High-Time-Resolution Measurements of the Polarization of the Crab Pulsar
  at 1.38 GHz
  Using the Westerbork Synthesis Radio Telescope (WSRT), we obtained
high-time-resolution measurements of the full (linear and circular)
polarization of the Crab pulsar. Taken at a resolution of 1/8192 of the 34-ms
pulse period (i.e., $4.1~\mu{\rm s}$), the 1.38-GHz linear-polarization
measurements are in general agreement with previous lower-time-resolution
1.4-GHz measurements of linear polarization in the main pulse (MP), in the
interpulse (IP), and in the low-frequency component (LFC). We find the MP and
IP to be linearly polarized at about $24\%$ and $21\%$, with no discernible
difference in polarization position angle. However, and contrary to theoretical
expectations and measurements in the visible, we find no evidence for
significant variation (sweep) in polarization position angle over the MP, the
IP, or the LFC. Although, the main pulse exhibits a small but statistically
significant quadratic variation in the degree of linear polarization. We
discuss the implications which appear to be in contradiction to theoretical
expectations. In addition, we detect weak circular polarization in the main
pulse and interpulse, and strong ($\approx 20\%$) circular polarization in the
low-frequency component, which also exhibits very strong ($\approx 98\%$)
linear polarization at a position angle about $40\degree$ from that of the MP
or IP. The pulse-mean polarization properties are consistent with the LFC being
a low-altitude component and the MP and IP being high-altitude caustic
components. Nevertheless, current models for the MP and IP emission do not
readily account for the observed absence of pronounced polarization changes
across the pulse. Finally, we measure IP and LFC pulse phases relative to the
MP that are consistent with recent measurements, which have shown that the
phases of these pulse components are evolving with time.
