Aligning VLBI images of active galactic nuclei at different frequencies

Sarah M. Croke, D.C. Gabuzda

Research output: Contribution to journalArticle

41 Citations (Scopus)

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 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.
LanguageEnglish
Pages619-626
Number of pages7
JournalMonthly Notices of the Royal Astronomical Society
Volume386
Issue number2
DOIs
Publication statusPublished - 11 May 2008

Fingerprint

very long base interferometry
very long baseline interferometry
active galactic nuclei
frequency dependence
radio
misalignment
interferometer
extragalactic radio sources
wavelength
radio emission
angular resolution
cross correlation
interferometers
sensitivity
comparison
wavelengths

Keywords

  • techniques
  • image processing
  • galaxies
  • active

Cite this

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title = "Aligning VLBI images of active galactic nuclei at different frequencies",
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 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.",
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Aligning VLBI images of active galactic nuclei at different frequencies. / Croke, Sarah M.; Gabuzda, D.C.

In: Monthly Notices of the Royal Astronomical Society, Vol. 386, No. 2, 11.05.2008, p. 619-626.

Research output: Contribution to journalArticle

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AU - Croke, Sarah M.

AU - Gabuzda, D.C.

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AB - 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.

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