Millimeter-VLBI Observations of Low-luminosity Active Galactic Nuclei with Source-frequency Phase Referencing

We report millimeter-VLBI results of low-luminosity active galactic nuclei (M84 and M87) up to 88 GHz with source-frequency phase-referencing observations. We detected the weak VLBI core and obtained the first image of M84 at 88 GHz. The derived brightness temperature of the M84 core was about 7.2 × 109 K, which could serve as a lower limit because the core down to 30 Schwarzschild radii was still unresolved in our 88 GHz observations. We successfully determined the core shifts of M87 at 22–44 GHz and 44–88 GHz through the source-frequency phase-referencing technique. The jet apex of M87 could be deduced at ∼46 μas upstream of the 43 GHz core from core-shift measurements. The estimated magnetic field strength of the 88 GHz core of M87 is 4.8 ± 2.4 G, which is at the same magnitude of 1–30 G near the event horizon probed by the Event Horizon Telescope.

A bias in VLBI measurements of the core shift effect in AGN jets

ABSTRACT The Blandford and Königl model of active galactic nuclei (AGN) jets predicts that the position of the apparent opaque jet base – the core – changes with frequency. This effect is observed with radio interferometry and is widely used to infer parameters and structure of the innermost jet regions. The position of the radio core is typically estimated by fitting a Gaussian template to the interferometric visibilities. This results in a model approximation error, i.e. a bias that can be detected and evaluated through simulations of observations with a realistic jet model. To assess the bias, we construct an artificial sample of sources based on the AGN jet model evaluated on a grid of the parameters derived from a real VLBI flux-density-limited sample and create simulated VLBI data sets at 2.3, 8.1, and 15.4 GHz. We found that the core position shifts from the true jet apex are generally overestimated. The bias is typically comparable to the core shift random error and can reach a factor of 2 for jets with large apparent opening angles. This observational bias depends mostly on the ratio between the true core shift and the image resolution. This implies that the magnetic field, the core radial distance, and the jet speed inferred from the core shift measurements are overestimated. We present a method to account for the bias.

Studies of stationary features in jets: BL Lacertae

Context. Monitoring of BL Lacertae at 15 GHz with the Very Long Baseline Array (VLBA) has revealed a quasi-stationary radio feature in the innermost part of the jet, at 0.26 mas from the radio core. Stationary features are found in many blazars, but they have rarely been explored in detail. Aims. We aim to study the kinematics, dynamics, and brightness of the quasi-stationary feature of the jet in BL Lacertae based on VLBA monitoring with submilliarcsecond resolution (subparsec-scales) over 17 years. Methods. We analysed position uncertainties and flux leakage effects of the innermost quasi-stationary feature and developed statistical tools to distinguish the motions of the stationary feature and the radio core. We constructed a toy model to simulate the observed emission of the quasi-stationary component. Results. We find that trajectories of the quasi-stationary component are aligned along the jet axis, which can be interpreted as evidence of the displacements of the radio core. The intrinsic motions of the core and quasi-stationary component have a commensurate contribution to the apparent motion of the stationary component. During the jet-stable state, the core shift significantly influences the apparent displacements of the stationary component, which shows orbiting motion with reversals. The quasi-stationary component has low superluminal speeds on time scales of months. On time-scales of few years, the apparent mean speeds are subrelativistic, of about 0.15 the speed of light. We find that the brightness profile of the quasi-stationary component is asymmetric along and transverse to the jet axis, and this effect remains unchanged regardless of epoch. Conclusions. Accurate positional determination, a high cadence of observations, and a proper accounting for the core shift are crucial for the measurement of the trajectories and speeds of the quasi-stationary component. Its motion is similar to the behaviour of the jet nozzle, which drags the outflow in a swinging motion and excites transverse waves of different amplitudes travelling downstream. A simple modelling of the brightness distribution shows that the configuration of twisted velocity field formed at the nozzle of the jet in combination with small jet viewing angle can account for the observed brightness asymmetry.

Opacity, variability, and kinematics of AGN jets

ABSTRACT Synchrotron self-absorption in active galactic nuclei (AGN) jets manifests itself as a time delay between flares observed at high and low radio frequencies. It is also responsible for the observing frequency-dependent change in size and position of the apparent base of the jet, aka the core shift effect, detected with very long baseline interferometry (VLBI). We measure the time delays and the core shifts in 11 radio-loud AGN to estimate the speed of their jets without relying on multi-epoch VLBI kinematics analysis. The 15–8 GHz total flux density time lags are obtained using Gaussian process regression, the core shift values are measured using VLBI observations and adopted from the literature. A strong correlation is found between the apparent core shift and the observed time delay. Our estimate of the jet speed is higher than the apparent speed of the fastest VLBI components by the median coefficient of 1.4. The coefficient ranges for individual sources from 0.5 to 20. We derive Doppler factors, Lorentz factors, and viewing angles of the jets, as well as the corresponding de-projected distance from the jet base to the core. The results support evidence for acceleration of the jets with bulk motion Lorentz factor Γ ∝ R0.52±0.03 on de-projected scales R of 0.5–500 parsecs.

Optical Fiber Cladding SPR Sensor Based on Core-Shift Welding Technology

The typical structure of an optical fiber surface plasmon resonance (SPR) sensor, which has been widely investigated, is to produce the SPR phenomenon by the transmission of light in a fiber core. The traditional method is to peel off the fiber cladding by complex methods such as corrosion, polishing, and grinding. In this paper, the transmitted light of a single-mode fiber is injected into three kinds of fiber cladding by core-shift welding technology to obtain the evanescent field directly between the cladding and the air interface and to build the Kretschmann structure by plating with a 50-nm gold film. The SPR sensing phenomenon is realized in three kinds of fiber cladding of a single-mode fiber, a graded-index multimode fiber, and a step-index multimode fiber. For the step-index multimode fiber cladding SPR sensor, all the light field energy is coupled to the cladding, leading to no light field in the fiber core, the deepest resonance valley, and the narrowest full width at half maximum. The single-mode fiber cladding SPR sensor has the highest sensitivity, and the mean sensitivity of the probe reaches 2538 nm/RIU (refractive index unit) after parameter optimization.

Core-shifts and proper-motion constraints in the S5 polar cap sample at the 15 and 43 GHz bands

We have studied a complete radio sample of active galactic nuclei with the very-long-baseline-interferometry (VLBI) technique and for the first time successfully obtained high-precision phase-delay astrometry at Q band (43 GHz) from observations acquired in 2010. We have compared our astrometric results with those obtained with the same technique at U band (15 GHz) from data collected in 2000. The differences in source separations among all the source pairs observed in common at the two epochs are compatible at the 1σ level between U and Q bands. With the benefit of quasi-simultaneous U and Q band observations in 2010, we have studied chromatic effects (core-shift) at the radio source cores with three different methods. The magnitudes of the core-shifts are of the same order (about 0.1 mas) for all methods. However, some discrepancies arise in the orientation of the core-shifts determined through the different methods. In some cases these discrepancies are due to insufficient signal for the method used. In others, the discrepancies reflect assumptions of the methods and could be explained by curvatures in the jets and departures from conical jets.

The correlation between the total jet power and the Poynting flux at the jet base

The magneto hydrodynamic models of relativistic jets from active galactic nuclei predict the jet power transported by the Poynting flux at the jet base, setting the correlation between the jet power and the total magnetic flux. For highly collimated jets taking the transversal structure into account allows to rewrite this correlation through the observed jet properties such as spectral flux and core shift. Applying this method we find that, for the sample of 48 sources, their jet power distribution is well peaked at the theoretically predicted level.