Studying the continuum spectrum of the parsec scale jets by multi-frequency VLBI measurements

TL;DR

This paper presents a model-fitting spectral extraction method for multi-frequency VLBI observations of AGN jets, enabling detailed analysis of jet features' spectra and physical properties despite observational limitations.

Contribution

The paper introduces a novel spectral extraction technique that leverages high-frequency source structure to analyze broadband spectra at lower frequencies in VLBI data.

Findings

Detected a strong magnetic field gradient across 3C273's jet.

Measured spectra and sizes of jet features at 5-86 GHz.

Derived magnetic flux density and electron energy density in jet regions.

Abstract

Multi-frequency VLBI observations allow studies of the continuum spectrum in the different parts of the parsec scale jets of AGN, providing information on the physical properties of the plasma and magnetic fields in them. Since VLBI networks cannot be scaled, the range of spatial frequencies observed differs significantly between the different observing frequencies, which makes it difficult to obtain a broadband spectrum of the individual emission features in the jet. In this paper we discuss a model-fitting based spectral extraction method, which can significantly relieve this problem. The method uses a priori knowledge of the source structure, measured at high frequencies, to allow at lower frequencies the derivation of the sizes and flux densities of even those emission features that have mutual separations significantly less than the Rayleigh limit at the given frequency. We have successfully used this method in the analysis of 5-86 GHz VLBA data of 3C273. The spectra and sizes of several individual jet features were measured, thus allowing derivation of the magnetic flux density and the energy density of the relativistic electrons in the different parts of the jet. We discuss the results, which include e.g. a detection of a strong gradient in the magnetic field across the jet of 3C273.