Frequency-Dependent Core Shifts in Ultracompact Quasars

Voitsik, P. A.; Pushkarev, A. B.; Kovalev, Y. Y.; Plavin, A. V.; Lobanov, A. P.; Ипатов, А. В.

doi:10.1134/s1063772918110094

Cited by 15 publications

(14 citation statements)

References 25 publications

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“…One way to achieve this alignment is to use phasereferencing to a nearby calibrator source (Marcaide & Shapiro 1984;Fomalont 1995;Voitsik et al 2018) resolving the core shift for targets and calibrators. The position of a target source can then be robustly located relative to the calibrator position.…”

Section: Image Alignmentmentioning

confidence: 99%

Significant core shift variability in parsec-scale jets of active galactic nuclei

Plavin

Kovalev

Pushkarev

et al. 2019

Monthly Notices of the Royal Astronomical Society

104

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The apparent position of jet base (core) in radio-loud active galactic nuclei changes with frequency because of synchrotron self-absorption. Studying this 'core shift' effect enables us to reconstruct properties of the jet regions close to the central engine. We report here results from core shift measurements in AGNs observed with global VLBI at 2 and 8 GHz at epochs from 1994 to 2016. Our sample contains 40 objects observed at least 10 times during that period. The core shift is determined using a new automatic procedure introduced to minimize possible biases. The resulting multiple epoch measurements of the core position are employed for examining temporal variability of the core shift. We argue that the core shift variability is a common phenomenon, as established for 33 of 40 AGNs we study. Our analysis shows that the typical offsets between the core positions at 2 and 8 GHz are about 0.5 mas and they vary in time. Typical variability of the individual core positions is about 0.3 mas. The measurements show a strong dependence between the core position and its flux density, suggesting that changes in both are likely related to the nuclear flares injecting denser plasma into the flow. We determine that density of emitting relativistic particles significantly increases during these flares, while relative magnetic field changes less and in the opposite direction.

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Section: Image Alignmentmentioning

confidence: 99%

Significant core shift variability in parsec-scale jets of active galactic nuclei

Plavin

Kovalev

Pushkarev

et al. 2019

Monthly Notices of the Royal Astronomical Society

104

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“…The method requires sub-milliarcsecond imaging and astrometry with very-long-baseline interferometry (VLBI). The above technique, commonly known as the 'core-shift' measurement, has been increasingly used to infer jet magnetic field strengths over the past decade (Kovalev et al 2008;O'Sullivan & Gabuzda 2009;Sokolovsky et al 2011;Pushkarev et al 2012Pushkarev et al , 2018Kutkin et al 2014;Voitsik et al 2018;Plavin et al 2019). Another, closely related method for measuring the jet magnetic field strength is to use multi-frequency VLBI to resolve the size of the emission region at its synchrotron turnover frequencythis gives the magnetic flux density without an equipartition assumption but tends to be more difficult in practice than the core-shift measurement (e.g., Savolainen et al 2008;Hodgson et al 2017).…”

Section: Introductionmentioning

confidence: 99%

Testing the magnetic flux paradigm for AGN radio loudness with a radio-intermediate quasar

Chamani

Savolainen

Hada

et al. 2021

A&A

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For understanding the diversity of jetted active galactic nuclei (AGN) and especially the puzzling wide range in their radio loudness, it is important to understand what role the magnetic fields play in setting the power of relativistic jets in AGN. We have performed VLBA phase-referencing observations of the radio-intermediate quasar IIIZw 2 to estimate jet magnetic flux by measuring the core-shift effect. Multi-frequency observations at 4 GHz, 8 GHz, 15 GHz, and 24 GHz were made using three nearby calibrators as reference sources. By combining the self-referencing core shift of each calibrator with the phase-referencing core shifts, we obtained an upper limit of 0.16 mas for the core shift between 4 and 24 GHz in IIIZw 2. By assuming equipartition between magnetic and particle energy densities and adopting the flux-freezing approximation, we further estimated the upper limit for both the magnetic field strength and poloidal magnetic flux threading the black hole. We find that the upper limit to the measured magnetic flux is smaller by at least a factor of five compared to the value predicted by the magnetically arrested disk (MAD) model. An alternative way to derive the jet magnetic field strength from the turnover of the synchrotron spectrum leads to an even smaller upper limit. Hence, the central engine of IIIZw 2 has not reached the MAD state, which could explain why it has failed to develop a powerful jet even though the source harbours a fast-spinning black hole. However, it generates an intermittent jet, which is possibly triggered by small-scale magnetic field fluctuations, as predicted by the magnetic flux paradigm. We propose here that combining black hole spin measurements with magnetic field measurements from the very-long-baseline-interferometry core-shift observations of AGN over a range of jet powers could provide a strong test for the dominant factor that sets the jet power relative to the available accretion power.

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“…Hovatta et al 2014;Plavin et al 2019) or phase-referencing on a nearby compact calibrator (e.g. Marcaide & Shapiro 1984;Voitsik et al 2018). This paper uses the former of these methods because the absolute position of the optically thin components in the jet does not depend on frequency.…”

Section: Spectral Index Mapsmentioning

confidence: 99%

Parsec-scale properties of the peculiar gigahertz-peaked spectrum quasar 0858-279

Kosogorov,

Kovalev,

Perucho

et al. 2021

Preprint

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We performed multi-frequency studies on the gigahertz-peaked spectrum high-redshift quasar 0858−279. Initially, the source presented itself at early VLBI images as a very peculiar resolved blob. We observed the quasar with the VLBA at 1.4−24 GHz in dual-polarization mode. The high spatial resolution and spectral index maps enabled us to resolve the core-jet structure and locate a weak and compact core by its inverted spectrum. The dominant jet component 20 parsecs away from the core was found to be optically thin above 10 GHz and opaque below it. We also estimated an uncommonly strong magnetic field in the bright jet feature which turned out to be around 3 G. Faraday rotation measure maps revealed high RM values over 6000 rad/m 2 . Additionally, these maps allowed us to follow the magnetic field direction in the bright jet feature being perpendicular to the propagation direction of the jet. All the results strongly indicated the formation of a shock wave in the dominant component arising from an interaction with the surrounding matter. Using the proposed hypothesis as well as the core shift approach, we discovered that the magnetic field in the core region is of the order of 0.1 G.

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Frequency-Dependent Core Shifts in Ultracompact Quasars

Cited by 15 publications

References 25 publications

Significant core shift variability in parsec-scale jets of active galactic nuclei

Significant core shift variability in parsec-scale jets of active galactic nuclei

Testing the magnetic flux paradigm for AGN radio loudness with a radio-intermediate quasar

Parsec-scale properties of the peculiar gigahertz-peaked spectrum quasar 0858-279

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