Origin of radio polarization in pulsar polar caps

Abstract

It is crucial to know the polarization properties of coherent radio waves that escape from pulsar polar caps to calculate the radiative transfer through the magnetosphere and to predict observable radio properties. We describe pair cascades in the pulsar polar cap, and we determine for the first time the Stokes parameters of the escaping radio waves from first-principle kinetic simulations for a pulsar with a magnetic obliquity of $60^{\circ}$. We present 3D particle-in-cell kinetic simulations that include quantum-electrodynamic pair cascades in a charge-limited flow from the stellar surface. Our model quantitatively and qualitatively explains the observed pulsar radio powers and spectra, the pulse profiles, polarization curves, their temporal variability, the strong Stokes-$L$ and weak Stokes-$V$ polarization components, the decline in the linear polarization with frequency, and the nonexistence of a radius-to-frequency relation. The observable properties of radio emission from the polar cap can vary and include single- or double-peaked profiles. Most of the Stokes~$V$ curves from our simulations appear to be antisymmetric, but symmetric curves are also present at some viewing angles. Although the polarization-angle (PA) swing of the radiation from the polar cap fits the rotating vector model (RVM) for most viewing angles, the angles obtained from the RVM do not correspond to the dipole geometry of the magnetic field. Instead, the PA is directly related to the plasma flows in the polar cap. Our simulations demonstrate that pair discharges close to the surface of the polar cap cause the radio emission of pulsars and determine the majority of their typically observed properties. The merits of RVM for estimations of the magnetic field geometry from observations need to be reevaluated.

Figures (9)

• Figure 1: Scheme of the polar cap in the our model (not to scale).
• Figure 2: Magnetospheric current profile across the polar cap at the stellar surface.
• Figure 3: Plasma number density in the polar cap at the simulation end. (a) Electron density, (b) positron density, (c) proton density, and (d) total density. One half of the domain is selected for $z > 0$. The quantities in closed field lines are set to zero.
• Figure 4: Same as Fig. \ref{['fig3']}, but for parallel electric fields (a) and Poynting flux (b), both parallel to the magnetic field.
• Figure 5: Stokes parameters $I$, $V$, $Q$, and $U$ (a--d) of the escaping radiation captured in a plane at a height $H \approx 2150$ m, averaged over the time interval $T \in [34.7,43.4]\,\mu$s, and normalized to the maximum value of the Stokes $I$ value. The dashed magenta line denotes the last open field line, and the magenta plus shows the dipole axis. The black plus denotes the maximum of the average plasma density. The black squares in (a) denote regions for which the spectra are analyzed below. The color scales differ between figures.