arXiv subjects

A. B. Pushkarev

Publications and source records attributed to A. B. Pushkarev.

At least 19 recordsLinked to original sources

arXiv2026

A broadband outburst of the compact steep-spectrum quasar 3C 138 in 2024-2026

After several decades of relative quiescence, the compact steep-spectrum quasar 3C 138 entered an active phase in 2024-2026, exhibiting strong broadband flaring. We investigate its multiwavelength behaviour using dense multifrequency radio monitoring at 1-22 GHz with RATAN-600 and RT-32, optical R-band observations with Zeiss-1000 and AS-500/2, X-ray measurements with Swift/XRT and SRG/ART-XC, and the Fermi-LAT $γ$-ray light curve. The radio brightening accelerated after 2022 and was strongest at the highest frequencies. The radio spectra hardened markedly, with the 11-22 GHz spectral index evolving from steep to flat or inverted during the active phase. The X-ray flux increased by more than a factor of three during 2025-2026, while the photon index hardened from $Γ_{\rm X}\simeq 1.6$ to $Γ_{\rm X}\simeq 0.9$ and softened back after the peak. Flare decomposition revealed five $γ$-ray flares and a sequence of optical subflares during the later stages of the activity. The $γ$-ray, X-ray, and optical maxima occur within a $\simeq 13$-day interval, suggesting a common activity episode, whereas the radio brightens more gradually and in a frequency-dependent manner. Under the adopted compact-zone geometries, the sparse two-state spectral energy distributions (SEDs) can be represented by one-zone synchrotron self-Compton (SSC) solutions, while the relative contribution of external Compton (EC) remains geometry dependent. The flare shifts the modelled energy partition towards relativistic electrons. These results favour a longer-lived, core-dominated activity phase, with later high-energy and optical flares superposed on the opacity-driven radio evolution of an emerging synchrotron component.

astro-ph.HE↗

arXiv2026

A hidden ultra-relativistic spine in the jet of a neutrino-associated blazar

Supermassive black holes launch powerful jets of plasma that can accelerate particles to extreme energies, but the physical conditions required to produce high-energy neutrinos remain unknown. In the blazar TXS 0506+056, the first source individually linked to a high-energy neutrino, radio images had seemed to reveal a jet too slow to sustain the extreme conditions required for neutrino production. Here we resolve this tension using long-term radio monitoring, particularly Very Long Baseline Interferometry (VLBI) imaging. We uncover a disturbance in emission propagating with an apparent speed of 21+-1 times the speed of light, that is masked by slower, radio-bright features that dominated earlier analyses. We interpret this as the signature of a stratified jet: an ultra-relativistic spine with Lorentz factor Gamma>20 embedded within a slower outer sheath. As the disturbance travels along the spine, it progressively illuminates the sheath, producing the delayed radio flare and naturally accounting for the years-long offset between neutrino and radio emission. The same pattern recurs in a second neutrino-associated event, pointing to a repeatable multi-messenger engine. These findings challenge the standard interpretation of VLBI jet speeds and establish a concrete, testable framework connecting structured jets to the sources of the Universe's highest-energy neutrinos.

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arXiv2026

Parsec-scale polarimetry and kinematics of a spine-sheath jet in the neutrino-blazar TXS 0506+056

In 2017, the blazar TXS 0506+056 showed a remarkable gamma-ray outburst simultaneously with a high-energy neutrino detection by IceCube from the same sky region. The significance of this association was found to be on the order of 3 sigma thus providing a strong link between the neutrino emission and the blazar flare. The high-energy flare in TXS 0506+056 was followed by a delayed radio flare, peaking in $\sim2020$ in the aftermath of the neutrino event. We investigate the parsec-scale jet structure and dynamics of TXS 0506+056 using 15 GHz full-polarization VLBI observations obtained between 2009 and 2025 with the Very Long Baseline Array. Our kinematic analysis reveals moderate superluminal jet speeds of $\sim(1-2)$c and two quasi-stationary components. A new jet component with comparable speed was ejected contemporaneously with the 2017 IceCube neutrino event. The stacked polarization map is consistent with a stratified spine-sheath jet structure: an inner spine with EVPA aligned with the jet, surrounded by a sheath layer with perpendicular EVPA. Variability in total intensity and polarization further indicates interaction between these layers, particularly evident in characteristic linear polarization flares, associated with EVPA rotations of the quasi-stationary components. The multi-layered jet configuration is consistent with previous studies that were able to explain the neutrino emission in this TXS 0506+056 through a spine-sheath jet structure. We suggest that TXS 0506+056 represents an archetypal case and that similar polarization signatures and geometric light curve flares may be present in other neutrino-emitting sources, potentially offering a solution to the Doppler-crisis phenomenon observed in TeV-emitting blazars.

astro-ph.HE↗

arXiv2026

Unique Science Opportunities for Space VLBI Systems with the SKA Telescopes

To date, two dedicated Space Very Long Baseline Interferometry (SVLBI) missions, the VLBI Space Observatory Programme (VSOP) and RadioAstron, have provided groundbreaking insights into the Universe at angular resolutions as fine as ~10 microarcseconds. The phased SKA-Mid, with its exceptional sensitivity and broad frequency coverage, will form a unique ground-based anchor for future SVLBI missions, driving major advances into previously unexplored regions of the angular resolution-sensitivity parameter space. The discovery of extreme brightness temperatures in blazars by RadioAstron demands detailed investigation with next-generation SVLBI. Such studies are crucial for understanding particle (re-)acceleration mechanisms, with direct implications for the search for high-energy neutrino sources. Combining centimeter-wavelength SVLBI with millimeter ground-based VLBI at comparable resolutions will enable detailed studies of plasma stratification and instabilities in Active Galactic Nuclei (AGN) jets, as well as the processes of jet formation, acceleration, collimation, and magnetic field evolution, for example through Faraday rotation mapping. The unprecedented sensitivity of the SKA-Mid will allow observations of active galactic nuclei to very high redshifts, tracing their evolution and overcoming opacity caused by the (1+z) shift of intrinsic emission frequencies. Future centimeter SVLBI experiments will also probe scattering in the interstellar medium through pulsar, maser, and AGN observations. Finally, the combination of multiple tied-array beams from the SKA telescopes and the extremely long SVLBI baselines will enable ultra-precise astrometry using the next-generation MultiView technique, allowing measurements of extragalactic parallaxes of pulsars and megamasers, proper motions of supermassive black holes, and even the astrometric detection of exoplanets.

astro-ph.HE↗

arXiv2026

Multi-messenger flare in the quasar PKS 0446+11

The physical mechanisms driving neutrino and electromagnetic flares in blazars remain poorly understood. We investigate a prominent multi-messenger flare in the quasar PKS 1424+240 to identify the processes responsible for its high-energy emission. We analyze the IceCube-240105A high-energy neutrino event together with contemporaneous observations in the gamma-ray, X-ray, optical, and radio bands. The on- and off-flare spectral energy distributions (SEDs) are modeled within a single-zone leptohadronic framework. Multi-epoch VLBA observations from the MOJAVE program provide parsec-scale polarization data that complement the multi-wavelength light curves. No significant time delay is detected between the neutrino arrival and the flares in different energy bands. This is consistent with an extremely small jet viewing angle below 1 deg, inferred from the parsec-scale polarization structure. The flare can be reproduced by the injection of a proton population and an increase of the Doppler factor from 18 to 24. We also detect an approximately 90 deg rotation of the EVPA in the parsec-scale core during the initial phase of the flare, indicating the emergence of a shock formed by the change in the bulk plasma speed. Our comprehensive multi-messenger analysis demonstrates that the extreme beaming and sub-degree viewing angle of this distant blazar can account for the observed neutrino and electromagnetic activity. These findings strengthen the case for blazars as efficient accelerators of hadrons and as significant contributors to the observed high-energy neutrino flux.

astro-ph.HE↗

arXiv2026

Heavy interstellar scattering toward the near end of the Galactic bar

We present results of a pilot observational wide-field VLBI campaign on probing scattering properties of the partly ionized interstellar medium towards the Galactic plane sky region between $28^\circ<l<36^\circ$ and $|b|<1^\circ$. This covers the region where the Galactic bar connects to the spiral arms and where a lot of star formation is currently ongoing. The Very Long Baseline Array (VLBA) observations of the whole region were performed in a special mode with multiple phase centers at L-band (1.4 -- 1.8 GHz) during April-June 2022 and a year later complemented by sessions at S (2.2 -- 2.4 GHz) and C-band (4.6 -- 5.0 GHz) partially covering the pilot region. We found compelling evidence that target sources are subject to scattering. The total detection rate in L, S and C-bands is 1.5, 3.4 and 9.2 per cent, respectively, and approximately scales with the square of the observation frequency. The low rate values imply that scattering is strong. Its power is non-uniform across the Galactic plane and it can be approximated by a Gaussian with a width of about $2^\circ$ peaking at the Galactic mid-plane. One of the brightest sources of the field shows anisotropic scattering, with a $λ^2$ dependence of its observed angular size, along a position angle of $26^\circ$ aligned with the line of constant Galactic latitude. We estimate the turbulence dissipation scale $r_\text{in}\approx1500$ km toward the source J1833+0015.

astro-ph.GA↗

arXiv2025

Studies of stationary features in jets: 3C 279 quasar I. On-sky scattering and dynamics

A recent study on the dynamics of the quasi-stationary component (QSC) in the jet of BL~ Lacertae highlighted its significance in evaluating the physical properties of relativistic transverse waves in the parsec-scale jet. Motivated by this finding, we selected a different type of blazar, the flat-spectrum radio quasar (FSRQ) 3C~279, which hosts a QSC at an angular median distance of 0.35~mas from the radio core, as revealed by 27 years of VLBA monitoring data at 15~GHz. We investigate the positional scatter and dynamics of a QSC in the 3C~279 jet, aiming to detect the presence of a relativistic transverse wave and estimate its characteristics. We employ an analytical statistical method to estimate the mean intrinsic speed of the QSC, while moving average and refinement methods are used to smooth its trajectory. Analysis of the QSC position scatter shows that the jet axis change its direction by about $21^{\circ}$ over 27 years and jet mean intrinsic full opening angle is $\approx 0.30^{\circ} \pm 0.03^{\circ}$. The apparent displacement vectors of the QSC exhibit strong asymmetry and anisotropy along the jet direction, indicating pronounced anisotropic displacements of the core along the jet axis. We estimated the mean intrinsic speed of the QSC to be superluminal, $\overline{β_{\rm s}} \approx 10$ in units of the speed of light, which, within the framework of the seagull-on-wave model, is interpreted as evidence for a relativistic transverse wave propagating through the QSC. Analysis of the reversing trajectory of the QSC enables the classification and characterisation of reversal patterns, which, in turn, allows the determination of key transverse wave parameters such as frequency, amplitude, inclination angle, and magnetic energy of the wave (abbrev.).

astro-ph.GA↗

arXiv2025

Revealing a ribbon-like jet in OJ 287 with RadioAstron

We present space-based very long baseline interferometry observations of the BL Lac type object OJ 287 taken with RadioAstron at 22 GHz on April 25, 2016, in conjunction with a ground array comprising 27 radio telescopes. We detect ground-space fringes at projected baselines extending up to 4.6 Earth diameters, which allowed us to image the jet in OJ 287 with an angular resolution of ~47 μas. Applying an advanced regularized maximum likelihood imaging method, we resolved the innermost jet structure with a complex morphology at a resolution of ~15 μas (~0.1 pc projected distance). For the first time, due to a favorable geometrical position of the jet in tandem with high data quality, we detect multiple sharp bends that form a "ribbon-like" jet structure that extends down to 1 mas. Two-dimensional Gaussian model-fitting reveals regions of the jet with brightness temperatures of more than 10^13 K, indicative of strong Doppler boosting. Polarimetric imaging reveals that the electric vector position angles are predominantly perpendicular to the innermost jet direction, implying a dominant poloidal magnetic field component near the central engine. Complementary multi-epoch Very Long Baseline Array observations at 43 GHz provide a multifrequency view of the jet evolution. Ridgeline analysis of the 43 GHz data shows significant variations in the jet position angle from 2014 to 2017, behavior consistent with a rotating helical jet structure. Finally, we confirm the emergence of a new jet component (B15 or K), which may be associated with the source's first TeV flare, and offer new observational constraints relevant to models involving a supermassive black hole binary.

astro-ph.HE↗

arXiv2025

Looking into the Jet Cone of the Neutrino-Associated Very High Energy Blazar PKS 1424+240

The acceleration process of massive particles as well as the production of very high energy (VHE) photons and neutrinos remains a fundamental challenge in astrophysics. We investigate the parsec-scale jet structure and magnetic field of the blazar PKS 1424+240, that has been selected on the basis of strong VHE gamma-ray emission and identified with one of the highest peaks in the IceCube 9-year neutrino sky. We analyze 15 GHz VLBA observations of this BL Lac object, stacking 42 polarization-sensitive images collected in 2009-2025 to enhance the signal and reveal persistent parsec-scale structure. Our observations uncover a rare scenario. The object is viewed inside the jet cone, very close to the axis of its relativistic jet, with a viewing angle of <0.6 deg. This effectively maximizes Doppler boosting to values ~30 and enhances both electromagnetic and neutrino emission in the observer's direction. Based on polarimetric observations, we unambiguously detect a net toroidal component of the jet's magnetic field, indicating a current carrying jet flowing almost directly towards our line of sight. Blazars with very small jet viewing angles offer a solution to the longstanding mismatch between Doppler factors inferred from low VLBI apparent jet speed and those derived from VHE observations -- the so-called "Doppler factor crisis". We show that relativistic beaming plays the critical role in the gamma-ray and neutrino emission of blazars, with direct implications for models of their multi-messenger emission.

astro-ph.HE↗

arXiv2025

Probing plasma scattering screens towards the quasar 2005+403 with long-term RATAN-600 observations

We continue investigating the observed properties of the quasar 2005$+$403 seen through the highly turbulent plasma in the Cygnus region. Our earlier study Koryukova+2023 revealed a great influence of propagation effects on the observed data, e.g. numerous episodes of multiple imaging formation, angular broadening of the source size, and the detection of an extreme scattering event (ESE) in 2019, making it a good probe for thermal plasma in the interstellar medium (ISM). We report the first detection of multi-frequency and multi-epoch ESEs revealed with the RATAN-600 daily light curves of the quasar 2005$+$403. The most prominent ESE flux density modulations are found in 2011, 2015, and 2020 and each lasted about $4-5$ months. We fitted the detected ESEs jointly at 4.7/4.8, 7.7/8.2, and 11.2~GHz using the models, which allowed us to constrain the angular and linear size of the scattering lens averaged over the three ESEs $0.3\pm0.1$ mas and $0.6\pm0.1$ au, proper motion $8.3\pm0.7$ mas yr$^{-1}$ and transverse velocity $70.1\pm5.7$ km s$^{-1}$ assuming that the lens distance is 1.8 kpc, maximum free-electron density on the line of sight $1183\pm124$ cm$^{-3}$ and lens mass $(0.8\pm0.4)\times10^{-15}\,\rm M_\odot$. Using the fitting results, we reconstructed the intrinsic unscattered angular size of the quasar at the ESE epochs. We also report on the first detection of up to six ESEs in a row that occurred in the period of $2015-2016$, apparently created by multiple lenses successively crossing the line of sight.

astro-ph.GA↗

arXiv2025

MOJAVE -- XXII. Brightness temperature distributions and geometric profiles along parsec-scale AGN jets

Radial gradients of the brightness temperatures along the parsec-scale jets of Active Galactic Nuclei (AGN) can be used to infer the energy balance and to estimate the parameter range of physical conditions in these regions. In this paper, we present a detailed study of the brightness temperature gradients and geometry profiles of relativistic jets of 447 AGN based on 15 GHz Very Long Baseline Array observations performed between 1994 and 2019. We used models of the jet structure using two-dimensional Gaussian components and analysed variations in their brightness temperatures and sizes along the jets. The size of the jet components, R, increases with projected distance from the jet base, r, as $R\propto r^{1.02\pm0.03}$, i.e., typically following a conically expanding streamline and therefore indicating that the size of jet components is a good tracer of jet geometry. The brightness temperature gradients along the jets typically follow a power-law $T_\mathrm{b} \propto r^{-2.82\pm0.07}$. Half of the sample sources show non-monotonic R(r) or Tb(r) profiles and their distributions were characterised by a double power-law model. We found at least six scenarios to explain the enhancement of the brightness temperature by a presence of inhomogeneities (shocks, jet recollimation) or curvature effects (helical structures, helical magnetic field, non-radial motion, bent jets). Our results are consistent with the scenario that the jet features can be simplified as optically thin moving blobs. In the sources demonstrating transition from a conical to parabolic jet shape, the gradient of the Tb(R) changes at the position of the break consistent with the model of magneto-hydrodynamic acceleration.

astro-ph.HE↗

arXiv2024

Studies of stationary features in jets: BL Lacertae II. Trajectory reversals and superluminal speeds on sub-parsec scales

High-resolution VLBI observations revealed a quasi-stationary component (QSC) in the relativistic jets of many blazars, which represents a standing recollimation shock. The VLBA monitoring of the BL Lacertae jet at 15~GHz shows the QSC at a projected distance of about 0.26~mas from the radio core.} We study the trajectory and kinematics of the QSC in BL Lacertae on sub-parsec scales using 15~GHz VLBA data of 164 observations over 20 years from the MOJAVE program and 2~cm VLBA Survey. To reconstruct the QSC's intrinsic trajectory, we use moving average and trajectory refinement procedures to smooth out the effects of core displacement and account for QSC positioning errors. We identified 22 QSC reversal patterns with a frequency of $\sim 1.5$ per year. Most reversals have an acute angle $<90\degr$ and few have a loop-shaped or arc-shaped trajectory. Where observed, combinations of reversals show reversible and quasi-oscillatory motion. We propose a model in which a relativistic transverse wave passes through the QSC, generating a short-lived reverse motion, similar to the transverse motion of a seagull on a wave. According to the model, relativistic waves are generated upstream and the reverse motion of the QSC is governed by the amplitude, velocity and tilt of the wave as it passes through. The apparent superluminal speeds of the QSC ($\sim 2\,c$) are then due to the relativistic speed of the jet's transverse wave ($<0.3\,c$ in the host galaxy rest frame) combined with the relativistic motion towards the observer. The measured superluminal speeds of QSC indirectly indicate the presence of relativistic transverse waves, and the size of the QSC scattering on the sky is proportional to the maximum amplitude of the wave. (abbreviated)

astro-ph.GA↗

arXiv2024

Simulations of linear polarization of precessing AGN jets at parsec scales

The latest results of the most detailed analysis of multi-epoch polarization-sensitive observations of active galactic nuclei (AGN) jets at parsecs scales by very long baseline interferometry (VLBI) reveal several characteristic patterns of linear polarization distribution and its variability (Pushkarev et al., 2023; Zobnina et al., 2023). Some of the observed profiles can be reproduced by a simple model of a jet threaded by a helical magnetic field. However, none of the models presented to date can explain the observed polarization profiles with an increase in its degree towards the edges of the jet, and accompanied by a 'fountain' type electric vector pattern and its high temporal variability in the center. Based on simulations of the VLBI observations of relativistic jets, we show here that the observed transverse linear polarization profiles, atypical for the simple magnetic field models can be naturally produced assuming the finite resolution of VLBI arrays and precession of a jet on ten-years scales, observational indications of which are found in an increasing number of AGN. In our simulations, we qualitatively reproduce the distribution of the electric vector and its variability, though the polarization images are characterized by a bright spine due to weak smearing, which is poorly consistent with observations. More effective depolarization can be obtained in models with the suppressed emission of the jet spine.

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arXiv2023

High-energy neutrino-induced cascade from the direction of the flaring radio blazar TXS 0506+056 observed by Baikal-GVD in 2021

The existence of high-energy astrophysical neutrinos has been unambiguously demonstrated, but their sources remain elusive. IceCube reported an association of a 290-TeV neutrino with a gamma-ray flare of TXS 0506+056, an active galactic nucleus with a compact radio jet pointing to us. Later, radio-bright blazars were shown to be associated with IceCube neutrino events with high statistical significance. These associations remained unconfirmed with the data of independent experiments. Here we report on the detection of a rare neutrino event with the estimated energy of 224+-75 TeV from the direction of TXS 0506+056 by the new Baikal Gigaton Volume Detector (Baikal-GVD) in April 2021. This event is the highest-energy cascade detected so far by the Baikal-GVD neutrino telescope from a direction below horizon. The result supports previous suggestions that radio blazars in general, and TXS 0506+056 in particular, are the sources of high-energy neutrinos, and opens up the cascade channel for the neutrino astronomy.

astro-ph.HE↗

arXiv2023

Residual Entropy as a Diagnostic and Stopping Metric for CLEAN

We propose the use of entropy, measured from the spatial and flux distribution of pixels in the residual image, as a potential diagnostic and stopping metric for the CLEAN algorithm. Despite its broad success as the standard deconvolution approach in radio interferometry, finding the optimum stopping point for the iterative CLEAN algorithm is still a challenge. We show that the entropy of the residual image, measured during the final stages of CLEAN, can be computed without prior knowledge of the source structure or expected noise levels, and that finding the point of maximum entropy as a measure of randomness in the residual image serves as a robust stopping criterion. We also find that, when compared to the expected thermal noise in the image, the maximum entropy of the residuals is a useful diagnostic that can reveal the presence of data editing, calibration, or deconvolution issues that may limit the fidelity of the final CLEAN map.

astro-ph.IM↗

arXiv2023

Multiple imaging of the quasar 2005+403 formed by anisotropic scattering

We report on the low Galactic latitude ($b=4.3^\circ$) quasar 2005$+$403, the second active galactic nuclei, in which we detected a rare phenomenon of multiple imaging induced by refractive-dominated scattering. The manifestation of this propagation effect is revealed at different frequencies ($\lesssim8$ GHz) and epochs of VLBA observations. The pattern formed by anisotropic scattering is stretched out along the line of constant Galactic latitude with a local $\mathrm{PA}\approx40^\circ$ showing one-two sub-images, often on either side of the core. Analysing the multi-frequency VLBA data ranging from 1.4 to 43.2 GHz, we found that both the angular size of the apparent core component and the separation between the primary and secondary core images follow a wavelength squared dependence, providing convincing evidence for a plasma scattering origin for the multiple imaging. Based on the OVRO long-term monitoring data at 15 GHz obtained for 2005$+$403, we identified the characteristic flux density excursions occurred in April-May 2019 and attributed to an extreme scattering event (ESE) associated with the passage of a plasma lens across the line of sight. Modeling the ESE, we determined that the angular size of the screen is 0.4 mas and it drifts with the proper motion of 4.4 mas yr$^{-1}$. Assuming that the scattering screen is located in the highly turbulent Cygnus region, the transverse linear size and speed of the lens with respect to the observer are 0.7 AU and 37 km s$^{-1}$, respectively.

astro-ph.GA↗

arXiv2023

Baikal-GVD Astrophysical Neutrino Candidate near the Blazar TXS~0506+056

We report on the observation of a rare neutrino event detected by Baikal-GVD in April 2021. The event GVD210418CA is the highest-energy cascade observed by Baikal-GVD so far from the direction below the horizon. The estimated cascade energy is $224\pm75$~TeV. The evaluated signalness parameter of GVD210418CA is 97.1\% using an assumption of the E$^{-2.46}$ spectrum of astrophysical neutrinos. The arrival direction of GVD210418CA is near the position of the well-known radio blazar TXS~0506+056, with the angular distance being within a 90\% directional uncertainty region of the Baikal-GVD measurement. The event was followed by a radio flare observed by the RATAN-600 radio telescope, further strengthening the case for the neutrino-blazar association.

astro-ph.HE↗

arXiv2023

Probing neutrino production in blazars by millimeter VLBI

The advancement of neutrino observatories has sparked a surge in multi-messenger astronomy. Multiple neutrino associations among blazars are reported while neutrino production site is located within their central (sub)parsecs. Yet many questions remain on the nature of those processes. The next generation Event Horizon Telescope (ngEHT) is uniquely positioned for these studies, as its high frequency and resolution can probe both the accretion disk region and the parsec-scale jet. This opens up new opportunities for connecting the two regions and unraveling the proton acceleration and neutrino production in blazars. We outline observational strategies for ngEHT and highlight what it can contribute to the multi-messenger study of blazars.

astro-ph.HE↗