HIGH-PRECISION ASTROMETRIC MILLIMETER VERY LONG BASELINE INTERFEROMETRY USING A NEW METHOD for MULTI-FREQUENCY CALIBRATION

Richard Dodson, Maria J. Rioja , Sol N. Molina, José L. Gómez

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Abstract

In this paper we describe a new approach for millimeter Very Long Baseline Interferometry (mm-VLBI) calibration that provides bona-fide astrometric alignment of the millimeter-wavelength images from a single source, for the measurement of frequency-dependent effects, such as "core-shifts" near the black hole of active galactic nucleus jets. We achieve our astrometric alignment by solving first for the ionospheric (dispersive) contributions using wide-band centimeter-wavelength observations. Second, we solve for the tropospheric (non-dispersive) contributions by using fast frequency-switching at the target millimeter-wavelengths. These solutions can be scaled and transferred from low frequency to the high frequency. To complete the calibration chain an additional step is required to remove a residual constant phase offset on each antenna. The result is an astrometric calibration and the measurement of the core-shift between 22 and 43 GHz for the jet in BL Lacertae to be -8 ±5, 20 ±6 μas, in R.A. and decl., respectively. By comparison to conventional phase referencing at centimeter-wavelengths we are able to show that this core shift at millimeter-wavelengths is significantly less than what would be predicted by extrapolating the low-frequency result, which closely followed the predictions of the Blandford & Kö nigl conical jet model. As such it would be the first demonstration for the association of the VLBI core with a recollimation shock, normally hidden at low frequencies due to the optical depth, which could be responsible for the γ-ray production in blazar jets.

Original languageEnglish
Article number177
JournalAstrophysical Journal
Volume834
Issue number2
DOIs
Publication statusPublished - 10 Jan 2017

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very long baseline interferometry
interferometry
wavelength
wavelengths
low frequencies
calibration
shift
alignment
very long base interferometry
active galactic nuclei
optical thickness
ionospherics
optical depth
antenna
rays
antennas
shock
method
broadband
prediction

Cite this

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title = "HIGH-PRECISION ASTROMETRIC MILLIMETER VERY LONG BASELINE INTERFEROMETRY USING A NEW METHOD for MULTI-FREQUENCY CALIBRATION",
abstract = "In this paper we describe a new approach for millimeter Very Long Baseline Interferometry (mm-VLBI) calibration that provides bona-fide astrometric alignment of the millimeter-wavelength images from a single source, for the measurement of frequency-dependent effects, such as {"}core-shifts{"} near the black hole of active galactic nucleus jets. We achieve our astrometric alignment by solving first for the ionospheric (dispersive) contributions using wide-band centimeter-wavelength observations. Second, we solve for the tropospheric (non-dispersive) contributions by using fast frequency-switching at the target millimeter-wavelengths. These solutions can be scaled and transferred from low frequency to the high frequency. To complete the calibration chain an additional step is required to remove a residual constant phase offset on each antenna. The result is an astrometric calibration and the measurement of the core-shift between 22 and 43 GHz for the jet in BL Lacertae to be -8 ±5, 20 ±6 μas, in R.A. and decl., respectively. By comparison to conventional phase referencing at centimeter-wavelengths we are able to show that this core shift at millimeter-wavelengths is significantly less than what would be predicted by extrapolating the low-frequency result, which closely followed the predictions of the Blandford & K{\"o} nigl conical jet model. As such it would be the first demonstration for the association of the VLBI core with a recollimation shock, normally hidden at low frequencies due to the optical depth, which could be responsible for the γ-ray production in blazar jets.",
keywords = "astrometry, quasars: individual (BL-Lac), radio continuum: galaxies, techniques: interferometric",
author = "Richard Dodson and Rioja, {Maria J.} and Molina, {Sol N.} and G{\'o}mez, {Jos{\'e} L.}",
year = "2017",
month = "1",
day = "10",
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language = "English",
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HIGH-PRECISION ASTROMETRIC MILLIMETER VERY LONG BASELINE INTERFEROMETRY USING A NEW METHOD for MULTI-FREQUENCY CALIBRATION. / Dodson, Richard; Rioja , Maria J.; Molina, Sol N.; Gómez, José L.

In: Astrophysical Journal, Vol. 834, No. 2, 177, 10.01.2017.

Research output: Contribution to journalArticle

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KW - astrometry

KW - quasars: individual (BL-Lac)

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