On Measuring Accurate 21-cm Line Profiles with the Robert C. Byrd Green Bank Telescope

TL;DR

This paper investigates the inaccuracies in 21 cm line profiles measured with the Green Bank Telescope, identifying calibration errors, sidelobe effects, and proposing correction schemes to improve measurement accuracy.

Contribution

It introduces a detailed analysis of GBT measurement inaccuracies and presents a correction scheme for more accurate 21 cm line profile measurements.

Findings

~10% calibration gain errors identified

Significant sidelobe contributions affect measurements

Proposed correction scheme improves profile accuracy

Abstract

We use observational data to show that 21 cm line profiles measured with the Green Bank Telescope (GBT) are subject to significant inaccuracy. These include ~10% errors in the calibrated gain and significant contribution from distant sidelobes. In addition, there are ~60% variations between the GBT and Leiden/Argentine/Bonn 21 cm line profile intensities, which probably occur because of the high main-beam efficiency of the GBT. Stokes V profiles from the GBT contain inaccuracies that are related to the distant sidelobes. We illustrate these problems, define physically motivated components for the sidelobes, and provide numerical results showing the inaccuracies. We provide a correction scheme for Stokes I 21 cm line profiles that is fairly successful and provide some rule-of-thumb comments concerning the accuracy of Stokes V profiles.