Abstract
The limit on the intrinsic brightness temperature, attributed to ‘Compton catastrophe’, has been established being 1012 K. Somewhat lower limit of the order of 1011.5 K is implied if we ...assume that the radiating plasma is in equipartition with the magnetic field – the idea that explained why the observed cores of active galactic nuclei (AGNs) sustained the limit lower than the ‘Compton catastrophe’. Recent observations with unprecedented high resolution by the RadioAstron have revealed systematic exceed in the observed brightness temperature. We propose means of estimating the degree of the non-equipartition regime in AGN cores. Coupled with the core-shift measurements, the method allows us to independently estimate the magnetic field strength and the particle number density at the core. We show that the ratio of magnetic energy to radiating plasma energy is of the order of 10−5, which means the flow in the core is dominated by the particle energy. We show that the magnetic field obtained by the brightness temperature measurements may be underestimated. We propose for the relativistic jets with small viewing angles the non-uniform magnetohydrodynamic model and obtain the expression for the magnetic field amplitude about two orders higher than that for the uniform model. These magnetic field amplitudes are consistent with the limiting magnetic field suggested by the ‘magnetically arrested disc’ model.
ABSTRACT
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 and 1.4 GHz. This search has found 10 out of 29 nearby jets at redshifts z < 0.07 with a transition from a parabolic to conical shape, while the full analysed sample is dominated by distant AGN with a typical z ≈ 1. The ten AGN are UGC 00773, NGC 1052, 3C 111, 3C 120, TXS 0815−094, Mrk 180, PKS 1514+00, NGC 6251, 3C 371, 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 1H 0323+342 and M87, we estimate that the break occurs at 105–106 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 support our model predictions on the jet acceleration and properties of the break point.
ABSTRACT
We use the observed jet boundary transition from parabolic to conical shape, which was previously discovered to be a possible common property in active galactic nuclei, to estimate various ...parameters for black holes, jets and the ambient medium. We previously explained the geometry transition as a consequence of a change in the jet properties: a transition from a magnetically dominated to an equipartition regime. This interpretation allows us to estimate the black hole spin, the black hole mass and the 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 sources with redshift z < 2 from spin evolution modelling. We find that the method of black hole mass determination based on the relationship between the broad-line region size and its luminosity may underestimate the masses of sources with large jet viewing angles. We propose a new method for the determination of the black hole mass, obtaining masses in the interval 108–1010 M⊙. The range of 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 Fanaroff–Riley II sources.
Abstract
Recent progress in very long baseline interferometry (VLBI) observations of relativistic jets outflowing from active galactic nuclei gives us direct information about jet width rjet(l) ...dependence on the distance l from the ‘central engine’. Being the missing link in previous works, this relation opens the possibility of determining the internal structure of a jet. In this article, we consider a relativistic jet submerged in an external medium with finite gas pressure Pext. Neither an external magnetic field nor an infinitely thin current sheet will be assumed. This approach allows us to construct a reasonable solution in which both the magnetic field and the flow velocity vanish at the jet boundary r = rjet. In particular, the connection between external gas pressure and internal structure of a relativistic jet is determined.
Abstract
We propose a new method of estimating the mass of a supermassive black hole residing in the centre 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 supermassive black hole has a mass somewhat larger than typically accepted so far.
Core shift in parabolic accelerating jets Nokhrina, E E; Pushkarev, A B
Monthly notices of the Royal Astronomical Society,
01/2024, Letnik:
528, Številka:
2
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
The core-shift method is a powerful method to estimate the physical parameters in relativistic jets from active galactic nuclei. The classical approach assumes a conical geometry of jets and ...a constant plasma speed. However, recent observations have shown that neither may hold close to the central engine, where the plasma in the jet is effectively accelerating and the jet geometry is quasi-parabolic. We modify the classical core-shift method to account for these jet properties. We show that the core-shift index may assume values in the range 0.8−1.2 or 0.53−0.8 depending on the jet geometry and viewing angle, and indices close to both values are indeed observed. We obtain expressions to estimate the jet magnetic field and the total magnetic flux in a jet. We show that the obtained magnetic field value can be easily recalculated down to the gravitational radius scale. For M87 and NGC 315, these values are in good agreement with those obtained by different methods.
ABSTRACT
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 non-thermal electrons are preferably located at the jet edges or are distributed according to Ohmic heating.
CLEAN imaging systematics of M87 radio jet Pashchenko, I N; Kravchenko, E V; Nokhrina, E E ...
Monthly notices of the Royal Astronomical Society,
05/2023, Letnik:
523, Številka:
1
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
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 towards outer ridges. This could imply optical depth effects, lower energy cut-off, 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 method for the compensation of the systematics using only the observed data.
The problem of the efficiency of particle acceleration for a paraboloidal poloidal magnetic field is considered within the approach of steady axisymmetric magnetohydrodynamic (MHD) flow. For the ...large Michel magnetization parameter σ 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 & Rafikov for a monopole magnetic field. It is shown that, on the fast magnetosonic surface (FMS), the particle Lorentz factor γ does not exceed the standard value σ1/3. On the other hand, in the supersonic region, the Lorentz factor grows with the distance z from the equatorial plane as γ≈ (z/RL)1/2 up to the distance z≈σ2RL, where RL=c/ΩF is the radius of the light cylinder. Thus, the maximal Lorentz factor is γmax≈σ, which corresponds to almost the full conversion of the Poynting energy flux into the particle kinetic one.
The analysis of the frequency dependence of the observed shift of the cores of relativistic jets in active galactic nuclei (AGNs) allows us to evaluate the number density of the outflowing plasma ... ...and, hence, the multiplicity parameter ..., where ... is the Goldreich-Julian number density. We have obtained the median value for ... and the median value for the Michel magnetization parameter ... 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 centre of the jet than near the jet boundary. The MHD model can also explain the typical observed level of jet acceleration. Finally, casual connectivity of strongly collimated jets is discussed. (ProQuest: ... denotes formulae/symbols omitted.)
PDF
