The Polarization Convention of the uGMRT in Band 4

Abstract

We present an experiment performed to understand the polarization convention adopted for band 4 (550--900 MHz) of the upgraded Giant Metrewave Radio Telescope (uGMRT). For that we observed the pulsar B1702--19 in this band, both in interferometry and pulsar modes, and compare the results with its already known Stokes $I$, $Q$, $U$, $V$ profiles obtained with the Lovell telescope. We find that the results obtained from interferometry and pulsar modes of the uGMRT agree with each other. However, although the Stokes $U$ profile obtained with the uGMRT match with that obtained by the Lovell telescope, Stokes $Q$ and $V$ do not. This can be explained if the $X$ and $Y$ dipoles in this band, from which $R$ and $L$ are derived, are swapped w.r.t. the IAU convention. The swapping makes $RR^*$ and $LL^*$ of the uGMRT band 4 to be $LL^*$ and $RR^*$ respectively according to the IEEE convention. This implies that if we need to compare polarization measurements obtained in band 4 of the uGMRT with telescopes like Lovell, Parkes, Very Large Array etc. (all follow IEEE convention for defining right and left hand circular polarization), we must interchange $RR^*$ and $LL^*$, and change the sign of Stokes $Q$ for the uGMRT data. Note that this is the current convention for uGMRT band 4, and is likely to change in future once the swapping of the dipoles is taken care of. Once it is done, it will be notified in another technical report.