Polarization of pulsar radiation in the two - pole caustic model and in the outer gap model ∗

We present linear polarization characteristics of pulsar radiation as predicted by two models of high-energy radiation from extended regions in the outer magnetosphere: the recently proposed two-pole caustic 1 Two-pole caustic model vs outer gap model The two-pole caustic model differs from the outer gap model in that its emission region extends below the null charge surface toward the polar cap (Fig. 1). The TPC model discards emission from regions close to the light cylinder (ie. from ρ > ∼ 0.8R lc where ρ is the distance from the rotation axis and R lc is the radius of the light cylinder), because the dynamics of electrons and the geometry of the magnetic field is not known there whereas the OG model includes this emission near the light cylinder. The TPC model assumes that the bulk of photon emission occurs on the last open field lines (r ovc = 1) whereas the OG model employs r ovc = 0.8 − 0.9, where r ovc is the " open volume coordinate " corresponding to the location of the gamma-ray emitting magnetic field lines at the star surface (r ovc = 1 – rim of the polar cap; r ovc ∼ 0 – central parts of the polar cap). Due to these differences in geometry of the emission region, the TPC model uses the double-pole interpretation of the widely separated double peaks of some gamma-ray pulsars (Crab, Vela, Geminga, B1951+32, B1046−58). The OG model offers the single-pole interpretation. Strong differences between the emission patterns of the two models, and between predicted lightcurves are shown in Fig. 2.