Frequency evolution of pulsar emission: Further evidence for fan beam model

Abstract

Aims. We explore frequency-dependent changes in pulsar radio emission by analyzing their profile widths and emission heights, assessing whether the simple radius-to-frequency mapping (RFM) or the fan beam model can describe the data. Methods. Using wideband (704-4032 MHz) Murriyang (Parkes) observations of over 100 pulsars, we measured profile widths at multiple intensity levels, fit Gaussian components, and used aberration-retardation effects to estimate emission altitudes. We compared trends in width evolution and emission height with a fan beam model. Results. Similar to other recent studies, we find that while many pulsars show profiles narrowing with increasing frequency, a substantial fraction show the reverse. The Gaussian decomposition of the profiles reveals that the peak locations of the components vary little with frequency. However, the component widths do, in general, narrow with increasing frequency. This argues that propagation effects are responsible for the width evolution of the profiles rather than emission height. Overall, the evolution of the emission height with frequency is unclear, and clouded by the assumptions in the model. Spin-down luminosity correlates weakly with profile narrowing but not with emission height. Conclusions. The classic picture where pulsars emit at a single emission height which decreases with increasing observing frequency cannot explain the diversity in behavior observed here. Instead, pulsar beams likely originate from extended regions at multiple altitudes, with fan-beam or patchy structures dominating their frequency evolution. Future models must incorporate realistic plasma physics and multi-altitude emission to capture the range of pulsar behaviors.

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