Emission Altitude from Relativistic Phase Shift Due to Plasma Corotation in Pulsars

Abstract

The radio emission from a relativistic plasma in a pulsar magnetosphere experiences a relativistic phase shift due to spin and difference in the emission altitude. Using the single pulse data of PSR B0329 + 54 at 325 MHz and 606 MHz we studied the structure of the pulsar emission beam. The distribution of the pulse components around the central core component indicates that the emission beam consists of four nested cones. The asymmetry in the location of the conal components in the leading versus trailing parts of the profile is interpreted as being due to aberration and retardation in the pulsar magnetosphere. These measurements allow us to determine the precise location of the 4 conal rings of emission. We find that the successive outer cones are emitted at higher altitudes in the magnetosphere. Further, for any given cone, the emission height at the lower frequency is found to be larger than that at the higher frequency. We discuss the implications of our new findings on our understanding of the pulsar emission geometry and its impact on the emission mechanisms. The relativistic plasma moving along the dipolar magnetic field lines emit coherent curvature radiation. The beamed emission occurs in the direction of tangents to the field lines, and to receive it, the sight line must align with the tangent within the beaming angle 1/γ, where γ is the particle Lorentz factor. By solving the viewing geometry in an inclined and rotating dipole magnetic field, we show that, at any given pulse phase, the observer tends to receive radiation only from the specific heights allowed by the geometry.