Journal Article

Aligning VLBI images of active galactic nuclei at different frequencies

S. M. Croke and D. C. Gabuzda

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 386, issue 2, pages 619-626
Published in print May 2008 | ISSN: 0035-8711
Published online April 2008 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2008.13087.x
Aligning VLBI images of active galactic nuclei at different frequencies

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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 are 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 multifrequency data comparison techniques can be applied. We consider the origins for the aforementioned 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.

Keywords: techniques: image processing; galaxies: active

Journal Article.  4784 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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