Dual-high-frequency VLBI study of blazar-jet brightness-temperature gradients and collimation profiles

Abstract

Context. On the kiloparsec scale, extragalactic radio jets show two distinct morphologies related to their power: collimated high-power jets ending in a bright termination shock and low-power jets opening up close to the core and showing a more diffuse surface brightness distribution. The emergence of this morphological dichotomy on the parsec scale at the innermost jet regions can be studied with very-long-baseline interferometry (VLBI) radio observations of blazars in which the jet emission is strongly Doppler boosted due to relativistic bulk motion at small angles between the jet direction and the line of sight. Aims. We seek to characterize the geometry and emission profiles of the parsec-scale radio jets of flat-spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs) on parsec scales to derive properties of the magnetic field, environment, and energetics for different classes of extragalactic jets. Methods. We analyze the VLBI radio data of 15 FSRQs, 11 BL Lacs, and two radio galaxies contained in both the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments data archive and the Boston University blazar group sample archive at 15 GHz and 43 GHz, repectively. We derived the brightness-temperature and jet-width gradients along the jet axis from parameterizations of the jets using 2D Gaussian brightness distributions. Results. In most BL Lac objects, the diameter and brightness-temperature gradients along the jet axis can generally be described well by single power laws, while the jets of FSRQs show more complex behavior and remain more strongly collimated on larger physical scales. We find evidence for a transition of the global jet geometry from a parabolic to a conical shape in the BL Lac objects 3C 66A, Mrk 421 and BL Lacertae, the radio galaxy 3C 111 and the FSRQs CTA 26, PKS 0528+134, 4C +71.07, 4C +29.45, and 3C 279 outside the Bondi sphere. Conclusions. Our results combined with findings from kinematic VLBI studies that the jets of FSRQs exhibit larger bulk Lorentz factors than BL Lacs are in agreement with relativistic magnetohydrodynamical jet-disk simulations in which the flattening of the jet magnetization profile due to magnetic fields from the accretion disk leads to a more persistent collimation in high-accretion-rate blazars.

First GMVA observations with the upgraded NOEMA facility: VLBI imaging of BL Lacertae in a flaring state

“…The best-fit parameters of the fits are on average: a = 0.21 ± 0.02 and k = 0.74 ± 0.05 (with 1σ uncertainty); the three fits resulted in the index k of 0.73-0.75, thus remaining consistent with respect to each other within the errorbars. Previous works measured the collimation profile of the jet at various observing frequencies with different approaches: stacked VLBI images (Kovalev et al 2020;Casadio et al 2021) and model-fit components of single-epoch images over a longterm period (Burd et al 2022). Their results on the inner jet region span a wide range; k spans the interval 0.5-1.2, larger at higher frequencies.…”

First Adiabatic Invariant and the Brightness Temperature of Relativistic Jets

“…However, despite the large volume of accumulated information, many issues still await their resolution. In particular, this concerns the recently detected break in the dependence of the brightness temperature T br on the distance to the "central engine" z ( Kadler et al 2004;Baczko et al 2019;Burd et al 2022) that occurs in many cases at distances ∼ 1 pc, i.e., exactly in the region of the transition from parabolic to conical shape. This break is observed already in dozens of objects, with the exponents a in the power law T br ∝ z −a being confined in a wide range, a 1 = 3.0 ± 1.0, (1) a 2 = 2.7 ± 1.0,…”