On the local magnetorotational instability of astrophysical jets

We present the local linear stability analysis of rotating jets confined by a toroidal magnetic field. Under the thin flux tube approximation, we derive the equation of motion for slender magnetic flux tubes. In addition to the terms responsible for the conventional instability of the toroidal magnetic field, a term related to the magnetic buoyancy and a term corresponding to the differential rotation become relevant for the stability properties. We find that the rigid rotation stabilizes while the differential rotational destabilizes the jet in a way similar to the Balbus–Hawley instability. Within the frame of our local analysis, we find that if the azimuthal velocity is of the order of or higher than the Alfvén azimuthal speed, the rigidly rotating part of the jet interior can be completely stabilized, while the strong shearing instability operates in the transition layer between the rotating jet interior and the external medium. This can explain the limb-brightening effect observed in several jets. However, it is still possible to find jet equilibria that are stable all across the jet, even in the presence of differential rotation. We discuss observational consequences of these results.     

Azimuthally symmetric magnetohydrodynamic and two-fluid equilibria with arbitrary flows

Magnetohydrodynamic (MHD) and two-fluid quasi-neutral equilibria with azimuthal symmetry, gravity and arbitrary ratios of (non-relativistic) flow speed to acoustic and Alfvén speeds are investigated. In the two-fluid case, the mass ratio of the two species is arbitrary, and the analysis is therefore applicable to electron–positron plasmas. The methods of derivation can be extended in an obvious manner to several charged species. Generalized Grad–Shafranov equations, describing the equilibrium magnetic field, are derived. Flux-function equations and Bernoulli relations for each species, together with Poisson's equation for the gravitational potential, complete the set of equations required to determine the equilibrium. These are straightforward to solve numerically. The two-fluid system, unlike the MHD system, is shown to be free of singularities. It is demonstrated analytically that there exists a class of incompressible MHD equilibria with magnetic field-aligned flow. A special subclass first identified by S. Chandrasekhar, in which the flow speed is everywhere equal to the local Alfvén speed, is compatible with virtually any azimuthally symmetric magnetic configuration. Potential applications of this analysis include extragalactic and stellar jets, accretion discs, and plasma structures associated with active late-type stars.     

A diffuse bubble-like radio-halo source MRC 0116+111: imprint of AGN feedback in a low-mass cluster of galaxies

We present detailed observations of MRC 0116+111, revealing a luminous, miniradio halo of ∼240-kpc diameter located at the centre of a cluster of galaxies at redshift   z = 0.131  . Our optical and multiwavelength Giant Metrewave Radio Telescope and Very Large Array radio observations reveal a highly unusual radio source: showing a pair of giant (∼100-kpc diameter) bubble-like diffuse structures, that are about three times larger than the analogous extended radio emission observed in M87 – the dominant central radio galaxy in the Virgo cluster. However, in MRC 0116+111 we do not detect any ongoing active galactic nucleus (AGN) activity, such as a compact core or active radio jets feeding the plasma bubbles. The radio emitting relativistic particles and magnetic fields were probably seeded in the past by a pair of radio jets originating in the AGN of the central cD galaxy. The extremely steep high-frequency radio spectrum of the north-western bubble, located ∼100 kpc from cluster centre, indicates radiation losses, possibly because having detached, it is rising buoyantly and moving away into the putative hot intracluster medium. The other bubble, closer to the cluster centre, shows signs of ongoing particle re-acceleration. We estimate that the radio jets which inflated these two bubbles might have also fed enough energy into the intracluster medium to create an enormous system of cavities and shock fronts, and to drive a massive outflow from the AGN, which could counter-balance and even quench a cooling flow. Therefore, this source presents an excellent opportunity to understand the energetics and the dynamical evolution of radio jet inflated plasma bubbles in the hot cluster atmosphere.     

Buoyant radio plasma in clusters of galaxies

Radio galaxies are known to inflate lobes of hot relativistic plasmas into the intergalactic medium. Here we present hydrodynamical and magnetohydrodynamical simulations of these hot plasma bubbles in FR II objects. We focus on the later stages of their evolution after the jet has died down and after the bow shock that surrounded the lobes at earlier stages has vanished. We investigate the evolution of the plasma bubbles as they become subject to Rayleigh–Taylor instabilities. From our simulations we calculate the radio and X-ray emissivities of the bubbles and discuss their appearance in observations. Finally, we investigate the influence of large-scale magnetic fields on the evolution of the bubbles. The issues of re-acceleration and diffusion of relativistic particles are briefly discussed.     

The radio galaxy Centaurus B

Centaurus B (PKS B1343−601) is one of the brightest and closest radio galaxies, with flux density ∼250 Jy at 408 MHz and redshift 0.01215, but it has not been studied much because of its position (i) close to the Galactic plane (it is also known as G309.6+1.7 and Kes 19) and (ii) in the southern sky. It has recently been suggested as the centre of a highly obscured cluster behind the Galactic plane. We present radio observations made with the Australia Telescope Compact Array and Molonglo Observatory Synthesis Telescope to study the jets and lobes. The total intensity and polarization radio images of the FR I jets are used to determine the jet brightness and width variations, magnetic field structure and fractional polarization. The equipartition pressure calculated along the jets declines rapidly over the first 1 arcmin from the galaxy reaching a constant pressure of 10⁻¹³  h ^−4/7 Pa in the lobes blown in the intracluster medium.     

Multifrequency observations of the jets in the radio galaxy NGC 315

We present images of the jets in the nearby radio galaxy NGC 315 made with the Very Large Array at five frequencies between 1.365 and 5 GHz with resolutions between 1.5 and 45 arcsec. Within 15 arcsec of the nucleus, the spectral index of the jets is  α= 0.61  . Further from the nucleus, the spectrum is flatter, with significant transverse structure. Between 15 and 70 arcsec from the nucleus, the spectral index varies from ≈0.55 on-axis to ≈0.44 at the edge. This spectral structure suggests a change of dominant particle acceleration mechanism with distance from the nucleus and the transverse gradient may be associated with shear in the jet velocity field. Further from the nucleus, the spectral index has a constant value of 0.47. We derive the distribution of Faraday rotation over the inner ±400 arcsec of the radio source and show that it has three components: a constant term, a linear gradient (both probably due to our Galaxy) and residual fluctuations at the level of 1–2 rad m⁻². These residual fluctuations are smaller in the brighter (approaching) jet, consistent with the idea that they are produced by magnetic fields in a halo of hot plasma that surrounds the radio source. We model this halo, deriving a core radius of ≈225 arcsec and constraining its central density and magnetic field strength. We also image the apparent magnetic field structure over the first ±200 arcsec from the nucleus.     

The X-ray jets of active galaxies

Jet physics is again flourishing as a result of Chandra’s ability to resolve high-energy emission from the radio-emitting structures of active galaxies and separate it from the X-ray-emitting thermal environments of the jets. These enhanced capabilities have coincided with an increasing interest in the link between the growth of super-massive black holes and galaxies, and an appreciation of the likely importance of jets in feedback processes. I review the progress that has been made using Chandra and XMM-Newton observations of jets and the medium in which they propagate, addressing several important questions, including: Are the radio structures in a state of minimum energy? Do powerful large-scale jets have fast spinal speeds? What keeps jets collimated? Where and how does particle acceleration occur? What is jet plasma made of? What does X-ray emission tell us about the dynamics and energetics of radio plasma/gas interactions? Is a jet’s fate determined by the central engine?     

The hydrodynamics of dead radio galaxies

We present a numerical investigation of dead, or relic, radio galaxies and the environmental impact that radio galaxy activity has on the host galaxy or galaxy cluster. We perform axisymmetric hydrodynamical calculations of light, supersonic, back-to-back jets propagating in a β -model galaxy/cluster atmosphere. We then shut down the jet activity and let the resulting structure evolve passively. The dead source undergoes an initial phase of pressure driven expansion until it achieves pressure equilibrium with its surroundings. Thereafter, buoyancy forces drive the evolution and lead to the formation of two oppositely directed plumes that float high into the galaxy/cluster atmosphere. These plumes entrain a significant amount of low entropy material from the galaxy/cluster core and lift it high into the atmosphere. An important result is that a large fraction (at least half) of the energy injected by the jet activity is thermalized in the interstellar medium (ISM)/intracluster medium (ICM) core. The whole ISM/ICM atmosphere inflates in order to regain hydrostatic equilibrium. This inflation is mediated by an approximately spherical disturbance which propagates into the atmosphere at the sound speed. The fact that such a large fraction of the injected energy is thermalized suggests that radio galaxies may have an important role in the overall energy budget of rich ISM/ICM atmospheres. In particular, they may act as a strong and highly time-dependent source of negative feedback for galaxy/cluster cooling flows.     

Jets in the Fanaroff–Riley class I radio galaxies PKS B1234−723, MRC 1452−517 and PKS B2148−555

We present radio observations made with the Australia Telescope Compact Array to study the jets and lobes of three Fanaroff–Riley class I (FR I) radio galaxies: PKS B1234−723, 1452−517 and B2148−555. The total intensity and polarization radio images of the FR I jets are used to determine jet brightness and width variations, magnetic field structure and fractional polarization. The equipartition pressure is determined as a function of distance from the galaxies to probe the intergalactic medium.     

The bulk kinetic power of radio jets in active galactic nuclei

Based on the Königl's inhomogeneous jet model, we estimate the jet parameters, such as bulk Lorentz factor Γ, viewing angle θ and electron number density n _e from radio very long-baseline interferometry and X-ray data for a sample of active galactic nuclei (AGNs) assuming that the X-rays are from the jet rather than the intracluster gas. The bulk kinetic power of jets is then calculated using the derived jet parameters. We find a strong correlation between the total luminosity of broad emission lines and the bulk kinetic power of the jets. This result supports the scenario that the accretion process is tightly linked with the radio jets, though how the disc and jet are coupled is not revealed by present correlation analysis. Moreover, we find a significant correlation between the bulk kinetic power and radio extended luminosity. This implies that the emission from the radio lobes is closely related with the energy flux transported through jets from the central part of AGNs.     

Dynamic confinement of jets by magnetotorsional oscillations

Many quasars and active galactic nuclei (AGN) appear in radio, optical and X-ray maps as bright nuclear sources from which emerge single or double long, thin jets. When observed with high angular resolution, these jets show evidence of structure, with bright knots separated by relatively dark regions. High percentages of polarization, sometimes more then 50 per cent, indicate the non-thermal nature of the radiation, which is well explained as the synchrotron radiation of the relativistic electrons in an ordered magnetic field.
A strong collimation of jets is probably connected with ordered magnetic fields. The mechanism of magnetic collimation first suggested by Bisnovatyi-Kogan et al. was based on the initial charge separation, which led to the creation of an oscillating electrical current, which in turn produced an azimuthal magnetic field, preventing jet expansion and disappearance. Here we consider magnetic collimation associated with the torsional oscillations of a cylinder with an elongated magnetic field. Instead of initial blobs with charge separation, we consider a cylinder with a periodically distributed initial rotation around the cylinder axis. The stabilizing azimuthal magnetic field is created by torsional oscillations, meaning that charge separation is unnecessary. An approximate simplified model is developed, and an ordinary differential equation is derived and solved numerically, making it possible to estimate quantitatively the range of parameters for which jets may be stabilized by torsional oscillations.     

A Chandra and XMM–Newton study of the wide-angle tail radio galaxy 3C 465

We have observed the prototypical wide-angle tail (WAT) radio galaxy 3C 465 with Chandra and XMM–Newton . X-ray emission is detected from the active nucleus and the inner radio jet, as well as a small-scale, cool component of thermal emission, a number of the individual galaxies of the host cluster (Abell 2634), and the hotter thermal emission from the cluster itself. The X-ray detection of the jet allows us to argue that synchrotron emission may be an important mechanism in other well-collimated, fast jets, including those of classical double radio sources. The bases of the radio plumes are not detected in the X-ray, which supports the model in which these plumes are physically different from the twin jets of lower-power radio galaxies. The plumes are in fact spatially coincident with deficits of X-ray emission on large scales, which argues that they contain little thermal material at the cluster temperature, although the minimum pressures throughout the source are lower than the external pressures estimated from the observed thermal emission. Our observations confirm both spatially and spectrally that a component of dense, cool gas with a short cooling time is associated with the central galaxy. However, there is no evidence for the kind of discontinuity in external properties that would be required in many models of the jet–plume transition in WATs. Although the WAT jet–plume transition appears likely to be related to the interface between this central cool component and the hotter intracluster medium, the mechanism for WAT formation remains unclear. We revisit the question of the bending of WAT plumes, and show that the plumes can be bent by plausible bulk motions of the intracluster medium, or by motion of the host galaxy with respect to the cluster, as long as the plumes are light.     

Multiwavelength study of Cygnus A – I. Precession and slow jet speeds from radio observations

We study the jet and counterjet of the powerful classical double Fanaroff–Riley type II (FR II) radio galaxy Cygnus A as seen in the 5-, 8- and 15-GHz radio bands using the highest spatial resolution and signal-to-noise ratio archival data available. We demonstrate that the trace of the radio knots that delineate the jet and counterjet deviates from a straight line and that the inner parts can be satisfactorily fitted with the precession model of Hjellming & Johnston. The parameter values of the precession model fits are all plausible although the jet speed is rather low (≲ 0.5 c ) but, on investigation, found to be consistent with a number of other independent estimates of the jet speed in Cygnus A. We compare the masses and precession periods for sources with known precession and find that for the small number of active galactic nuclei with precessing jets the precession periods are significantly longer than those of microquasars.     

Models for jet power in elliptical galaxies: a case for rapidly spinning black holes

The power of jets from black holes is expected to depend on both the spin of the black hole and the structure of the accretion disc in the region of the last stable orbit. We investigate these dependencies using two different physical models for the jet power: the classical Blandford–Znajek (BZ) model and a hybrid model developed by Meier. In the BZ case, the jets are powered by magnetic fields directly threading the spinning black hole while in the hybrid model, the jet energy is extracted from both the accretion disc as well as the black hole via magnetic fields anchored to the accretion flow inside and outside the hole's ergosphere. The hybrid model takes advantage of the strengths of both the Blandford–Payne and BZ mechanisms, while avoiding the more controversial features of the latter. We develop these models more fully to account for general relativistic effects and to focus on advection-dominated accretion flows (ADAFs) for which the jet power is expected to be a significant fraction of the accreted rest mass energy.
We apply the models to elliptical galaxies, in order to see if these models can explain the observed correlation between the Bondi accretion rates and the total jet powers. For typical values of the disc viscosity parameter  α∼ 0.04 –0.3  and mass accretion rates consistent with ADAF model expectations, we find that the observed correlation requires   j ≳ 0.9  ; that is, it implies that the black holes are rapidly spinning. Our results suggest that the central black holes in the cores of clusters of galaxies must be rapidly rotating in order to drive jets powerful enough to heat the intracluster medium and quench cooling flows.     

Chandra finds that X-ray jets are common in low-power radio galaxies

We present results for the first three low-power radio galaxies from the B2 bright sample to have been observed with Chandra . Two have kiloparsec-scale radio jets, and in both Chandra resolves jet X-ray emission, and detects soft X-ray core emission and an X-ray-emitting galaxy-scale atmosphere of luminosity a few ×10⁴¹ erg s⁻¹. These are the first detections of X-ray jets in low-power radio galaxies more distant than Centaurus A and M87. The cooling time of the galaxy-scale gas implies mass infall rates of the order of 1 M_⊙ yr⁻¹. The gas pressure near the jets is comparable to the minimum pressure in the jets, implying that the X-ray-emitting gas may play an important role in jet dynamics. The third B2 radio galaxy has no kiloparsec-scale radio jet, and here only soft X-ray emission from the core is detected. The ratio of X-ray to radio flux is similar for the jets and cores, and the results favour a synchrotron origin for the emission. Kiloparsec-scale radio jets are detected in the X-ray in ∼7-ks exposures with Chandra more readily than in the optical via Hubble Space Telescope snapshot surveys.     

The infrared jet in 3C 66B

We present images of infrared (IR) emission from the radio jet in 3C 66B. Data at three wavelengths (4.5, 6.75 and 14.5 μm) were obtained using the Infrared Space Observatory . The 6.75-μm image clearly shows an extension aligned with the radio structure. The jet was also detected in the 14.5-μm image, but not at 4.5 μm. The radio–infrared–optical spectrum of the jet can be interpreted as synchrotron emission from a population of electrons with a high-energy break of 4×10¹¹ eV. We place upper limits on the IR flux from the radio counter-jet. A symmetrical, relativistically beamed twin-jet structure is consistent with our results if the jets consist of multiple components.     

X-ray synchrotron emission from the oblique shock in the jet of the powerful radio galaxy 3C 346

We report the first detection, with Chandra , of X-ray emission from the jet of the powerful narrow-line radio galaxy 3C 346. X-rays are detected from the bright radio and optical knot at which the jet apparently bends by approximately 70°. The Chandra observation also reveals a bright galaxy-scale atmosphere within the previously known cluster and provides a good X-ray spectrum for the bright core of 3C 346. The X-ray emission from the knot is synchrotron radiation, as seen in lower-power sources. In common with these sources, there is evidence of morphological differences between the radio/optical and X-ray structures, and the spectrum is inconsistent with a one-component continuous-injection model. We suggest that the X-ray-bright knot is associated with a strong oblique shock in a moderately relativistic, light jet, at ∼ 20° to the line of sight, and that this shock is caused by the jet interacting with the wake in the cluster medium behind the companion galaxy of 3C 346. The general jet curvature can result from pressure gradients in the cluster atmosphere.     

An Inhomogeneous Jet Model and Its Application in BL Lacertae Objects

In the inhomogeneous conical jet model, the electron number den- sity and magnetic ?eld strength have a power-law distribution with the distance from the jet apex. This model can interpret successfully the ?at-spectrum radio radiation from the core regions of active galactic nuclei. But the existing model calculation suits only the situation when the enclosed angle between the moving direction of a jet and the line of sight is very large, hence, we need to build a formula for calculating the radiation of inhomogeneous conical jets with any viewing angles. It is generally believed that the enclosed angle between the di- rection of jet motion and the line of sight is very small in BL Lac objects. With the extended inhomogeneous jet model, we have made ?tting on the observed radio spectra of three BL Lac objects, and obtained the physical parameters, such as the electron number density and magnetic ?eld strength in their jets. The result indicates that the nearest distance of the conical jet from the black hole is determined by the transition frequency of the observed radiation spec- trum, and that the nearest distance between the conical jet and the black hole is approximately the Schwarzschild radius for the three BL Lac objects.     

Interactions of radio galaxies and the intracluster medium in Abell 160 and Abell 2462

We present Chandra and Very Large Array observations of two galaxy clusters, Abell 160 and Abell 2462, whose brightest cluster galaxies (BCGs) host wide angle tailed radio galaxies (WATs). We search for evidence of interactions between the radio emission and the hot, X-ray emitting gas, and we test various jet termination models. We find that both clusters have cool BCGs at the cluster centre, and that the scale of these cores (∼30–40 kpc for both sources) is of approximately the same scale as the length of the radio jets. For both sources, the jet flaring point is coincident with a steepening in the host cluster's temperature gradient, and similar results are found for 3C 465 and Hydra A. However, none of the published models of WAT formation offers a satisfactory explanation as to why this may be the case. Therefore, it is unclear what causes the sudden transition between the jet and the plume. Without accurate modelling, we cannot ascertain whether the steepening of the temperature gradient is the main cause of the transition, or merely a tracer of an underlying process.