra.