Spectral Variation across Pulsar Profile due to Coherent Curvature Radiation

Abstract

Abstract The pulsar profile is characterized by two distinct emission components, the core and the cone. The standard model of a pulsar radio emission beam originating from dipolar magnetic fields places the core at the center surrounded by concentric layers of inner and outer conal components. The core emission is expected to have steeper spectra compared to the cones. We present a detailed analysis of the relative differences between the spectra of the core and conal emission from a large sample of 53 pulsars over a wide frequency range between 100 MHz and 10 GHz. The core spectra were seen to be much steeper than those of the cones, particularly between 100 MHz and 1 GHz, with a relative difference between the spectral index of Δ α core / cone ∼ −1.0. In addition we found the spectra of the outer conal components to be steeper than those of the inner cone with a relative difference in the spectral index of Δα in/out ∼ +0.5. The flattening of the spectra from the magnetic axis toward the edge of the open field line region with increasing curvature of the field lines is a natural outcome of the coherent curvature radiation from charged soliton bunches and explains the difference in spectra between the core and the cones. In addition, due to the relativistic beaming effect, the radiation is only visible when it is directed toward the observer over a narrow angle θ ≤ 1/γ, where γ is the Lorentz factor of the outflowing plasma clouds. This restricts the emission particularly from outer cones that are associated with field lines with larger curvature, thereby making their spectra steeper than those of the inner cones.