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We studied the pc-scale core shift effect using radio light curves for three blazars, S5 0716+714, 3C 279 and BL Lacertae, which were monitored at five frequencies ($ u$) between 4.8 GHz and 36.8 GHz using the University of Michigan Radio Astronomical Observatory (UMRAO), the Crimean Astrophysical Observatory (CrAO), and Metsahovi Radio Observatory for over 40 years. Flares were Gaussian fitted to derive time delays between observed frequencies for each flare ($Delta t$), peak amplitude ($A$), and their half width. Using $A propto u^{alpha}$ we infer $alpha$ in the range $-$16.67 to 2.41 and using $Delta t propto u^{1/k_r}$, we infer $k_r sim 1$, employed in the context of equipartition between magnetic and kinetic energy density for parameter estimation. From the estimated core position offset ($Omega_{r u}$) and the core radius ($r_{rm core}$), we infer that opacity model may not be valid in all cases. The mean magnetic field strength at 1 pc ($B_1$) and at the core ($B_{rm core}$), are in agreement with previous estimates. We apply the magnetically arrested disk model to estimate black hole spins in the range $0.15-0.9$ for these blazars, indicating that the model is consistent with expected accretion mode in such sources. The power law shaped power spectral density has slopes $-$1.3 to $-$2.3 and is interpreted in terms of multiple shocks or magnetic instabilities.
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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.
Opacity-driven shifts of the apparent VLBI core position with frequency (the core shift effect) probe physical conditions in the innermost parts of jets in active galactic nuclei. We present the first detailed investigation of this effect in the brightest gamma-ray blazar 3C454.3 using direct measurements from simultaneous 4.6-43 GHz VLBA observations, and a time lag analysis of 4.8-37 GHz lightcurves from the UMRAO, CrAO, and Metsahovi observations in 2007-2009. The results support the standard Konigl model of jet physics in the VLBI core region. The distance of the core from the jet origin r_c(nu), the core size W(nu), and the lightcurve time lag DT(nu) all depend on the observing frequency nu as r_c(nu)~W(nu)~ DT(nu)~nu^-1/k. The obtained range of k=0.6-0.8 is consistent with the synchrotron self-absorption being the dominating opacity mechanism in the jet. The similar frequency dependence of r_c(nu) and W(nu) suggests that the external pressure gradient does not dictate the jet geometry in the cm-band core region. Assuming equipartition, the magnetic field strength scales with distance r as B = 0.4(r/1pc)^-0.8 G. The total kinetic power of electron/positron jet is about 10^44 ergs/s.
The Blandford and K{o}nigl model of 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 two 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.
Estimates of magnetic field strength in relativistic jets of active galactic nuclei (AGN), obtained by measuring the frequency-dependent radio core location, imply that the total magnetic fluxes in those jets are consistent with the predictions of the magnetically-arrested disk (MAD) scenario of jet formation. On the other hand, the magnetic field strength determines the luminosity of the synchrotron radiation, which forms the low-energy bump of the observed blazar spectral energy distribution (SED). The SEDs of the most powerful blazars are strongly dominated by the high-energy bump, which is most likely due to the external radiation Compton (ERC) mechanism. This high Compton dominance may be difficult to reconcile with the MAD scenario, unless 1) the geometry of external radiation sources (broad-line region, hot-dust torus) is quasi-spherical rather than flat, or 2) most gamma-ray radiation is produced in jet regions of low magnetization, e.g., in magnetic reconnection layers or in fast jet spines.
We report the first systematic search for blazars among broad-absorption-line (BAL) quasars. This is based on our intranight optical monitoring of a well-defined sample of 10 candidates selected on the criteria of a flat spectrum and an abnormally high linear polarization at radio wavelengths. A small population of BAL blazars can be expected in the polar model of BAL quasars. However, no such case is found, since none of our 30 monitoring sessions devoted to the 10 candidates yielded a positive detection of intra-night optical variability (INOV), which is uncharacteristic of blazars. This lack of INOV detection contrasts with the high duty cycle of INOV observed for a comparison sample of 15 normal (i.e., non-BAL) blazars. Some possible implications of this are pointed out.
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