Improving pulsar polarization and timing measurements with the Nançay Radio Telescope

Abstract: Accurate polarimetric calibration of the radio pulse profiles from pulsars is crucial for studying their radiation properties at these wavelengths. Inaccurate calibration can also distort recorded pulse profiles, introducing noise in time of arrival (TOA) data and thus degrading pulsar timing analyses. One method for determining the full polarimetric response of a given telescope is to conduct observations of bright polarized pulsars over wide ranges of parallactic angles, to sample different orientations of their polarization angle and determine the cross-couplings between polarization feeds. The Nan\c{c}ay decimetric Radio Telescope (NRT) is a 94m equivalent meridian telescope, capable of tracking a given pulsar for approximately one hour around transit. In November 2019, we began conducting regular observations of the bright and highly linearly polarized pulsar PSR~J0742$-$2822, in a special mode where the feed horn rotates by $\sim 180^\circ$ over the course of the one hour observation, mimicking wide parallactic angle variations and enabling us to determine the polarimetric response of the NRT at 1.4~GHz. The improved polarimetric response of the NRT as determined from these observations was applied to observations of a selection of MSPs with published polarimetric properties. We find that the new polarimetric profiles and polarization position angles are consistent with previous findings, unlike NRT polarimetric results obtained with the previously used method of calibration. The analysis of timing data on J1730$-$2304, J1744$-$1134, and J1857+0953 shows that the new calibration method improves the quality of the timing, and the Matrix Template Matching (MTM) method proves very effective at reducing noise from imperfect calibration. For pulsars with sufficient degrees of polarization, the MTM method appears to be the preferred method for extracting TOAs from NRT observations.