The polarimetric response of the Nan\c{c}ay Radio Telescope and its impact on precision pulsar timing

TL;DR

This study investigates the polarimetric response of the Nan extc{a}cy Radio Telescope, demonstrating that it does not vary with observation angle and that a new calibration method improves pulsar timing accuracy and data quality.

Contribution

The paper introduces a new calibration approach for NRT pulsar data before November 2019 and confirms the stability of the telescope's polarimetric response across different observation angles.

Findings

Polarimetric response does not vary with hour angle or declination.

New METM calibration significantly improves pre-2019 data quality.

Enhanced calibration leads to more accurate and homogeneous pulsar timing measurements.

Abstract

In \citet{Guillemot2023} we presented a new method for calibrating pulsar observations conducted with the Nan\c{c}ay decimetric Radio Telescope (NRT), which significantly improved NRT polarimetric measurements and pulsar timing quality for data taken after this method was developed, in November 2019. Results hinted at a dependence of the polarimetric response of the NRT on the observed direction. We investigated this potential dependence, since unaccounted variations of the instrumental response could degrade polarimetric measurements. Additionally, we aimed to develop a method for properly calibrating NRT pulsar observations conducted before November 2019. We conducted three series of observations of bright pulsars over wide declination ranges, in a special observation mode in which the feed horn rotates by $\sim$ 180$^\circ$ degrees across the observation, enabling us to determine the full polarimetric response of the NRT while modeling potential variations of calibration parameters with hour angle and declination. In addition, we used the METM technique to improve the calibration of pre-November 2019 data. From the analysis of the series of observations of bright pulsars with horn rotation, we found that the polarimetric response of the NRT does not appear to vary with hour angle or declination. On the other hand, the new METM-based calibration method appears to significantly improve the calibration of pre-November 2019 data. By analyzing NRT data on a selection of millisecond pulsars we found that the new polarimetric profiles are more homogeneous, they generally have larger signal-to-noise ratios, and found that the TOA data for these MSPs are more accurate and contain lower levels of noise, especially when combining the new calibration method with the \textit{Matrix Template Matching} (MTM) method for extracting TOAs from pulsar observations.