Quasar Jets Research Articles

Witnessing the Gradual Slowdown of Powerful Extragalactic Jets: The X-Ray-Optical-Radio Connection

A puzzling feature of the Chandra-detected quasar jets is that their X-ray emission decreases faster along the jet than their radio emission, resulting in an outward-increasing radio-to-X-ray ratio. In some sources this behavior is so extreme that the radio emission peak is located clearly downstream of that of the X-rays. This is a rather unanticipated behavior given that the inverse Compton nature of the X-rays and the synchrotron radio emission are attributed to roughly the same electrons of the jet's nonthermal electron distribution. In this Letter we show that this morphological behavior can result from the gradual deceleration of a relativistic flow and that the offsets in peak emission at different wavelengths carry the imprint of this deceleration. This notion is consistent with another recent finding, namely, that the jets feeding the terminal hot spots of powerful radio galaxies and quasars are still relativistic with Lorentz factors Γ ~ 2-3. The picture of the kinematics of powerful jets emerging from these considerations is that they remain relativistic as they gradually decelerate from kiloparsec scales to the hot spots, where, in a final collision with the intergalactic medium, they slow down rapidly to the subrelativistic velocities of the hot spot advance speed.

On Multiwavelength Emission of Large‐Scale Quasar Jets

We discuss morphological properties of the large-scale jets in powerful radio sources, which are now observed at radio, optical, and X-ray frequencies, in order to determine the origin of their X-ray radiation and the nature of the particle acceleration responsible for their multiwavelength emission. We show that modeling knots of these objects as stationary regions of energy dissipation within the uniform and continuous jet flow leads to several problems. Such problems are especially pronounced if the observed X-ray emission is due to inverse Compton scattering of the cosmic microwave background radiation. We explore another possibility, namely, that the knots represent moving and separate portions of the jet matter with excess kinetic power. We suggest a possible connection between this scenario and the idea of intermittent/highly modulated jet activity. The proposed model can explain some morphological properties of quasar jets—such as high knot-to-interknot brightness contrasts, frequency-independent knot profiles, and the almost universal extents of the knot regions—independently of the exact emission mechanism responsible for producing the X-rays. In this context, we consider different possibilities for the production of such X-ray radiation and discuss the related issue of particle acceleration. We conclude that the appropriate process cannot be designated for certain yet. We suggest, however, that X-ray observations of the jet in 3C 120 (in which intermittent jet activity is quite distinct) already seem to support the synchrotron origin of the X-ray emission, at least in this object.

Brightness Suppression of Relativistic Radio Jets of Quasars: The Role of the Lower Electron Energy Cutoff

Although the possibility of a low-energy cutoff (LEC) of the relativistic electron population (Emin ~ 0.1 GeV) in the different components of radio galaxies has been discussed in the literature, from both theoretical and observational viewpoints, its possible role in causing a distinct spectral flattening, and thus reducing the apparent brightness temperature of the radio nucleus, has thus far not been explicitly discussed. Here we point out that such an effect may, in fact, be quite significant for the parsec-scale, superluminal emission regions associated with the inner radio jets. This is because the observed frequency of the spectral turnover arising from the LEC increases linearly with the bulk Doppler factor (δ) of the jet flow, whereas the frequency of synchrotron self-absorption (the well-known cause of spectral flattening and turnover at low frequencies) shows a mild decrease as δ rises. For an observationally relevant range of source parameters, we investigate here the role of the LEC. We argue that some statistical trends revealed by the recent very long baseline interferometry (VLBI) surveys can, in fact, be understood in terms of effects arising from LEC, including the faster superluminal motion found in the VLBI surveys at higher frequencies and the apparent inability of most radio cores to even attain the brightness temperatures consistent with the equipartition condition. We also point out some possible implications of the LEC for X-ray observations of kiloparsec-scale relativistic jets of quasars.

Dispersion relations in relativistic astrophysical outflows

In this work we delineate the physical picture for the propagation of electromagnetic oscillations in a moving plasma. We present calculations devoted to describing the dispersion features of the radiation that propagates within a magnetoactive plasma. Both classical and relativistic treatments have been employed. We have concluded that if the outflow is moving with small velocities, the general motion of the plasma does not affect the propagation of transverse electromagnetic waves. The waves are refracted according to the ordinary refraction law deduced for non-moving magnetoactive plasmas. The global movement does not change the dispersion relations, and the shapes of the corresponding curves remain unaltered. In the relativistic formulation, we have derived the equations that determine the electromagnetic wave spectrum for a moving magnetoactive plasma. In this case, it is found that the flow velocity affects the plasma dispersion properties and is responsible for new dispersion branches: transverse and longitudinal perturbations. The ordinary and extraordinary electromagnetic modes are modified and described by dispersion relations whose points of resonance are also in explicit dependence on the plasma velocity. Furthermore, the relativistic motion causes the split of electromagnetic modes into two independent waves that propagate at different rates. This effect is remarkable for highly relativistic velocities. The results presented in this work may be applied in order to incorporate suitable corrections to the dispersion models, developed to explain the anomalous features of observed lines or continuum radiation, associated with astrophysical relativistic outflows which are present in gamma-ray bursts, blazars, relativistic jets in AGNs (active galactic nuclei), quasars and microquasars.

Model of large scale jet powered by very-high energy gamma rays

The radiative cooling of electrons responsible for the nonthermal synchrotron emission of large scale jets of radiogalaxies and quasars requires quasi continuous (in time and space) production of relativistic electrons throughout the jets over the scales exceeding 100 kpc. While in the standard paradigm of large scale jets this implies in situ acceleration of electrons, here we propose a principally different ‘non-acceleration’ origin of these electrons, assuming that they are implemented all over the length of the jet through effective development of electromagnetic cascades initiated by extremely high energy γ-rays injected into the jet from the central object. This scenario provides a natural and very economic way to power the jets up to distances of 100 kpc and beyond [PhRvL 89 (2002) 051101].

CHANDRA Observations of X-ray jet structure on kpc to Mpc scales

With its exquisite spatial resolution of better than 0.5 arcsec, the Chandra observatory is uniquely capable of resolving and studying the spatial structure of extragalactic X-ray jets on scales of a few to a few hundred kilo-parsec. Our analyses of four recent Chandra images of quasar jets interpret the X-ray emission as inverse Compton scattering of high energy electrons on the cosmic microwave background. We infer that these jets are in bulk relativistic motion, carrying kinetic powers upwards of 10 46 ergs s −1 to distances of hundreds of kpc, with very high efficiency.

Relativistic parsec–scale jets: II. Synchrotron emission

We calculate the optically thin synchrotron emission of fast electrons and positrons in a spiral stationary magnetic field and a radial electric field of a rotating relativistic strongly magnetized force–free jet consisting of electron-positron pair plasma. The magnetic field has a helical structure with a uniform axial component and a toroidal component that is maximal inside the jet and decreasing to zero towards the boundary of the jet. Doppler boosting and swing of the polarization angle of synchrotron emission due to the relativistic motion of the emitting volume are calculated. The distribution of the plasma velocity in the jet is consistent with the electromagnetic field structure. Two spatial distributions of fast particles are considered: uniform, and concentrated in the vicinity of the Alfvén resonance surface. The latter distribution corresponds to the regular acceleration by an electromagnetic wave in the vicinity of its Alfvén resonance surface inside the jet. The polarization properties of the radiation have been obtained and compared with the existing VLBI polarization measurements of parsec–scale jets in BL Lac sources and quasars. Our results give a natural explanation of the observed bimodality in the alignment between the electric field vector of the polarized radiation and the projection of the jet axis on the plane of the sky. We interpret the motion of bright knots as a phase velocity of standing spiral eigenmodes of electromagnetic perturbations in a cylindrical jet. The degree of polarization and the velocity of the observed proper motion of bright knots depend upon the angular rotational velocity of the jet. The observed polarizations and velocities of knots indicate that the magnetic field lines are bent in the direction opposite to the direction of the jet rotation.

A View through Faraday’s Fog. II. Parsec‐Scale Rotation Measures in 40 Active Galactic Nuclei

Results from a survey of the parsec-scale Faraday rotation measure (RM) properties for 40 quasars, radio galaxies, and BL Lac objects are presented. Core RMs for quasars vary from approximately 500 to several thousand rad m-2. Quasar jets have RMs that are typically 500 rad m-2 or less. The cores and jets of the BL Lac objects have RMs similar to those found in quasar jets. The jets of radio galaxies exhibit a range of RMs from a few hundred to almost 10,000 rad m-2 for the jet of M87. Radio galaxy cores are generally depolarized, and only one of four radio galaxies (3C 120) has a detectable RM in the core. Several potential identities for the foreground Faraday screen are considered, and we believe the most promising candidate for all the active galactic nucleus types considered is a screen in close proximity to the jet. This constrains the path length to approximately 10 pc, and magnetic field strengths of approximately 1 μG can account for the observed RMs. For 27 out of 34 quasars and BL Lac objects, their optically thick cores have good agreement with a λ2 law. This requires the different τ = 1 surfaces to have the same intrinsic polarization angle independent of the observed frequency and distance from the black hole.

Clear Evidence for Intranight Optical Variability in Radio-quiet Quasars

We present new clues to the problem of the radio loudness dichotomy arising from an extensive search for intranight optical variability in seven sets of optically luminous radio-quiet quasars and (radio-loud) BL Lacertae objects, which are matched in optical luminosity and redshift. Our monitoring of radio-quiet quasars has for the first time clearly detected such intranight variability, with peak-to-peak amplitudes ~1%, occurring with a duty cycle of ~. The matched BL Lac objects have both higher variability amplitudes and duty cycles when observed in the same fashion. We show that the much less pronounced intranight variability of the radio-quiet quasars relative to BL Lac objects can be understood in terms of a modest misalignment of the jets in radio-quiet quasars from the line of sight. We thus infer that relativistic particle jets may well also emerge from radio-quiet quasars, but while traversing the short optical-emitting distances, they could be snuffed out, possibly through inverse Compton losses in the nuclear region.

Ion-dominated plasma and the origin of jets in quasars

Low cooling plasmas associated with large kinetic energies are likely to be the origin of the kpc-extended and well collimated extra-galactic jets. It is proposed that jets are launched from a layer, governed by a highly diffusive, super-Keplerian rotating and thermally dominated by virial-hot and magnetized ion-plasma. The launching layer is located between the accretion disk and the corona surrounding the nucleus. The matter in the layer is causally connected to both the disk and to the central engine. Moreover we find that coronae, in the absence of heating from below, are dynamically unstable to thermal ion-conduction, and that accretion disks become intrinsically advection-dominated. We confirm the capability of this multi-layer model to form jets by carrying out 3D axisymmetric quasi-stationary MHD calculations with high spatial resolution, and taking into account turbulent and magnetic diffusion. The new multi-layer topology accommodates several previously proposed elements for jet-initiation, in particular the ion-torus, the magneto-centrifugal and the truncated disk - advective tori models.

Large-scale extragalactic jets powered by very-high-energy gamma rays.

The radiative cooling of electrons responsible for the nonthermal synchrotron emission of large-scale jets of radiogalaxies and quasars requires quasicontinuous (in time and space) production of relativistic electrons throughout the jets over the scales exceeding 100 kpc. While in the standard paradigm of large-scale jets this implies in situ acceleration of electrons, we propose a different "nonacceleration" origin of these electrons, assuming that they are implemented all over the length of the jet through effective development of electromagnetic cascades initiated by extremely high-energy gamma rays injected into the jet from the central object.

Radiation fields of disk, BLR and torus in quasars and blazars: implications for γ-ray absorption

The radiation fields external to the jets and originating from within a few parsecs from the black hole, are discussed in this paper. They are the direct radiation from an accretion disk in symbiosis with jets, the radiation field from the broad line region (BLR) surrounding the accretion disk, and the infrared radiation from a dusty torus. The jet/disk symbiosis modifies the energetics in the central parsec of active galactic nuclei (AGN) such that for a given accretion rate, a powerful jet would occur at the expense of the disk luminosity, and consequently the disk would less efficiently ionize the BLR clouds or heat the dust in the torus, thereby affecting potentially important target photon fields for interactions of γ-rays, accelerated electrons and protons along the jet. Motivated by unification schemes of AGN, we briefly review the evidence for the existence of BLRs and small-scale dust tori in BL Lacs and Fanaroff-Riley Class I (FR-I) radio galaxies. We propose that an existing jet-accretion disk symbiosis can be extrapolated to provide a large-scale symbiosis between other important dusty constituents of the blazar/FR-I family. In the present paper, we discuss in the context of this symbiosis interactions of GeV and TeV γ-rays produced in the jet with the various radiation fields external to the jet in quasars and blazars, taking account the anisotropy of the radiation.

X‐Ray universe: Quasar's jet goes the distance

Science NewsVolume 161, Issue 7 p. 101-101 This Week X-Ray universe: Quasar's jet goes the distance Ron Cowen, Ron CowenSearch for more papers by this author Ron Cowen, Ron CowenSearch for more papers by this author First published: 02 July 2009 https://doi.org/10.2307/4013011AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume161, Issue716 February 2002Pages 101-101 RelatedInformation

Host galaxies and the unification of radio-loud AGN

Our HST WFPC2 survey of 110 BL Lac objects, from six complete X-ray-, radio-, and optically-selected catalogs, probes the host galaxies of low-luminosity radio sources in the redshift range 0< z<1.35. The host galaxies are luminous ellipticals, well matched in radio power and galaxy magnitude to FR I radio galaxies. Similarly, the host galaxies of high luminosity quasars occupy the same region of this plane as FR II radio galaxies (matched in redshift). This strongly supports the unification of radio-loud AGN, and suggests that studying blazars at high redshift is a proxy for investigating less luminous (to us) but intrinsically identical radio galaxies, which are harder to find at high z. Accordingly, the difference between low-power jets in BL Lac objects and high-power jets in quasars can then be related to the FR I/FR II dichotomy; and the evolution of blazar host galaxies or their nuclei (jets) should correspond to the evolution of radio galaxies.

О возможности определения скорости джетов радиогалактик и квазаров по исследованиям тонкой структуры их узлов при высоком угловом разрешении

О возможности определения скорости джетов радиогалактик и квазаров по исследованиям тонкой структуры их узлов при высоком угловом разрешении

Superluminal motion inastronomy

Several examples of `intrinsic-type' superluminal motion inastronomy are taken. A simple signal-delay transformationis devised and shown to be sufficient to explain thesuperluminal effect as resulting from differential signal delayacross an expanding source. The distinction betweenrelativistic motion and relativistic kinematicsis made. The key kinematical equation used to describesuperluminal motion is an alternative statement of the Dopplereffect. Relativistic transformations, which are relevant whenintervals in different reference frames are compared,then lead to the relativistic Doppler factor (δ), whichis applicable to measurements on a photographic image, forexample that of a relativistic quasar jet with superluminalcomponents.

Modeling the Production of Flares in Gamma‐Ray Quasars

Theories of high-energy radiation production in quasar jets can be verified by studies of both time-averaged spectra and variability patterns. While the former has been explored extensively, the latter is in its infancy. In this paper, we study the production of short-term flares in the shock-in-jet model. We examine how the flares' profiles depend on such parameters as shock/dissipation lifetime, electron-injection time profile, adiabaticity, and half-opening angle of the jet. In particular, we demonstrate the large difference between flare profiles produced in the radiative and adiabatic regimes. We apply our model to the ~1 day timescale flares observed in optically violently variable quasars, checking whether the external radiation Compton (ERC) model for γ-ray flares at energies greater than 30 MeV (EGRET range) can be reconciled with the flares observed at lower energies. Specifically, we show that the strict correlation between X-ray and γ-ray flares strongly supports the dominance of the synchrotron self-Compton mechanism in the X-ray band. We also derive conditions that must be satisfied by the ERC model in order to explain a lag of the γ-ray peak behind the optical one, as claimed to be observed in PKS 1406-076. Finally, we predict that in ERC models where the MeV peak is related to the break in electron distribution owing to inefficient cooling of electrons below a certain energy, the flares should decay significantly more slowly in the soft γ-ray band than at energies greater than 30 MeV.

Internal shocks in the jets of radio-loud quasars

The central engine causing the production of jets in radio sources may work intermittently, accelerating shells of plasma with different mass, energy and velocity. Faster but later shells can then catch up slower earlier ones. In the resulting collisions shocks develop, converting some of the ordered bulk kinetic energy into magnetic field and random energy of the electrons which then radiate. We propose that this internal shock scenario, which is the scenario generally thought to explain the observed gamma-ray burst radiation, can also work for radio sources in general, and for blazars in particular. We investigate in detail this idea, simulating the birth, propagation and collision of shells, calculating the spectrum produced in each collision, and summing the locally produced spectra from those regions of the jet which are simultaneously active in the observer's frame. We can thus construct snapshots of the overall spectral energy distribution, time-dependent spectra and light curves. This allows us to characterize the predicted variability at any frequency, study correlations between the emission at different frequencies, specify the contribution of each region of the jet to the total emission, and find correlations between flares at high energies and the birth of superluminal radio knots and/or radio flares. The model has been applied to reproduce qualitatively the observed properties of 3C 279. Global agreement in terms of both spectra and temporal evolution is found. In a forthcoming work, we will explore the constraints that this scenario sets on the initial conditions of the plasma injected in the jet and the shock dissipation for different classes of blazars.

The Association of Jet Production with Geometrically Thick Accretion Flows and Black Hole Rotation

A model is presented in which the strongest radio-emitting jet outflows are produced in black hole systems when the accretion is a geometrically thick (H/R ~ 1) inflow (e.g., an advection-dominated or a convection-dominated accretion flow) and if the black hole is rotating. For galactic black hole candidates, the model naturally accounts for the observed correlation of jet outflow with the black hole hard emission state and predicts an association of strong jets with rapid black hole rotation. When extended to the supermassive case, the model accounts for the highest radio galaxy and quasar jet powers and provides additional theoretical support for the spin paradigm, which asserts that radio-loud quasars are produced by Kerr holes and radio-quiet ones by Schwarzschild holes. In some cases, the angular momentum and energy outflow in the jet may be large enough to alter significantly the structure of the accretion flow from that predicted by current models.

III Zw 2: Superluminal Motion and Compact Lobe Expansion in a Seyfert Galaxy

So far all relativistically boosted jets with superluminal motion have only been detected in normal radio galaxies that have early-type host galaxies. We have now discovered superluminal motion in the Seyfert 1 galaxy III Zw 2, classified as a spiral. Spectral and spatial evolution are closely linked. Since III Zw 2 is part of a sample of so-called radio-intermediate quasars (RIQ), it confirms earlier predictions of superluminal motion for this source, based on the argument that RIQs could be relativistically boosted jets in radio-weak quasars and Seyfert galaxies.