Quasar Jets Research Articles

Evidence of Thick Obscuring Matter Revealed in the X‐Ray Spectrum of thez = 4.28 Quasar RX J1028.6−0844

We report the discovery of an unambiguous, substantial low-energy cutoff in the broadband X-ray spectrum of the radio quasar RX J1028.6-0844 at a redshift of 4.276 obtained with the ASCA satellite, which we preferably explained as an indication of excess X-ray absorption. The equivalent hydrogen column density of the absorbing matter, depending on the redshift and metallicity, ranges from 2.5 × 1021 cm-2 for local absorption up to 2.1 × 1023 cm-2 (solar metallicity) or 1.6 × 1024 cm-2 (10% solar metallicity) for absorption at the quasar redshift. Such a value is among the largest found for high-redshift radio quasars. The absorption, if interpreted as being produced close to the quasar, may indicate the presence of a remarkably large amount of obscuring matter in the quasar environment in the early universe. Implications of the result for the possible origins of the absorbing matter are discussed, concerning especially galactic intervening matter, cool intracluster gas, and ambient medium around the quasar jet. The quasar itself has an enormous apparent luminosity of at least about 2.6 × 1047 ergs-1 (H0 = 50 km s-1 Mpc-1, q0 = 0.5) and a power-law photon index of 1.67 in the 2-50 keV band in the source rest frame.

Comptonization of Infrared Radiation from Hot Dust by Relativistic Jets in Quasars

We demonstrate the importance of near-infrared radiation from hot dust for Compton cooling of electrons/positrons in quasar jets. In our model, we assume that the nonthermal radiation spectra observed in optically violent variable (OVV) quasars are produced by relativistic electrons/positrons accelerated in thin shells that propagate down the jet with relativistic speeds. We show that the Comptonization of the near-IR flux is likely to dominate the radiative output of OVV quasars in the energy range from tens of keV up to hundreds of MeV, where it exceeds that produced by Comptonization of the UV radiation reprocessed and rescattered in the broad emission line (BEL) region. The main reason for this lies in the fact that the jet encounters the ambient IR radiation over a relatively large distance as compared to the distance where the energy density of the BEL light peaks. In the soft to mid-energy X-ray band, the spectral component resulting from Comptonization of the near-IR radiation joins smoothly with the synchrotron self-Compton component, which may be responsible for the soft X-ray flux. At the highest observed γ-ray energies, in the GeV range, Comptonization of broad emission lines dominates over other components.

First-Epoch VSOP Observation of 3C 380: Kinematics of the Parsec-Scale Jet

Abstract The space-VLBI mission VSOP provides high-quality imaging capabilities to investigate fine structures in quasar jets. We report on the first-epoch VSOP observation of the superluminal quasar 3C 380 at λ 6 cm. A high-quality image with a dynamic range of 1:1200 at a resolution of 0.74 mas × 0.36 mas reveals precise structure along the pc-scale jet. Components C2, A, and F, have been identified with those in previous ground-VLBI observations, and an apparent superluminal motion has been confirmed. We find a very straight motion with a constant apparent velocity for components A and F. Each motion vector is almost parallel to each position vector. This suggests a ballistic ejection from the core. In spite of better position accuracy than previous ground-VLBI observations, we cannot confirm any evidence of acceleration or changes of the position angle which were reported in Polatidis and Wilkinson (1998, AAA 069.159.023). We confirmed that the bent jet structure has kept its similarity and direction, compared with previous images. This suggests an oblique shock in relativistic flow rather than the Kelvin-Helmholtz instability.

Radio-quiet quasar environments at 0.5 <= z <= 0.8

We have quantified the galaxy environments around a sample of 0.5 < z < 0.8 radio-quiet quasars using the amplitude of the spatial galaxy--quasar correlation function, B_gq. The quasars exist in a wide variety of environments, some sources are located in clusters as rich as Abell class 1--2 clusters, whereas others exist in environments comparable to the field. We find that on average, the quasars prefer poorer clusters of approximately Abell class 0, which suggests that quasars are biased tracers of mass compared to galaxies. The mean B_gq for the sample is found to be indistinguishable from the mean amplitude for a sample of radio-loud quasars matched in redshift and optical luminosity. These observations are consistent with recent studies of the hosts of radio-quiet quasars at low to intermediate redshifts, and suggest that the mechanism for the production of powerful radio jets in radio-loud quasars is controlled by processes deep within the active galactic nucleus itself, and is unrelated to the nature of the hosts or their environments.

On Pair Content and Variability of Subparsec Jets in Quasars

X-ray observations of blazars associated with optically violently variable (OVV) quasars put strong constraints on the e+e- pair content of radio-loud quasar jets. From those observations, we infer that jets in quasars contain many more e+e- pairs than protons, but dynamically are still dominated by protons. In particular, we show that pure e+e- jet models can be excluded, since they overpredict soft X-radiation; likewise, pure proton-electron jets can be excluded, since they predict too weak nonthermal X-radiation. An intermediate case is viable. We demonstrate that jets that are initially proton-electron ("protojets") can be pair-loaded via interaction with 100-300 keV photons produced in hot accretion disc coronae, likely to exist in active galactic nuclei in general. If the coronal radiation is powered by magnetic flares, the pair loading is expected to be nonuniform and nonaxisymmetric. Together with radiation drag, this leads to velocity and density perturbations in a jet and the formation of shocks, where the pairs are accelerated. Such a scenario can explain rapid (timescale of ~a day) variability observed in OVV quasars.

Quasar jets and their fields

Quasar jets and their fields

The contribution of unresolved radio-loud AGN to the extragalactic diffuse gamma-ray background

We present a calculation of the blazar contribution to the extragalactic diffuse γ-ray background (EGRB) in the EGRET energy range. Our model is based on inverse-Compton scattering as the dominant γ-ray production process in the jets of flat spectrum radio quasars (FSRQs) and BL Lac objects, and on the unification scheme of radio-loud AGN. According to this picture, blazars represent the beamed fraction of the Fanaroff–Riley radio galaxies (FR galaxies). The observed log N–log S distribution and redshift distribution of both FSRQs and BL Lacs constrain our model. Depending slightly on the evolutionary behaviour of blazars, we find that unresolved AGN underproduce the intensity of the extragalactic background radiation. With our model only 20–40 per cent of the extragalactic background emission can be explained by unresolved blazars if we integrate to a maximum redshift of Zmax=3. For Zmax=5, blazars could account for 40–80 per cent of the EGRB. Roughly 70–90 per cent of the AGN contribution to the EGRB would result from BL Lacs. While the systematic uncertainties in our estimate for the FSRQ contribution appear small, in the case of BL Lacs our model parameters are not consistent with the results from studies in other wavelength regimes, and therefore may have larger systematic uncertainties. Thus we end up with two possibilities, depending on whether we underpredict or overpredict the BL Lac contribution: either unresolved AGN cannot account for the entire EGRB, or unresolved BL Lacs produce the observed background. We predict a significant flattening of the γ-ray log N–log S function in the next two decades of flux below the EGRET threshold.

Magnetohydrodynamic Origin of Jets from Accretion Disks

A review is made of recent magnetohydrodynamic (MHD) theory and simulations of origin of jets from accretion disks. Many compact astrophysical objects emit powerful, highly-collimated, oppositely directed jets. Included are the extra galactic radio jets of active galaxies and quasars, and old compact stars in binaries, and emission line jets in young stellar objects. It is widely thought that these different jets arise from rotating, conducting accretion disks threaded by an ordered magnetic field. The twisting of the B field by the rotation of the disk drives the jets by magnetically extracting matter, angular momentum, and energy from the accretion disk. Two main regimes have been discussed theoretically, hydromagnetic winds which have a significant mass flux, and Poynting flux jets where the mass flux is negligible. Over the past several years, exciting new developments on models of jets have come from progress in MHD simulations which now allow the study of the origin -the acceleration and collimation - of jets from accretion disks. Simulation studies in the hydromagnetic wind regime indicate that the outflows are accelerated close to their region of origin whereas the collimation occurs at much larger distances.

On the spin paradigm and the radio dichotomy of quasars

We investigate whether models based on the assumption that jets in quasars are powered by rotating black holes can explain the observed radio dichotomy of quasars. We show that in terms of the ‘spin paradigm’ models, radio-loud quasars could be objects in which the rotation rate of the black hole corresponds to an equilibrium between spin-up by accretion and spin-down by the Blandford-Znajek mechanism. Radio-quiet quasars could be hosting black holes with an average spin much smaller than the equilibrium one. We discuss possible accretion scenarios which can lead to such a bimodal distribution of black hole spins.

How fast are the large-scale jets in quasars? Constraints on both Doppler beaming and intrinsic asymmetries

We analyse the jet-to-counterjet flux ratios measured from deep VLA images of 13 3CR quasars made by Bridle et aI., in order to derive constraints on the degree of Doppler boosting (and hence the characteristic jet speeds on kiloparsec scales) and on how much intrinsic asymmetry between jets and counterjets is permitted. The main result is that the large jet-counterjet brightness ratios measured by Bridle et al. must be attributed almost entirely to the Doppler effect, requiring characteristic jet speeds ? 0.6c on kiloparsec scales. Modest intrinsic asymmetries are permitted, but do not reduce the characteristic speed significantly, while large asymmetries are ruled out by VLBI data. The data also require that orientations close to the plane of the sky are excluded (e max < 75°), so at least some quasars masquerade as galaxies. For e max in the range 45° to 60°, as suggested in certain unified schemes, the best­ fitting jet speeds are in the range 0.6c to 0.7c. Analysis of jet-to-Iobe ratios for a complete sample of 3CR quasars leads to an upper limit on jet speed of < 0.95c, demonstrating that there is at least some deceleration between parsec and kiloparsec scales.

EVN + MERLIN Observations of Radio-intermediate Quasars: Evidence for Boosted Radio-weak Quasars

We present VLBI (EVN + MERLIN) observations of a sample of three low-redshift radio-intermediate PG quasars (RIQ) with flat and variable radio spectrum (III Zw 2, PG 1309+355, PG 2209+184). Their radio-to-optical flux ratio (R) is slightly lower than the average R for steep-spectrum quasars, but their radio spectral properties are those of core-dominated quasars. It was proposed previously that these sources might be relativistically boosted jets in radio-weak quasars. Our VLBI observations now indeed confirm the presence of a high brightness temperature core in all three of these objects—two of them have lower limits on TB well in excess of 1010 K. Moreover, we find no missing flux, which means that basically all the flux of these quasars is concentrated in the compact radio core. As the total radio flux is already at the low end for radio-loud quasars, we can place a strong limit on the presence of any extended emission. This limit is consistent with the extended emission in radio-weak quasars but excludes that the flat-spectrum RIQ reside in typical radio-loud quasars. The observations therefore strongly support the idea that relativistic jets are present in radio-weak quasars and, hence, that radio-loud and radio-weak quasars have very similar central engines.

Limits on the chirality of interstellar and intergalactic space

We raise the question of whether velocities of left and right circularly-polarized photons might be different (for reasons other than the well-known Faraday effect). Such a difference could manifest itself either in the time profiles of pulsed or bursting astronomical sources or in the rotation of the direction of polarization of linearly polarized radiation from them. The existing observations of pulsars, gamma ray bursters, and quasar jets are used to set limits to the difference in speed, |c(L) - c(R)|/ c between 10 -17 and 10 -32 .

Merlin and HST Observations of the Jet in 3C273

The jet of 3C273 is the only quasar jet visible at both radio and optical wavelengths. Investigations using ground-based observations suggest that the optical emission, like the radio, is produced by the synchrotron process. We present here a new radio map at 18 cm wavelength made with the improved MERLIN array (Fig. 1b) and compare it with a new optical image obtained with WFPC2 on the Hubble Space Telescope (Fig. 1a) This work has been carried out in collaboration with colleagues from Princeton, Pennsylvania State University and Caltech and is being published by Bancall et al. (1995).

A Tentative Sketch for BL Lacertae Objects

VLBI polarization data show magnetic field configuration parallel to the nuclear jet in quasars and perpendicular to it in BL Lac objects. It appears difficult to account for this contrast within the unified scheme for AGN. To investigate a direct explanation of this peculiarity of BL Lac objects, we study the possibilities of propagation and radiation of beams of particles in transverse ambient magnetic field. High energy streams with kinetic energy density larger than the ambient magnetic one, Ekin &gt; Emag, can easily propagate with enhancement of the transverse magnetic field at the leading edge of the stream and reconnection of magnetic lines in its wake. Synchrotron radiation in front shocks naturally leads to the observed polarization. Moreover self-polarization, with formation of charge layers and E × B drift velocity, allows substantial propagation for even lower energy streams with Ekin &lt; Emag, as long as their density no is large enough, typically κ = 4πnomic2/B2 &gt; 1. Such low energy streams are non diamagnetic and do not modify the ambient field. Any high energy tail of the total particle distribution in the jet therefore radiates in the transverse field pattern. This concerns for instance the BL Lac object W Comae if we assume a proton-electron jet with bulk velocity vo = 0.1c (as the source does not require relativistic beaming so far), an equipartition magnetic field B = 0.02 G and a density of radiating particles nr = 0.05cm−3 at about 7 pc from the nucleus (knot K3). For nr/no = 10−3, one gets the stream density no = 50cm−3 which allows good propagation as κ reaches 2 × 103, and still corresponds to a moderate mass outflow of 0.06 M⊙ /year for a VLBI jet cross-sectional area of 2pc2.

Structure and Emission of Parsec-Scale Jets in Quasars

The moving knots observed in VLBI of compact quasars are due to off-axis substructures in the parsec-scale jets which are dragged along by the underlying plasma flow. This is suggested by detailed MHD models for the structure of relativistic jets on the parsec–scale that are based on exact solutions of the nonlinear Grad–Schlüter–Shafranov equation in the asymptotic domain. These jet models are characterized by a current–carrying core and a current–free envelope. The corresponding core–radius Rc = upRL is related to the poloidal jet velocity up and the light cylinder radius RL generated by the rotation of the magnetic surfaces. The Poynting flux, which has an inhomogeneous distribution in the jet, provides an off–axis energy reservoir that could be tapped and converted into particle acceleration by non-axisymmetric instabilities in the core–jet structure. Evidence is presented that quasi-periodic synchrotron emission (in the IR–optical) and inverse Compton emission (in X– and γ–rays) from freshly accelerated electrons are due to a lighthouse effect of the rotating knots.

Extragalactic Radio Sources and the Role of Relativistic Jets

This paper summarises some of the ideas surrounding the role of relativistic jets in radio galaxies and quasars and describes work presented in two recent papers (Bicknell 1994a,b) relating relativistic jets to the Fanaroff~Riley classification of radio galaxies. I conclude with some speculation on the evolutionary connection between Fanaroff~Riley Class I and Class II radio galaxies and the relationship between mergers and radio galaxies, an idea which was discussed at the time of the discovery of Cygnus A and Ccntaurus A.

Viscous shear in general-relativistic astrophysical flows

Introducing the concept of general co-moving frames (gcmf) in [4] we have argued that it may become useful in a number of hydrodynamical and mhd applications. In [5] using thegcmf technique we have constructed a fully covariant, general-relativistic theory of strongly magnetized collisionless plasma. The approach proved itself to be highly convenient-it allowed us to find new equations of state for such a medium. In the present paper we have considered viscous shear in generalrelativistic astrophysical flows as an another example of the effective usage of orthonormal tetrads method. Namely, we have specified general corotating frames (gcrf)-subclass ofgcmf corresponding to the flows being in purely rotational motion. By means of gcrf we have been able to find expressions for nonzero components of shear tensor and turbulent viscosity tensor for the innermost region of a black hole accretion disc. We think that the method may be useful when considering analogous problems with astrophysical flows of more complicated geometry and/or dynamics. In particular, the method may become efficient for jets in active galactic nuclei (agn) and quasars [13], general-relativistic winds of compact objects [14] and the innermost regions of candidates for galactic black hole accretion discs [15]. To be sure, in some of these problems we have to use a more general set ofgcmf instead ofgcrf. Such problems, however, are beyond the scope of this paper, where we have only outlined the main background of the method and demonstrated its productivity in a simple case of quasi-keplerian accretion flow in a general-relativistic standard accretion disc.

Milliarcsecond Polarization Structure of 24 Objects from the Pearson-Readhead Sample of Bright Extragalactic Radio Sources. II. Discussion

view Abstract Citations (109) References (59) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Milliarcsecond Polarization Structure of 24 Objects from the Pearson-Readhead Sample of Bright Extragalactic Radio Sources. II. Discussion Cawthorne, T. V. ; Wardle, J. F. C. ; Roberts, D. H. ; Gabuzda, D. C. Abstract The observations (Paper I) of the milliarcsecond-scale linear polarization properties of compact extragalactic radio sources from the Pearson-Readhead sample are discussed. In general, the linear polarization properties correlate strongly with optical identification, and, for the sources in this sample, galaxies, quasars, and BL Lacertae objects are quite distinct. None of the compact radio sources identified with galaxies was detected in polarization on milliarcsecond scales. In contrast, most quasars and BL Lacertae objects have polarized structures, especially if they have prominent jets. We find systematic differences between quasars and BL Lacertae objects in the core polarizations, the orientations of the inferred magnetic fields in the jets, the visible lengths of the jets, and the behavior of the run of fractional polarization along the jets. Many of these differences may be caused by differences in the parsec-scale environment in quasars and BL Lacertae objects, such as in the density of the line-emitting gas. In BL Lacertae objects the inferred magnetic fields in the milliarcsecond scale jets are perpendicular to the jet axes, while observations of the best resolved quasars indicate that the fields are often quite closely parallel to the jet axes. This shows that BL Lacertae objects are not simply quasars whose jets make a particularly small angle to the line of sight, nor are they microlensed images of more distant quasars. There is also an anticorrelation between the degree of polarization and the total intensity of quasar jet components. Together, these results suggest a picture in which the knots of emission in the jets are associated with transverse shocks acting on a jet containing a mainly tangled magnetic field. In quasars the jets also contain a significant longitudinal component whose relative strength increases with distance from the core. If this is the case, the shocks in quasars must be weak so that the transverse field amplified by the shock never dominates the polarization. Our observations suggest that, in general, the magnetic field orientations in the milliarcsecond jets continue to scales intermediate between those resolved by VLBI and those accessible to the VLA. Publication: The Astrophysical Journal Pub Date: October 1993 DOI: 10.1086/173254 Bibcode: 1993ApJ...416..519C Keywords: GALAXIES: BL LACERTAE OBJECTS: GENERAL; GALAXIES: JETS; GALAXIES: MAGNETIC FIELDS; GALAXIES: QUASARS: GENERAL; POLARIZATION full text sources ADS | data products NED (46) SIMBAD (45) Related Materials (1) Part 1: 1993ApJ...416..496C

Linear polarization of radiation from oblique and conical shocks

A simple model for synchrotron radio emission from oblique and conical shock waves is described, and the polarization properties are examined, assuming that the magnetic field in plasma upstream from the shock is random in direction. Such structures have been suggested as the origin of radio emission from the knots in radio jets that emerge from active nuclei, particularly on VLBI scales. It is found that conical shock waves in jets of circular cross section can yield polarized radiation with electric vector either parallel or perpendicular to the jet axis. The results provide insight into the extent to which conical shock waves might account for the polarization structures of jets in BL Lac objects and quasars that have been revealed by recent VLBI polarimetry. 13 refs.

Vertical Magnetic Fields in Spiral Galaxies

Evolution of a vertical magnetic field which may have existed in the primeval galaxy is discussed based on a primordial-origin hypothesis for the bisymmetric spiral configuration of magnetic fields in spiral galaxies. The vertical field is accumulated toward the nucleus and forms a strong poloidal field, which may trigger activities like jets. Quasar jets are suggested to be the result of such strong vertical fields in the cores of primeval galaxies.