Aligning VLBI Images of Active Galactic Nuclei at Different Frequencies

TL;DR

This paper discusses the challenges of aligning VLBI images of active galactic nuclei at different frequencies, and introduces a cross-correlation based technique to improve image registration for multi-frequency analysis.

Contribution

It presents a novel cross-correlation based method for correcting misalignments in multi-frequency VLBI images of active galactic nuclei.

Findings

The technique effectively reduces image misalignments.

Improves accuracy of spectral-index and rotation-measure mapping.

Addresses effects of source variability and resolution differences.

Abstract

Many important techniques for investigating the properties of extragalactic radio sources, such as spectral-index and rotation-measure mapping, involve the comparison of images at two or more frequencies. In the case of radio interferometric data, this can be done by comparing the CLEAN maps obtained at the different frequencies. However, intrinsic differences in images due to the frequency dependence of the radio emission can be distorted by additional differences that arise due to source variability (if the data to be compared is obtained at different times), image misalignment, and the frequency dependence of the sensitivity to weak emission and the angular resolution provided by the observations (the resolution of an interferometer depends on the lengths of its baselines in units of the observing wavelength). These effects must be corrected for as best as possible before multi-frequency data comparison techniques can be applied. We consider the origins for the afore-mentioned factors, outline the standard techniques used to overcome these difficulties, and describe in detail a technique developed by us, based on the cross-correlation technique widely used in other fields, to correct for misalignments between maps at different frequencies.