arXiv subjects

Alexander B. Pushkarev

Publications and source records attributed to Alexander B. Pushkarev.

13 recordsLinked to original sources

arXiv2026

Search for refractive substructure in active galactic nuclei using ground-based VLBI observations

We present the results of the first systematic search for refractive scattering substructure in active galactic nuclei (AGN) using ground-based very long baseline interferometry (VLBI) data. We selected 1074 AGNs showing a steep frequency dependence of their angular sizes and compared the observed long-baseline visibility amplitudes with the predictions of the refractive substructure model for scatter-broadened images. Nine sources showed consistency with the expected level of refractive noise, including the quasar TXS 2005+403 (J2007+4029), which confirms previously reported results for this source. Remarkably, the majority of these AGNs are located in the Cygnus region -- one of the key ``hotspots'' of strong Galactic scattering. For five selected AGNs, we identified at least two epochs with sufficiently high data quality showing long-baseline detections, suggesting the presence of scattering substructure. We showed that refractive scattering substructure in AGN is a rare effect, requiring high sensitivity, sufficient source brightness, and lines of sight with particularly strong turbulence in the interstellar medium. Nevertheless, AGNs can serve as powerful probes of the turbulent interstellar medium. We discuss strategies for improving the sensitivity of VLBI observations and identify the most promising sources for reliable detection and investigation of refractive substructure in future observations.

astro-ph.HE↗

arXiv2026

Observations of a Possible Transient Magnetically Arrested Accretion State in a Nearby Quasar: OQ208

OQ208 is a nearby, partially obscured quasar (z=0.077) that is a young, bright, parsec scale radio source. We assemble archival and new high frequency VLBA and VLA observations and optical spectra to form a data-set spanning 39 years. Radio light curves covering 58 years were also compiled. We utilize new spectrophotmetry to calibrate previous spectroscopy using forbidden narrow lines that are expected to be stable on much longer time scales. VLBA and VLA observations of a light-year scale bright nuclear flare at 15.4~GHz and 22~GHz reveal a rise (fade) beginning in mid-1996 (early-2000). Quasi-contemporaneously, from 2/7/1997-6/3/2000, the H$α$ broad line equivalent widths (EWs) and fluxes dropped dramatically. In the context of the tendency of radio loud quasars to have a depressed extreme ultraviolet (EUV) continuum (the main source of ionizing flux for H$α$) relative to radio quiet quasars at matched UV luminosity (the EUV deficit of radio loud quasars), this may not be a coincidence. Analytic models previously developed to explain the relationship between jet power and the EUV deficit are consistent with (but not direct observational proof of) the small EWs being a consequence of transient magnetically arrested accretion states from $\sim1997-2001$. The 22 GHz VLBA nucleus gradually fades, in 2023 the flux density is $<5\%$ of its value in 2000. The environs of the nucleus also fade at 22 GHz, but in a time delayed fashion.

astro-ph.GA↗

arXiv2026

Revisiting the angular size-redshift cosmological test with milliarcsecond radio structures in active galactic nuclei

VLBI measurements of the sizes of compact extragalactic radio sources, jetted active galactic nuclei, provide data for probing the angular size--redshift relation, offering a complementary test to other distance--redshift methods. We analyse a significantly expanded dataset to reassess the angular size--redshift relation and its potential for constraining cosmological model parameters, focusing on the matter density parameter $Ω_{\mathrm{m}}$ in a flat $Λ$ Cold Dark Matter Universe. This is the first major update of the compact-source angular size test in the past quarter of a century, using a dataset an order of magnitude larger than in previous studies. MCMC analysis on real data and on multiple mock catalogues to evaluate parameter constraints in the presence of observational scatter. In addition, we conducted a test with 100 randomized catalogues created by shuffling redshifts while preserving other observables. We also explored how astrophysical parameters depend on fixed cosmological models with different $Ω_{\mathrm{m}}$ values. The randomization test showed that the posterior distributions from randomized data do not overlap with those from real observations, confirming that the measured angular size--redshift relation is physically meaningful. The astrophysical model parameter that describes the redshift dependence of the source angular size exhibits degeneracy with $Ω_{\mathrm{m}}$. Simulated mock catalogues indicate that the method is able to constrain $Ω_{\mathrm{m}}$ if the data scatter is below $\sim20\%$. Scaling estimates suggest that high-quality data of samples of several thousands to $\sim 100\,000$ sources, a standardisation calibration approach, and/or refining sample selection criteria are needed to fully exploit the potential of the angular size--redshift test with this type of objects (abridged).

astro-ph.CO↗

arXiv2026

Where within the 3C 84 jet are $γ$-rays produced?

The location of $γ$-ray creation and emission within extra-galactic jets is a matter of active debate. One particularly well-suited source to pinpoint the location is the nearby, bright radio galaxy 3C 84, harbouring a powerful jet. Here we investigate the origin of $γ$-rays measured during a recent $γ$-ray flare, by analysing the linear polarisation signal of close-in-time very long baseline interferometry (VLBI) observations at centimetre and millimetre wavelengths. While 3C 84 is overall almost unpolarised, we find that close-in-time to the $γ$-ray flare peak regions at parsec-scale distances from the central engine shows a fractional linear polarisation increase. Under the physically well-motivated assumption of a causal relation between this polarisation enhancement and the $γ$-ray flare, and combined with insights from concurrent X-ray polarisation measurements, the $γ$-rays being created in this region is a physically motivated scenario, in a process consistent with synchrotron self-Compton.

astro-ph.HE↗

arXiv2025

Revisiting 3C 279 jet morphology with space VLBI at 26 microarcsecond resolution

We present observations of the blazar 3C 279 at 22 GHz using the space VLBI mission RadioAstron on 2018 January 15. Images in both total intensity and fractional polarization are reconstructed using RML method implemented in the eht-imaging library. The electric vector position angles are found to be mostly aligned with the general jet direction, suggesting a predominantly toroidal magnetic field, in agreement with the presence of a helical magnetic field. Ground-space fringes were detected up to a projected baseline length of $\sim 8$G$λ$, achieving the angular resolution of around 26$μ$as. The fine-scale structure of the relativistic jet is found in our study extending to a projected distance of $\sim 180$ parsec from the radio core. However, the filamentary structure reported by previous RadioAstron observations of 2014 is not detected in our current study. We discuss potential causes for this phenomenon, together with a comparison using public 43 GHz data from the BEAM-ME program, showing a significant drop in the jet's total intensity. The optically thick core is observed with a brightness temperature of $ 1.6 \times 10^{12}$ K, consistent with equipartition between the energy densities of the relativistic particles and the magnetic field. This yields an estimated magnetic field strength of 0.2 G.

astro-ph.HE↗

arXiv2024

Unveiling the Bent Jet Structure and Polarization of OJ 287 at 1.7 GHz with Space VLBI

We present total intensity and linear polarization images of OJ287 at 1.68GHz, obtained through space-based VLBI observations with RadioAstron on April 16, 2016. The observations were conducted using a ground array consisting of the VLBA and the EVN. Ground-space fringes were detected with a maximum projected baseline length of 5.6 Earth's diameter, resulting in an angular resolution of 530 uas. With this unprecedented resolution at such a low frequency, the progressively bending jet structure of OJ287 has been resolved up to 10 pc of the projected distance from the radio core. In comparison with close-in-time VLBI observations at 15, 43, 86 GHz from MOJAVE and VLBA-BU-BLAZAR monitoring projects, we obtain the spectral index map showing the opaque core and optically thin jet components. The optically thick core has a brightness temperature of 10$^{13}$ K, and is further resolved into two sub-components at higher frequencies labeled C1 and C2. These sub-components exhibit a transition from optically thick to thin, with a SSA turnover frequency estimated to be 33 and 11.5 GHz, and a turnover flux density 4 and 0.7 Jy, respectively. Assuming a Doppler boosting factor of 10, the SSA values provide the estimate of the magnetic field strengths from SSA of 3.4 G for C1 and 1.0 G for C2. The magnetic field strengths assuming equipartition arguments are also estimated as 2.6 G and 1.6 G, respectively. The integrated degree of linear polarization is found to be approximately 2.5 %, with the electric vector position angle being well aligned with the local jet direction at the core region. This alignment suggests a predominant toroidal magnetic field, which is in agreement with the jet formation model that requires a helical magnetic field anchored to either the black hole ergosphere or the accretion disk. Further downstream, the jet seems to be predominantly threaded by a poloidal magnetic field.

astro-ph.HE↗

arXiv2023

Configuration of the global magnetic field in AGN parsec-scale jets

The magnetic field plays a significant role in the phenomenon of highly collimated jets of active galactic nuclei (AGN). Relativistic effects prevent the direct reconstruction of the magnetic field direction as transverse to electric vectors on radio maps. We determined the topology of the B-field by modeling the transverse distributions of the total and linearly polarized intensity, polarization degree, and deviation of the polarization direction from the local jet axis and by further comparison with observational data. We consider (i) a helical field with a different twist angle; (ii) a toroidal field on the jet axis surrounded by a sheath with a longitudinal field. In the latter scenario, we consider different sheath thickness relative to the spine. We assumed the sheath velocity is equal to or less than that of the spine. The relativistic effects have been considered for a general case, under which the axis and velocity vector of the jet and radial directions do not coincide. Our simulations reproduce the main features of the observed transverse profiles of polarization characteristics in parsec-scale AGN jets. The model transverse distribution shapes of the polarization properties are found to be strongly influenced by kinematic and geometric parameters of an outflow. We demonstrated it for three AGNs having different but typical polarization patterns revealed on radio maps. For each of these objects, we identified the model parameters, which provide a qualitative correspondence of theoretical profiles with those obtained from observations, indicating that the B-field is strongly ordered on parsec scales.

astro-ph.HE↗

arXiv2019

Inverse Compton Scattering of the Central Source Photons as an X-ray Emission Mechanism on Kiloparsec Scales in PKS 1127-145

The beamed inverse Compton/cosmic microwave background model has generally been used for the interpretation of X-ray radiation from kiloparsec-scale jets of the core-dominated quasars. Recent \textit{Fermi}-LAT and \textit{HST} observations have brought this model into question. We examine the assumption that X-rays from the kiloparsec-scale jet of the quasar PKS~1127$-$145 are produced by inverse Compton scattering of the central source emission. In this context, we show that both similarity and distinction between the observed radio and X-ray spectral indices for some of the jet knots can be explained under a single power-law electron energy distribution. We derive that the viewing angle of the kiloparsec-scale jet is about $35^\circ$ and the jet has a moderate relativistic speed of $\approx 0.8c$. The predicted gamma-ray flux of the jet is found to be a few orders of magnitude lower than the minimum flux level measured by \textit{Fermi}-LAT, further supporting our scenario.

astro-ph.HE↗

arXiv2015

The connection between the radio jet and the gamma-ray emission in the radio galaxy 3C 120

We present the analysis of the radio jet evolution of the radio galaxy 3C 120 during a period of prolonged gamma-ray activity detected by the Fermi satellite between December 2012 and October 2014. We find a clear connection between the gamma-ray and radio emission, such that every period of gamma-ray activity is accompanied by the flaring of the mm-VLBI core and subsequent ejection of a new superluminal component. However, not all ejections of components are associated with gamma-ray events detectable by Fermi. Clear gamma-ray detections are obtained only when components are moving in a direction closer to our line of sight.This suggests that the observed gamma-ray emission depends not only on the interaction of moving components with the mm-VLBI core, but also on their orientation with respect to the observer. Timing of the gamma-ray detections and ejection of superluminal components locate the gamma-ray production to within almost 0.13 pc from the mm-VLBI core, which was previously estimated to lie about 0.24 pc from the central black hole. This corresponds to about twice the estimated extension of the broad line region, limiting the external photon field and therefore suggesting synchrotron self Compton as the most probable mechanism for the production of the gamma-ray emission. Alternatively, the interaction of components with the jet sheath can provide the necessary photon field to produced the observed gamma-rays by Compton scattering.

astro-ph.HE↗

arXiv2015

Opening angles and shapes of parsec-scale AGN jets

We used 15 GHz VLBA observations of 366 sources having at least 5 epochs within a time interval 1995-2013 from the MOJAVE program and/or its predecessor, the 2 cm VLBA Survey. For each source we produced a corresponding stacked image averaging all available epochs for a better reconstruction of the cross section of the flow. We have analyzed jet profiles transverse to the local jet ridge line and derived both apparent and intrinsic opening angles of the parsec-scale outflows. The sources detected by the Fermi Large Area Telescope (LAT) during the first 24 months of operation show wider apparent jet opening angle and smaller viewing angles on a very high level of significance supporting our early findings. Analyzing transverse shapes of the outflows we found that most sources have conical jet geometry at parsec scales, though there are also sources that exhibit active jet collimation.

astro-ph.HE↗

arXiv2013

AGN jet physics and apparent opening angles

We present a new method to measure Gamma*theta_j in flux-limited samples of active galactic nuclei (AGN) jets, where Gamma is the bulk Lorentz factor and theta_j is the jet's half-opening angle. The Gamma*theta_j parameter is physically important for models of jet launching, and also determines the effectiveness of jet instabilities and magnetic reconnection. We measure Gamma*theta_j by analyzing the observed distribution of apparent opening angles in very long baseline interferometry (VLBI) flux-limited samples of jets, given some prior knowledge of the active galactic nuclei (AGN) radio luminosity function. We then apply this method to the MOJAVE flux-limited sample of radio loud objects and find Gamma*theta_j = 0.1 +- 0.03, which implies that AGN jets are subject to a variety of physical processes that require causal connection.

astro-ph.HE↗

arXiv2011

Multi-frequency synthesis algorithm based on Generalized Maximum Entropy Method. Application to 0954+658

We propose the multi-frequency synthesis (MFS) algorithm with spectral correction of frequency-dependent source brightness distribution based on maximum entropy method. In order to take into account the spectral terms of n-th order in the Taylor expansion for the frequency-dependent brightness distribution, we use a generalized form of the maximum entropy method suitable for reconstruction of not only positive-definite functions, but also sign-variable ones. The proposed algorithm is aimed at producing both improved total intensity image and two-dimensional spectral index distribution over the source. We consider also the problem of frequency-dependent variation of the radio core positions of self-absorbed active galactic nuclei, which should be taken into account in a correct multi-frequency synthesis. First, the proposed MFS algorithm has been tested on simulated data and then applied to four-frequency synthesis imaging of the radio source 0954+658 from VLBA observational data obtained quasi-simultaneously at 5, 8, 15 and 22 GHz.

astro-ph.CO↗

arXiv2010

Multifrequency Method for Mapping Active Galactic Nuclei with Allowance for the Frequency-Dependent Image Shift

We consider the problem of multifrequency VLBA image synthesis and spectral-index mapping for active galactic nuclei related to the necessity of taking into account the frequency-dependent image shift. We describe our generalized multifrequency synthesis algorithm with a spectral correction based on the maximum entropy method. The results of our processing of multifrequency VLBI data for the radio sources J2202+4216, J0336+3218, and J1419+5423 are presented.

astro-ph.CO↗