Radio structure of 3C147 determined by multi-element very long baseline interferometry

Interferometric Imaging Directly with Closure Phases and Closure Amplitudes

“…The results in §5.1 and §5.2 demonstrate that closure amplitudes and phases can be directly used in interferometric imaging to produce images that are insensitive to phase and amplitude calibration errors. Traditional selfcalibration and imaging loops require many iterations of CLEAN imaging and fitting complex gains to visibili-ties (e.g., Wilkinson et al 1977;Readhead & Wilkinson 1978;Readhead et al 1980;Schwab 1980;Cornwell & Wilkinson 1981;Pearson & Readhead 1984;Walker 1995;Cornwell & Fomalont 1999). These loops contain many tunable parameters including the choice of initial source model, the strategy for independent or concurrent calibration of amplitude and phase gains, the CLEAN convergence criterion, the choice of taper and weighting for the CLEAN visibilities, and the scales and regions to clean in each CLEAN iteration.…”

“…The self-calibration procedure starts from an initial model image and solves for the set of time-dependent complex gains in Equation (3) either by fixing a sufficient set of amplitudes or phases directly from the image and solving for the rest analytically (Wilkinson et al 1977;Readhead et al 1980), or by finding a set that minimizes the sum of squares of the differences between the measured and model visibilities (Schwab 1980;Cornwell & Wilkinson 1981). Self-calibration is often performed by first solving only for the phases of the complex gains, correcting the amplitudes at a later stage (Walker 1995;Cornwell & Fomalont 1999).…”

“…The standard algorithm used for interferometric imaging is CLEAN (Högbom 1974;Clark 1980), which deconvolves a dirty image produced by an inverse Fourier transform by decomposing it into point sources. When the calibration is uncertain, the usual approach is to iterate between imaging with CLEAN and deriving new calibration solutions using information from the previous image -a so-called "self calibration" or "hybrid mapping" loop (e.g., Wilkinson et al 1977;Readhead & Wilkinson 1978;Readhead et al 1980;Schwab 1980;Cornwell & Wilkinson 1981;Pearson & Readhead 1984;Walker 1995;Cornwell & Fomalont 1999;Thompson et al 2017). The results and time to convergence of this approach depend on many assumptions made in the course of this hybrid process, including the initial source model used for self-calibration, the choice of which regions to clean in a given iteration, the method used for deriving complex gains from a given image, and the choice of how frequently to re-calibrate the data.…”

Relativistic Jets from Active Galactic Nuclei

“…Structure and Kinematics. Three key observational facts emerged in the early years of VLBI imaging: (i) they are one-sided jets (Wilkinson et al 1977);(ii) they have a compact flat-spectrum core at one end of a steep-spectrum jet (Readhead et al 1978b); and (iii) components are often seen to expand or to move along the jet away from the core at superluminal speeds (Gubbay et al 1969;Zensus & Pearson 1987). By far the most comprehensive VLBI study monitoring blazars is that of the MOJAVE group (Lister et al 2016a;Lister 2016b), where the results of VLBI monitoring observations of 400 AGN jets spanning 20 years are presented.…”

The Milliarcsecond Core of 3C147 at 6 cm

“…3C147 is a compact 0^,1"), steep spectrum radio source identified with a quasar at z = 0.545 (OVOOl = 7.4 pc; c/H 0 = 6000 Mpc and q 0 = 0.5). The radio structure shown by VLBI observations at 18 cm (Readhead & Wilkinson, 1980;Simon et al, this volume), at 50 cm (Wilkinson et al, 1977), and at 90 cm (Simon et al, 1980 and1983) shows a bright 'core 1 ^0VOO8 (60 pc; at one end of a ! jet T ^0V2 (1.5 kpc) in length oriented in p.a.…”