High-precision Astrometric Millimeter Very Long Baseline Interferometry Using a New Method for Multi-frequency Calibration

DOI: 
10.3847/1538-4357/834/2/177
Publication date: 
01/01/2017
Main author: 
Dodson, Richard
IAA authors: 
Molina, Sol N.;Gómez, José L.
Authors: 
Dodson, Richard;Rioja, María J.;Molina, Sol N.;Gómez, José L.
Journal: 
The Astrophysical Journal
Refereed: 
Yes
Publication type: 
Article
Volume: 
834
Pages: 
177
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 & Königl 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.
Database: 
ADS
SCOPUS
URL: 
https://www.scopus.com/inward/record.uri?eid=2-s2.0-85010440539&doi=10.3847%2f1538-4357%2f834%2f2%2f177&partnerID=40&md5=96f603dcb6021e68331560b6f1366bd3
ADS Bibcode: 
2017ApJ...834..177D
Keywords: 
astrometry;quasars: individual: BL-Lac;radio continuum: galaxies;techniques: interferometric;Astrophysics - Instrumentation and Methods for Astrophysics;Astrophysics - Astrophysics of Galaxies