arXiv subjects

E. E. Nokhrina

Publications and source records attributed to E. E. Nokhrina.

18 recordsLinked to original sources

arXiv2026

Magnetic field and plasma number density from radio and millimeter core measurements in AGN jets

Understanding the mechanism for launching relativistic jets in active galactic nuclei relies upon measuring the magnetic field strength and emitting plasma number density, tracing their evolution along the jet, and determining the relation between their rest frame energy densities. This can be achieved using measurements of the size and brightness temperature of the compact region at the jet base (the ``core'') obtained with very long baseline interferometry (VLBI) across frequencies from 2 to 230~GHz. We develop a framework for independently estimating the magnetic field B* and the emitting plasma number density N* as functions of the jet width $d$, using multifrequency VLBI observations of the core size and brightness temperature. We apply the standard model of self-absorbed synchrotron emission, assuming power-law dependencies of the jet Doppler factor, Lorentz factor, magnetic field strength, and plasma density on the jet width. For an arbitrary jet boundary shape, we derive the dependencies B*(d) and N*(d), and explore a possible relation between the rest frame energy densities of the magnetic field and the emitting plasma. Analysis of core widths and brightness temperatures measured at multiple frequencies points to the possible presence of a magnetic flux decay and effective plasma acceleration within the observed cores at least in some sources of the sample.

astro-ph.HE↗

arXiv2025

Jet MHD analytical model - framework for radiative transfer

We developed the full magnetohydrodynamical analytical jet model that allows accurate reproducing of a transversal and longitudinal structure for a highly collimated relativistic jets. This model can be used as a setup for convenient solution of radiative transfer equations and modelling the total intensity and polarization maps. We show that the analytical fits are in excellent agreement with the numerical solutions of full magnetohydrodynamical equations. Our approach allows setting easily different models for an emitting plasma number density. For example, we show that the equipartition number density ranges from several to tens of percent of a total number density. We show that the Doppler-corrected emissivity distribution behave in such a way that we may expect a limb brightened intensity pattern on a sub-parsec scale and a spine-brightened structure downstream. We reproduce the broken power-law dependence of a jet pressure at its boundary from the jet radius. The corresponding power exponents are in agreement with the parabola-to-cone transition observed directly in nearby sources.

astro-ph.HE↗

arXiv2024

On the possible core shift break in relativistic jets

Measurement of a jet geometry transition region is an important instrument of assessing the jet ambient medium properties, plasma bulk motion acceleration, parameters of a black hole and location of a jet launching radius. In this work we explore the possibility of a presence of a core shift break, associated with the geometry and jet physical properties transition. We obtain the relations on the core shift offset jump due to a change in a core shift exponent. The condition of a proper frame magnetic field continuity and the core shift break can be used as an instrument to refine the magnetic field estimates upstream the break. This method is applied to the jet in NGC 315. We speculate that the localised in a flow plasma heating either by reconnection or due to particles acceleration at the shock will also lead to a core shift break, but of a different type, than the one observed in NGC 315. We propose to use the multi-frequency core shift measurements to increase the number of sources with a detected jet shape break and to boost the accuracy of assessing the properties of a jet geometry transition region.

astro-ph.HE↗

arXiv2024

On the M87 jet structure near the central engine

At present, there is no doubt that relativistic jets observed in active galactic nuclei pass from highly magnetized to weakly magnetized stage, which is observed as a break in the dependence on their width $d_{\rm jet}(z)$ on the distance $z$ to the central engine. In this paper, we discuss the possibility of observing another break, which should be located at shorter distances. The position of this break can be associated with the region of formation of the dense central core near the jet axis which was predicted both analytically and numerically more than a decade ago, but has not yet received sufficient attention. In this case, the observed width should be determined by the dense core, and not by the total transverse size of the jet. The calculations carried out in this paper, which took into account both the transverse electromagnetic structure of the jet and the change in the spectrum of emitting particles along its axis, indeed showed such behaviour. We also found the evidence of the predicted break in the jet expansion profile using stacked 15 GHz VLBA image of M87 radio jet and constrain the light cylinder radius.

astro-ph.GA↗

arXiv2023

CLEAN imaging systematics of M87 radio jet

The spectral index images of the jet in the nearby radio galaxy M87 have previously been shown with Very Long Baseline Interferometric arrays at 2-43 GHz. They exhibit flattening of the spectra at a location of inner (central) spine and toward outer ridges. This could imply optical depth effects, lower energy cutoff or stratification of the emitting particles energy distribution. In this paper we employ simulations of multifrequency VLBI observations of M87 radio jet with various model brightness distributions. CLEAN deconvolution errors produce significant features in the observed images. For intensity images they result in the appearance of the inner ridge line in the intrinsically edge brightened jet models. For spectral index images they flatten the spectra in a series of stripes along the jet. Another bias encountered in our simulations is steepening of the spectra in a low surface brightness jet regions. These types of the imaging artefacts do not depend on the model considered. We propose a methods for the compensation of the systematics using only the observed data.

astro-ph.GA↗

arXiv2023

Synchrotron intensity plots from a relativistic stratified jet

We examine the effect of a jet transversal structure from magnetohydrodynamic semi-analytical modelling on the total intensity profiles of relativistic jets from active galactic nuclei. In order to determine the conditions for forming double- and triple-peaked transverse intensity profiles, we calculate the radiative transfer for synchrotron emission with self-absorption from the jets described by the models with a constant angular velocity and with a total electric current closed inside a jet. We show that double-peaked profiles appear either in the models with high maximal Lorentz factors or in optically thick conditions. We show that triple-peaked profiles in radio galaxies constrain the fraction of the emitting particles in a jet. We introduce the possible conditions for triple-peaked profiles under the assumptions that nonthermal electrons are preferably located at the jet edges or are distributed according to Ohmic heating.

astro-ph.HE↗

arXiv2021

Parabolic jet shape on parsec scales in high redshift AGN

Geometry of relativistic jets in active galaxies provides important information about mechanisms of launching, collimation and acceleration of plasma flow. We propose a new method to probe a boundary shape of a jet on parsec scales -- in the vicinity of its radio core. Apparent speed of an outflow is derived from variability time delays and core shifts measured at the same jet region, providing a self-consistent estimate of the Lorentz factor $Γ$. We link together the distance along the jet $z$ with its transverse size assuming a constant flow acceleration. Our results indicate that jets have parabolic shape and sustain an effective acceleration in the core region, consistent with the Lorentz factor dependency $Γ\propto z^{0.5}$. The proposed method can be applied to the sources observed at small viewing angles as well as to the distant sources when direct measurements are impossible due to a limited angular resolution.

astro-ph.HE↗

arXiv2020

Physical parameters of active galactic nuclei derived from properties of jet geometry transition region

We use the observed jet boundary transition from parabolic to conical shape, which was earlier discovered as possibly a common effect in active galactic nuclei, to estimate a black hole, a jet and an ambient medium parameters. We explained earlier the geometry transition as a consequence of a change in the jet inner properties: a transition from a magnetically dominated to an equipartition regime. This interpretation allows us to estimate a black hole spin, a black hole mass and an ambient pressure amplitude, using the observed jet shape break position and the jet width at the transition point for 11 active galactic nuclei. The black hole spin values obtained using our method are consistent with the lower estimates for the sources with redshift $z<2$ from the spin evolution modelling. We find that the method of a black hole mass determination based on the relation between the broad-line region size and its luminosity may underestimate masses of the sources with large jet viewing angles. We propose a new method for the black hole mass determination, with the obtained masses being in interval $10^8-10^{10}\;M_{\odot}$. The range of the values of the ambient pressure amplitude points to the uniform medium conditions for the sources in our sample, with a tentative indication of higher pressure around FRII sources.

astro-ph.HE↗

arXiv2020

A transition from parabolic to conical shape as a common effect in nearby AGN jets

Observational studies of collimation in jets in active galactic nuclei (AGN) are a key to understanding their formation and acceleration processes. We have performed an automated search for jet shape transitions in a sample of 367 AGN using VLBA data at 15 GHz and 1.4 GHz. This search has found ten out of 29 nearby jets at redshifts z<0.07 with a transition from a parabolic to conical shape, while the full analyzed sample is dominated by distant AGN with a typical z about 1. The ten AGN are UGC00773, NGC1052, 3C111, 3C120, TXS0815-094, Mrk180, PKS1514+00, NGC6251, 3C371, and BL Lac. We conclude that the geometry transition may be a common effect in AGN jets. It can be observed only when sufficient linear resolution is obtained. Supplementing these results with previously reported shape breaks in the nearby AGN 1H0323+342 and M87, we estimate that the break occurs at 10^5-10^6 gravitational radii from the nucleus. We suggest that the jet shape transition happens when the bulk plasma kinetic energy flux becomes equal to the Poynting energy flux, while the ambient medium pressure is assumed to be governed by Bondi accretion. In general, the break point may not coincide with the Bondi radius. The observational data supports our model predictions on the jet acceleration and properties of the break point.

astro-ph.GA↗

arXiv2019

M87 black hole mass and spin estimate through the position of the jet boundary shape break

We propose a new method of estimating a mass of a super massive black hole residing in the center of an active galaxy. The active galaxy M87 offers a convenient test case for the method due to the existence of a large amount of observational data on the jet and ambient environment properties in the central area of the object. We suggest that the observed transition of a jet boundary shape from a parabolic to a conical form is associated with the flow transiting from the magnetically dominated regime to the energy equipartition between plasma bulk motion and magnetic field. By coupling the unique set of observations available for the jet kinematics, environment and boundary profile with our MHD modelling under assumption on the presence of a dynamically important magnetic field in the M87 jet, we estimate the central black hole mass and spin. The method leads us to believe that the M87 super massive black hole has a mass somewhat larger than typically accepted so far.

astro-ph.HE↗

arXiv2017

On the acceleration and deceleration of relativistic jets in active galactic nuclei - II. Mass-loading

The effect of a mass-loading of the magnetohydrodynamic (MHD) flow in a relativistic jet from active galactic nuclei (AGN) due to $γγ\rightarrow e^+e^-$ conversion is considered analytically. We argue that the effects of charge average separation due to specific initial pairs' motion lead to partial magnetic and electric fields screening or enhancement. The effect of the fields screening has not been considered earlier. The pairs with the center-of-mass moving faster or slower than the bulk plasma flow create a surface charge and a current which either screen or enhance both electric and magnetic fields in a pair creation domain. This impacts the bulk flow motion, which either accelerates or decelerates. The pairs with the center-of-mass moving with exactly the drift velocity does not induce the fields disturbance. In this case the flow decelerates due to pure mass-loading. For these different cases the Lorentz factor of the loaded outflow is calculated as a function of loading pairs number density. The effect may be important on sub-parsec to parsec scales due to conversion of TeV jet radiation on soft infra-red to ultra-violet external isotropic photon field. This leads to an outer shell acceleration. The conversion of MeV jet radiation on larger scales may account for the flow deceleration due to pure mass-loading. The proposed mechanism may be a source of internal shocks and instabilities in the pair creation region.

astro-ph.HE↗

arXiv2014

Intrinsic Physical Conditions and Structure of Relativistic Jets in Active Galactic Nuclei

The analysis of the frequency dependence of the observed shift of the cores of relativistic jets in active galactic nuclei (AGN) allows us to evaluate the number density of the outflowing plasma n and, hence, the multiplicity parameter, which is a ratio of particle number density to Goldreich-Julian number density. We have obtained the median value for the multiplicity parameter to be 3x10^13 and the median value for the Michel magnetization parameter is 8 from an analysis of 97 sources. Since the magnetization parameter can be interpreted as the maximum possible Lorentz factor of the bulk motion which can be obtained for relativistic magnetohydrodynamic (MHD) flow, this estimate is in agreement with the observed superluminal motion of bright features in AGN jets. Moreover, knowing these key parameters, one can determine the transverse structure of the flow. We show that the poloidal magnetic field and particle number density are much larger in the center of the jet than near the at boundary. The MHD model can also explain the typical observed level of jet acceleration. Finally, casual connectivity of strongly collimated jets is discussed.

astro-ph.HE↗

arXiv2011

Determination of a magnetization parameter of the parsec-scale AGN jets

The observed shift of the core of the relativistic AGN jets as a function of frequency allows us to evaluate the number density of outflowing plasma and, hence, the magnetization parameter. New analysis of more than 20 sources shows that for most of jets the magnetization parameter ~ 10 - 100. Since the magnetization parameter is the maximum possible bulk Lorentz-factor of the relativistic flow, this estimate is consistent with the observed superluminal motion.

astro-ph.HE↗

arXiv2008

On the central core in MHD winds and jets

We demonstrate that the 1D cylindrical version of the Grad-Shafranov equation is more rich than classical self-similar ones, and more suitable for the astrophysical jets we observe. In particular, it allows us to describe the central (and, hence, the most energetic) part of the flow. Both relativistic and non-relativistic versions are discussed. It is shown that taking into account the finite pressure of the external media one can determine the magnetic flux within the central core. We found as well that for non-relativistic flows which are magnetically dominated near the origin the solution can be constructed only in the presense of the oblique shock near the base of a jet where the additional heating is to take place.

astro-ph↗

arXiv2008

On the Cylindrical Grad-Shafranov Equation

The goal of this presentation is in paying attention to the 1D cylindrical version of the Grad-Shafranov (GS) equation. In our opinion, this approach is more rich than classical self-similar ones, and more suitable for astrophysical jets we observe. In particular, it allows us describing the central (and, hence, the most energetic) part of the flow.

astro-ph↗

arXiv2007

On the role of the current loss in radio pulsar evolution

The aim of this article is to draw attention to the importance of the electric current loss in the energy output of radio pulsars. We remind that even the losses attributed to the magneto-dipole radiation of a pulsar in vacuum can be written as a result of an Ampere force action of the electric currens flowing over the neutron star surface (Michel, 1991, Beskin et al., 1993). It is this force that is responsible for the transfer of angular momentum of a neutron star to an outgoing magneto-dipole wave. If a pulsar is surrounded by plasma, and there is no longitudinal current in its magnetosphere, there is no energy loss (Beskin et al., 1993, Mestel et al., 1999). It is the longitudinal current closing within the pulsar polar cap that exerts the retardation torque acting on the neutron star. This torque can be determined if the structure of longitudinal current is known. Here we remind of the solution by Beskin, Gurevitch & Istomin (1993) and discuss the validity of such an assumption. The behavior of the recently observed "part-time job" pulsar B1931+24 can be naturally explained within the model of current loss while the magneto-dipole model faces difficulties.

astro-ph↗

arXiv2007

On the role of the current loss mechanism in radio pulsar evolution

The aim of this article is to draw attention to the importance of the electric current loss in the energy output of radio pulsars. We remind that even the losses attributed to the magneto-dipole radiation of a pulsar in vacuum can be written as a result of Ampere force action of the electric currents flowing over the neutron star surface (Michel 1991, Beskin, Gurevich & Istomin 1993). It is this force that is responsible for the transfer of angular momentum of a neutron star to an outgoing magneto-dipole wave. If a pulsar is surrounded by plasma, and there is no longitudinal current in its magnetosphere, there is no energy loss (Beskin, Gurevich & Istomin 1993, Mestel, Panagi & Shibata 1999). It is the longitudinal current closing within the pulsar polar cap that exerts the retardation torque acting on the neutron star. This torque can be determined if the structure of longitudinal current is known. Here we remind of the solution by Beskin, Gurevich & Istomin (1993) and discuss the validity of such an assumption. Finally, it is shown that the behaviour of the recently observed "part-time job" pulsar B1931+24 can be naturally explained within the model of current loss while the magneto-dipole model faces difficulties.

astro-ph↗

arXiv2005

On the effective particle acceleration in the paraboloidal magnetic field

The problem of the efficiency of particle acceleration for parabolic poloidal magnetic field is considered within the approach of steady axisymmetric MHD flow. For the large Michel magnetization parameter it is possible to linearize the stream equation near the force-free solution and to solve the problem self-consistently as was done by Beskin, Kuznetsova and Rafikov (1998) for monopole magnetic field. It is shown that on the fast magnetosonic surface the particle Lorentz factor does not exceed the standard value. On the other hand, in the supersonic region the Lorentz factor grows linearly with the distance from the rotational axis, reaching almost its maximal value. This corresponds to almost the full conversion of the Poynting energy flux into the particle kinetic one.

astro-ph↗