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The prominent radio source Hercules A features complex structures in its radio lobes. Although it is one of the most comprehensively studied sources in the radio sky, the origin of the ring structures in the Hercules A radio lobes remains an open question. We present the first sub-arcsecond angular resolution images at low frequencies (<300 MHz) of Hercules A, made with the International LOFAR Telescope. With the addition of data from the Karl G. Jansky Very Large Array, we mapped the structure of the lobes from 144 MHz to 7 GHz. We explore the origin of the rings within the lobes of Hercules A, and test whether their properties are best described by a shock model, where shock waves are produced by the jet propagating in the radio lobe, or by an inner-lobe model, where the rings are formed by decelerated jetted plasma. From spectral index mapping our large frequency coverage reveals that the curvature of the different ring spectra increases with distance away from the central active galactic nucleus. We demonstrate that the spectral shape of the rings is consistent with synchrotron aging, which speaks in favor of an inner-lobe model where the rings are formed from the deposition of material from past periods of intermittent core activity., Comment: Accepted for a special issue of A&A on sub-arcsecond imaging with LOFAR

We model the kiloparsec-scale synchrotron emission from jets in 10 Fanaroff-Riley Class I radio galaxies for which we have sensitive, high-resolution imaging and polarimetry from the Very Large Array. We assume that the jets are intrinsically symmetrical, axisymmetric, decelerating, relativistic outflows and we infer their inclination angles and the spatial variations of their flow velocities, magnetic field structures and emissivities using a common set of fitting functions. The inferred inclinations agree well with independent indicators. The spreading rates increase rapidly, then decrease, in a flaring region. The jets then recollimate to form conical outer regions at distance r0 from the active galactic nucleus (AGN). The flaring regions are homologous when scaled by r0. At ~0.1r0, the jets brighten abruptly at the onset of a high-emissivity region and we find an outflow speed of ~0.8c, with a uniform transverse profile. Jet deceleration first becomes detectable at ~0.2r0 and the outflow often becomes slower at its edges than it is on-axis. Deceleration continues until ~0.6r0, after which the outflow speed is usually constant. The dominant magnetic-field component is longitudinal close to the AGN and toroidal after recollimation, but the field evolution is initially much slower than predicted by flux-freezing. In the flaring region, acceleration of ultrarelativistic particles is required to counterbalance the effects of adiabatic losses and account for observed X-ray synchrotron emission, but the brightness evolution of the outer jets is consistent with adiabatic losses alone. We interpret our results as effects of the interaction between the jets and their surroundings. (Slightly abridged)., Comment: 56 pages, 27 figures, MNRAS, in press. The full text and appendices are included in a single file. The appendices will appear as supplementary material in the version published on-line. Minor changes to match published version and correct typographical errors

We present accurate, spatially resolved imaging of radio spectra at the bases of jets in eleven low-luminosity (Fanaroff-Riley I) radio galaxies, derived from Very Large Array (VLA) observations. We pay careful attention to calibration and to the effects of random and systematic errors, and we base the flux-density scale on recent measurements of VLA primary amplitude calibrators by Perley & Butler (2013). We show images and profiles of spectral index over the frequency range 1.4 - 8.5 GHz, together with values integrated over fiducial regions defined by our relativistic models of the jets. We find that the spectral index decreases (the spectrum flattens) with distance from the nucleus in all of the jets. The mean spectral indices are 0.66 +/- 0.01 where the jets first brighten abruptly and 0.59 +/- 0.01 after they recollimate. The mean change in spectral index between these locations, which is independent of calibration and flux-density scale errors and is therefore more accurately and robustly measured, is -0.067 +/- 0.006. Our jet models associate the decrease in spectral index with a bulk deceleration of the flow from 0.8c to <0.5c. We suggest that the decrease is the result of a change in the characteristics of ongoing particle acceleration. One possible acceleration mechanism is the first-order Fermi process in mildly relativistic shocks: in the Bohm limit, the index of the electron energy spectrum, p, is slightly larger than 2 and decreases with velocity upstream of the shock. This possibility is consistent with our measurements, but requires shocks throughout the jet volume rather than at a few discrete locations. A second possibility is that two acceleration mechanisms operate in these jets: one (with p = 2.32) dominant close to the galactic nucleus and associated with high flow speeds, another (with p = 2.18) taking over at larger distances and slower flow speeds., Comment: 21 pages, 9 figures, MNRAS, in press

We show that asymmetries in total intensity and linear polarization between the radio jets and counter-jets in two lobed Fanaroff-Riley Class I (FR I) radio galaxies, B2 0206+35 (UGC 1651) and B2 0755+37 (NGC 2484), can be accounted for if these jets are intrinsically symmetrical, with decelerating relativistic outflows surrounded by mildly relativistic backflows. Our interpretation is motivated by sensitive, well-resolved Very Large Array imaging which shows that both jets in both sources have a two-component structure transverse to their axes. Close to the jet axis, a centrally-darkened counter-jet lies opposite a centrally-brightened jet, but both are surrounded by broader collimated emission that is brighter on the counter-jet side. We have adapted our previous models of FR I jets as relativistic outflows to include an added component of symmetric backflow. We find that the observed radio emission, after subtracting contributions from the extended lobes, is well described by models in which decelerating outflows with parameters similar to those derived for jets in plumed FR I sources are surrounded by backflows containing predominantly toroidal magnetic fields. These return to within a few kpc of the galaxies with velocities of roughly 0.25c and radiate with a synchrotron spectral index close to 0.55. We discuss whether such backflow is to be expected in lobed FR I sources and suggest ways in which our hypothesis can be tested by further observations., Comment: 22 pages, 17 figures, accepted for publication in MNRAS

We explore the magneto-ionic environment of the isolated radio galaxy B2 0755+37 using detailed imaging of the distributions of Faraday rotation and depolarization over the radio source from Very Large Array observations at 1385,1465 and 4860 MHz and new X-ray data from XMM-Newton. The Rotation Measure (RM) distribution is complex, with evidence for anisotropic fluctuations in two regions. The approaching lobe shows low and uniform RM in an unusual `stripe' along an extension of the jet axis and a linear gradient transverse to this axis over its Northern half. The leading edge of the receding lobe shows arc-like RM structures with sign reversals. Elsewhere, the RM structures are reasonably isotropic. The RM power spectra are well described by cut-off power laws with slopes ranging from 2.1 to 3.2 in different sub-regions. The corresponding magnetic-field autocorrelation lengths, where well-determined, range from 0.25 to 1.4 kpc. It is likely that the fluctuations are mostly produced by compressed gas and field around the leading edges of the lobes. We identify areas of high depolarization around the jets and inner lobes. These could be produced by dense gas immediately surrounding the radio emission containing a magnetic field which is tangled on small scales. We also identify four ways in which the well known depolarization (Faraday depth) asymmetry between jetted and counter-jetted lobes of extended radio sources can be modified by interactions with the surrounding medium., Comment: 16 pages, 13 figures, accepted for publication in MNRAS. Full resolution paper available at: ftp://ftp.ira.inaf.it/pub/outgoing/guidetti/ Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

We present deep, high-resolution imaging of the nearby Fanaroff-Riley Class I (FR I) radio galaxies NGC 193, B2 0206+35, B2 0755+37 and M 84 at frequencies of 4.9 and 1.4 GHz using new and archival multi-configuration observations from the Very Large Array. In addition, we describe lower-resolution observations of B2 0326+39 and a reanalysis of our published images of 3C 296. All of these radio galaxies show twin jets and well-defined lobes or bridges of emission, and we examine the common properties of this class of source. We show detailed images of total intensity, brightness gradient, spectral index, degree of polarization and projected magnetic-field direction. The jet bases are very similar to those in tailed twin-jet sources and show the characteristics of decelerating, relativistic flows. Except on one side of M 84, we find that the jets can be traced at least as far as the ends of the lobes, where they often form structures which we call "caps" with sharp outer brightness gradients. Continuing, but less well collimated flows back into the lobes from the caps can often be identified by their relatively flat spectral indices. The lobes in these radio galaxies are similar in morphology, spectral-index distribution and magnetic-field structure to those in more powerful (FR II) sources, but lack hot-spots or other evidence for strong shocks at the ends of the jets. M 84 may be an intermediate case between lobed and tailed sources, in which one jet does not reach the end of its lobe, but disrupts to form a "bubble"., Comment: 21 pages, 12 figures, accepted for publication in MNRAS

We present detailed imaging of Faraday rotation and depolarization for the radio galaxies 0206+35, 3C 270, 3C 353 and M 84, based on Very Large Array observations at multiple frequencies in the range 1365 to 8440 MHz. This work suggests a more complex picture of the magneto-ionic environments of radio galaxies than was apparent from earlier work. All of the sources show spectacular banded rotation measure (RM) structures with contours of constant RM perpendicular to the major axes of their radio lobes. We give a comprehensive description of the banded RM phenomenon and present an initial attempt to interpret it as a consequence of interactions between the sources and their surroundings. We show that the material responsible for the Faraday rotation is in front of the radio emission and that the bands are likely to be caused by magnetized plasma which has been compressed by the expanding radio lobes. A two-dimensional magnetic structure in which the field lines are a family of ellipses draped around the leading edge of the lobe can produce RM bands in the correct orientation for any source orientation. We also report the first detections of rims of high depolarization at the edges of the inner radio lobes of M 84 and 3C 270. These are spatially coincident with shells of enhanced X-ray surface brightness, in which both the field strength and the thermal gas density are likely to be increased by compression., Comment: 21 pages, 15 figures, accepted for publication in MNRAS. Full resolution paper available at http://www.ira.inaf.it/~guidetti/bands/ Subjects: Astrophysics (astro-ph)

We use high-quality VLA images of the Fanaroff & Riley Class I radio galaxy 3C 31 at six frequencies in the range 1365 to 8440MHz to explore the spatial scale and origin of the rotation measure (RM) fluctuations on the line of sight to the radio source. We analyse the distribution of the degree of polarization to show that the large depolarization asymmetry between the North and South sides of the source seen in earlier work largely disappears as the resolution is increased. We show that the depolarization seen at low resolution results primarily from unresolved gradients in a Faraday screen in front of the synchrotron-emitting plasma. We establish that the residual degree of polarization in the short-wavelength limit should follow a Burn law and we fit such a law to our data to estimate the residual depolarization at high resolution. We show that the observed RM variations over selected areas of 3C 31 are consistent with a power spectrum of magnetic fluctuations in front of 3C 31 whose power-law slope changes significantly on the scales sampled by our data. The power spectrum can only have the form expected for Kolmogorov turbulence on scales <5 kpc. On larger scales we find a flatter slope. We also compare the global variations of RM across 3C 31 with the results of three-dimensional simulations of the magnetic-field fluctuations in the surrounding magnetoionic medium. We show that our data are consistent with a field distribution that favours the plane perpendicular to the jet axis - probably because the radio source has evacuated a large cavity in the surrounding medium. We also apply our analysis techniques to the case of Hydra A, where the shape and the size of the cavities produced by the source in the surrounding medium are known from X-ray data. (Abridged), Comment: 33 pages, 25 figures, accepted for publication in MNRAS

We present high-quality VLA images of the FR I radio galaxy 3C 31 in the frequency range 1365 to 8440 MHz with angular resolutions from 0.25 to 40 arcsec. Our new images reveal complex, well resolved filamentary substructure in the radio jets and tails. We also use these images to explore the spectral structure of 3C 31 on large and small scales. We infer the apparent magnetic field structure by correcting for Faraday rotation. Some of the intensity substructure in the jets is clearly related to structure in their apparent magnetic field: there are arcs of emission where the degree of linear polarization increases, with the apparent magnetic field parallel to the ridges of the arcs. The spectral indices are significantly steeper (0.62) within 7 arcsec of the nucleus than between 7 and 50 arcsec (0.52 - 0.57). The spectra of the jet edges are also slightly flatter than the average for their surroundings. At larger distances, the jets are clearly delimited from surrounding larger-scale emission both by their flatter radio spectra and by sharp brightness gradients. The spectral index of 0.62 in the first 7 arcsec of 3C 31's jets is very close to that found in other FR I galaxies where their jets first brighten in the radio and where X-ray synchrotron emission is most prominent. Farther from the nucleus, where the spectra flatten, X-ray emission is fainter relative to the radio. The brightest X-ray emission from FR I jets is therefore not associated with the flattest radio spectra, but with a particle-acceleration process whose characteristic energy index is 2.24. The spectral flattening with distance from the nucleus occurs where our relativistic jet models require deceleration, and the flatter-spectra at the jet edges may be associated with transverse velocity shear. (Slightly abridged), Comment: 17 pages, 13 figures, accepted for publication in MNRAS

We describe very accurate imaging of radio spectral index for the inner jets in three FR I radio galaxies. Where the jets first brighten, there is a remarkably small dispersion around a spectral index of 0.62. This is also the region where bright X-ray emission is detected. Further from the nucleus, the spectral index flattens slightly to 0.50 - 0.55 and X-ray emission, although still detectable, is fainter relative to the radio. The brightest X-ray emission from the jets is therefore not associated with the flattest radio spectra, but rather with some particle-acceleration process whose characteristic energy index is 2.24. The change in spectral index occurs roughly where our relativistic jet models require rapid deceleration. Flatter-spectrum edges can be seen where the jets are isolated from significant surrounding diffuse emission and we suggest that these are associated with shear., Comment: 5 pages, 3 figures, to appear in Extragalactic Jets: Theory and Observation from Radio to Gamma Ray, T. A. Rector and D. S. De Young (eds.), ASP Conference Series

There is now unequivocal evidence that the jets in FR I radio galaxies are initially relativistic, decelerating flows. On the assumption that they are axisymmetric and intrinsically symmetrical (a good approximation close to the nucleus), we can make models of their geometry, velocity, emissivity and field structure whose parameters can be determined by fitting to deep VLA observations. Mass entrainment - either from stellar mass loss within the jet volume or via a boundary layer at the jet surface - is the most likely cause for deceleration. This idea is quantitatively consistent with the velocity field and geometry inferred from kinematic modelling and the external gas density and pressure profiles derived from X-ray observations. The jets must initially be very light, perhaps with an electron-positron composition., Comment: 10 pages, 9 figures, to appear in Extragalactic Jets: Theory and Observation from Radio to Gamma Ray, T. A. Rector and D. S. De Young (eds.), ASP Conference Series

We present an analysis of the magnetic-field fluctuations in the magnetoionic medium in front of the radio galaxy 3C 31 derived from rotation-measure (RM) fits to high-resolution polarization images. We first show that the Faraday rotation must be due primarily to a foreground medium. We determine the RM structure functions for different parts of the source and infer that the simplest form for the power spectrum is a power law with a high-frequency cutoff. We also present three-dimensional simulations of RM produced by a tangled magnetic field in the hot plasma surrounding 3C 31, and show that the observed RM distribution is consistent with a spherical plasma distribution in which the radio source has produced a cavity., Comment: 5 pages, 4 figures, to appear in Extragalactic Jets: Theory and Observation from Radio to Gamma Ray, T. A. Rector and D. S. De Young (eds.), ASP Conference Series

We present Chandra X-ray results for the jet, nucleus, and gaseous atmosphere of NGC 315, a nearby radio galaxy whose jet kinematics are known through deep radio mapping. Diffuse X-ray synchrotron emission is detected from the jet out to 30 arcsec from the nucleus, through regions both of fast bulk flow and deceleration. The X-ray to radio flux ratio drops considerably where the flow decelerates, but the X-ray and radio emissions show similar transverse extents throughout, requiring distributed particle acceleration to maintain the supply of X-ray-emitting electrons. A remarkable knotty filament within the jet is seen in both the radio and X-ray, contributing roughly 10 per cent of the diffuse emission along its extent at both wavelengths. No completely satisfactory explanation for the filament is found, though its oscillatory appearance, roughly aligned magnetic field, and requirements for particle acceleration, suggest that it is a magnetic strand within a shear layer between fast inner and slower outer flow., Comment: Accepted for publication in the MNRAS. 13 pages,14 figures (some in colour)

We present new, deep 8.5-GHz VLA observations of the nearby, low-luminosity radio galaxy 3C 296 at resolutions from 0.25 to 5.5 arcsec. These show the intensity and polarization structures of the twin radio jets in detail. We also derive the spectral-index and rotation-measure distributions. We apply our intrinsically symmetrical, decelerating relativistic jet model to the new observations. An optimized model accurately fits the data in both total intensity and linear polarization. We infer that the jets are inclined by 58 deg to the line of sight. On-axis, they decelerate from a (poorly-constrained) initial velocity of 0.8c to approximately 0.4c around 5 kpc from the nucleus, the velocity thereafter remaining constant. The speed at the edge of the jet is low everywhere. The longitudinal profile of proper emissivity has three principal power-law sections and the emission is centre-brightened. Our observations rule out a globally-ordered, helical magnetic-field configuration. Instead, we model the field as random on small scales but anisotropic, with toroidal and longitudinal components only. The ratio of longitudinal to toroidal field falls with distance along the jet, qualitatively but not quantitatively as expected from flux freezing, so that the field is predominantly toroidal far from the nucleus. The toroidal component is relatively stronger at the edges of the jet. A simple adiabatic model fits the emissivity evolution only in the outer region after the jets have decelerated and recollimated; closer to the nucleus, it predicts far too steep an emissivity decline with distance. We also interpret the morphological differences between brightness enhancements ("arcs") in the main and counter-jets as an effect of relativistic aberration. (Abridged), Comment: 30 pages, 27 figures, accepted by MNRAS

Our modelling of FR I radio jets as decelerating, relativistic flows allows us to derive their orientations accurately. We present images of Faraday rotation for two of these these objects (3C 31 and NGC 315) and show that the fluctuations of rotation measure (RM) are larger in the fainter (receding) jets, as expected if the rotation occurs in the hot galaxy/group halos. The gas density is much lower in NGC 315 and the RM fluctuations are only just detectable., Comment: 3 pages, 2 figures, requires an.cls. To appear in the Proceedings of the International Conference: "The Origin and Evolution of Cosmic Magnetism"; Bologna 29 August - 2 September 2005; eds R. Beck, G. Brunetti, L. Feretti, and B. Gaensler (Astronomische Nachrichten, 2006)

We present images of the jets in the nearby radio galaxy NGC 315 made with the VLA at five frequencies between 1.365 and 5 GHz with resolutions between 1.5 and 45 arcsec FWHM. Within 15 arcsec of the nucleus, the spectral index of the jets is 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 approximately 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., Comment: 20 pages, 17 figures, accepted for publication in MNRAS. Minor corrections to match published version, including a short note on emission from the background galaxy FGC0110

The question of the degree of order in the magnetic fields of relativistic jets is important to any understanding of their production. Both vector-ordered (e.g. helical) and disordered, but anisotropic fields can produce the high observed degrees of polarization. We outline our models of jets in FR I radio galaxies as decelerating relativistic flows. We then present theoretical calculations of the synchrotron emission from different field configurations and compare them with observed emission from FR I jets. We show that large-scale helical fields (with significant poloidal and toroidal components) are inconsistent with observations. The combination of an ordered toroidal and disordered poloidal component is consistent with our data, as is an entirely disordered field. Jets must also contain small, but significant amounts of radial field., Comment: 5 pages, 6 figures, requires an.cls. To appear in the Proceedings of the International Conference: "The Origin and Evolution of Cosmic Magnetism"; Bologna 29 August - 2 September 2005; eds R. Beck, G. Brunetti, L. Feretti, and B. Gaensler (Astronomische Nachrichten, 2006)

We apply our intrinsically symmetrical, decelerating relativistic jet model to deep VLA imaging of the inner 140 arcsec of the giant low-luminosity radio galaxy NGC 315. An optimized model accurately fits the data in both total intensity and linear polarization. We infer that the velocity, emissivity and field structure in NGC 315 are very similar to those of the other low-luminosity sources we have modelled, but that all of the physical scales are larger by a factor of about 5. We derive an inclination to the line of sight of 38 degrees for the jets. Where they first brighten, their on-axis velocity is approximately v/c = 0.9. They decelerate to v/c = 0.4 between 8 and 18 kpc from the nucleus and the velocity thereafter remains constant. The speed at the edge of the jet is roughly 0.6 of the on-axis value where it is best constrained, but the transverse velocity profile may deviate systematically from the Gaussian form we assume. The proper emissivity profile is split into three power-law regions separated by shorter transition zones. In the first of these, at 3 kpc (the flaring point) the jets expand rapidly at constant emissivity, leading to a large increase in the observed brightness on the approaching side. At 10 kpc, the emissivity drops abruptly by a factor of 2. Where the jets are well resolved their rest-frame emission is centre-brightened. The magnetic field is modelled as random on small scales but anisotropic and we rule out a globally ordered helical configuration. To a first approximation, the field evolves from a mixture of longitudinal and toroidal components to predominantly toroidal, but it also shows variations in structure along and across the jets, with a significant radial component in places. Simple adiabatic models fail to fit the emissivity variations., Comment: 20 pages, 17 figures, MNRAS (in press)

We have observed the twin-jet radio galaxy 3C296 with Chandra. X-ray emission is detected from the nucleus, from the inner parts of the radio jet, and from a small-scale thermal environment around the jet deceleration region. As we have found in previous observations of other twin-jet radio galaxies, the X-ray jet and a steep pressure gradient in the external thermal environment are associated with the region where strong bulk deceleration of the jet material is suggested by radio observations. Our observations provide additional evidence that the inner jets of twin-jet objects are always associated with a relatively cool, dense central X-ray emitting component with a short cooling time., Comment: 9 pages, 7 figures. MNRAS accepted

We summarise the recent progress of a project to determine physical conditions in the jets of low-luminosity (FR I) radio galaxies. We model the jets as intrinsically symmetrical, relativistic, decelerating flows. By fitting to deep, high-resolution VLA images in total intensity and linear polarization, we have been able to derive for the first time the three-dimensional distributions of velocity, rest-frame emissivity and magnetic-field structure. We describe our results for four sources and outline the implications for theories of jet deceleration. We also summarise a conservation-law analysis of 3C 31, which gives profiles of internal pressure, density, Mach number and entrainment rate along the jets, again for the first time. Finally, we discuss the applicability of adiabatic models to FR I jets and identify the regions where distributed particle acceleration is necessary, Comment: 14 pages, 13 figures, to appear in "X-ray and radio connections", eds Sjouwerman, L.O., Dyer, K.K., published electronically at http://www.aoc.nrao.edu/events/xraydio/

We present a general approach to the modelling of the brightness and polarization structures of adiabatic, decelerating relativistic jets, based on the formalism of Matthews & Scheuer (1990). We compare the predictions of adiabatic jet models with deep, high-resolution observations of the radio jets in the FR I radio galaxy 3C 31. Adiabatic models require coupling between the variations of velocity, magnetic field and particle density. They are therefore more tightly constrained than the models previously presented for 3C 31 by Laing & Bridle (2002). We show that adiabatic models provide a poorer description of the data in two crucial respects: they cannot reproduce the observed magnetic-field structures in detail, and they also predict too steep a brightness decline along the jets for plausible variations of the jet velocity. We find that the innermost regions of the jets show the strongest evidence for non-adiabatic behaviour, and that the adiabatic models provide progressively better descriptions of the jet emission at larger distances from the galactic nucleus. We briefly discuss physical processes which might contribute to this non-adiabatic behaviour. In particular, we develop a parameterized description of distributed particle injection, which we fit to the observed total intensities. We show that particles are preferentially injected where bright X-ray emission is observed, and where we infer that the jets are over-pressured., Comment: 16 pages, 11 figures, accepted for publication in MNRAS

We model jets in low-luminosity (FR I) radio galaxies as intrinsically symmetrical, axisymmetric, decelerating relativistic flows with transverse velocity gradients. This allows us to derive velocity fields and the three-dimensional distributions of magnetic-field ordering and rest-frame emissivity. A conservation-law analysis, combining the kinematic model with X-ray observations of the surrounding IGM, gives the profiles of internal density, pressure, Mach number and entrainment rate along the jets. We summarize our recently-published results on 3C 31 and outline new work on other sources and adiabatic jet models., Comment: 4 pages, 2 figures, to appear in The Physics of Relativistic Jets in the Chandra and XMM Era, eds G. Brunetti, D.E. Harris, R.M. Sambruna, G. Setti, New Astronomy Reviews

During the course of an investigation of the interaction of the radio galaxy M84 and its ambient cluster gas, we found excess X-ray emission aligned with the northern radio jet. The emission extends from the X-ray core of the host galaxy as a weak bridge and then brightens to a local peak coincident with the first detectable radio knot at approx. 2.5" from the core. The second radio knot at 3.3" is brighter in both radio and X-rays. The X-ray jet terminates 3.9" from the core. Although all the evidence suggests that Doppler favoritism augments the emission of the northern jet, it is unlikely that the excess X-ray emission is produced by inverse Compton emission. We find many similarities between the M84 X-ray jet and recent jet detections from Chandra data of low luminosity radio galaxies. For most of these current detections synchrotron emission is the favored explanation for the observed X-rays., Comment: 6 pages. Tentatively scheduled to appear in the ApJ vol. 580, 2002 Nov 20 issue

We present a dynamical analysis of the flow in the jets of the low-luminosity radio galaxy 3C 31 based on our earlier geometrical and kinematic model and on estimates of the external pressure and density distributions from Chandra observations. We apply conservation of particles, energy and momentum to derive the variations of pressure and density along the jets and show that there are self-consistent solutions for deceleration by injection of thermal matter. We initially take the jets to be in pressure equilibrium with the external medium at large distances from the nucleus and the momentum flux to be energy flux/c; we then progressively relax these constraints. We infer that the jets are over-pressured compared with the external medium where they start to expand rapidly. Local minima in the density and pressure and maxima in the mass injection rate and Mach number occur at approximately 3 kpc. Further out, the jets decelerate smoothly with a Mach number close to 1. The mass injection rate we infer is comparable with that expected from stellar mass loss at 1 - 2 kpc, but significantly exceeds it at large distances. We conclude that entrainment from the galactic atmosphere is the dominant mass input process far from the nucleus, but that stellar mass loss may also contribute closer in. The occurrence of a significant over-pressure where expansion begins leads us to suggest that there is a stationary shock system, perhaps caused by reconfinement of an initially free jet. Our results are compatible with a jet consisting of electron-positron plasma on parsec scales which picks up thermal matter from stellar mass loss to reach the inferred density and mass flux at 1 kpc, but we cannot rule out an electron-proton composition with a low-energy cut-off. [Abridged], Comment: 23 pages, 22 figures, accepted for publication in MNRAS

We show that the principal differences in appearance of the main and counter-jets within 30 arcsec of the nucleus of the low-luminosity radio galaxy 3C 31 can result entirely from the effects of relativistic aberration in two symmetrical, antiparallel, axisymmetric, time-stationary relativistic flows. We develop empirical parameterized models of the jet geometry and the three-dimensional distributions of the velocity, emissivity and magnetic-field structure and optimize their parameters by fitting the predicted synchrotron intensity and polarization to deep 8.4-GHz VLA observations. We conclude that the jets are at roughly 52 degrees to the line of sight, that they decelerate and that they have transverse velocity gradients. They have three regions with distinct kinematics: a narrow inner region; a flaring region of rapid expansion followed by recollimation and a conical outer region. There is a discontinuity in the flow between inner and flaring regions. The on-axis velocity is close to 0.8c until the end of the flaring region, where it drops abruptly to 0.55c, thereafter falling more slowly to 0.25c at the end of the modelled region. Throughout the flaring and outer regions, the velocity at the edge of the jet is 0.7 of its on-axis value. The magnetic field has primarily toroidal and longitudinal components except in the flaring region, where there is a significant radial component at the jet edge. Simple adiabatic models fail in the inner and flaring regions. The inferred transverse velocity profiles and field structure in the flaring region support the idea that the jets decelerate by entraining the external medium. We demonstrate the appearance of our model at other angles to the line of sight and argue that other low-luminosity radio galaxies resemble 3C 31 seen at different orientations. [Abridged], Comment: 29 pages, 25 figures, accepted by MNRAS. For additional material, including higher-resolution figures and animations, see http://www.cv.nrao.edu/~abridle/3c31free/

We have used a deep Chandra observation of the central regions of the twin-jet FRI radio galaxy 3C31 to resolve the thermal X-ray emission in the central few kiloparsecs of the host galaxy, NGC 383, where the jets are thought to be decelerating rapidly. This allows us to make high-precision measurements of the density, temperature and pressure distributions in this region, and to show that the X-ray emitting gas in the centre of the galaxy has a cooling time of only 5 times 10^7 years. In a companion paper these measurements are used to place constraints on models of the jet dynamics. A previously unknown one-sided X-ray jet in 3C31, extending up to 8 arcsec from the nucleus, is detected and resolved. Its structure and steep X-ray spectrum are similar to those of X-ray jets known in other FRI sources, and we attribute the radiation to synchrotron emission from a high-energy population of electrons. In situ particle acceleration is required in the region of the jet where bulk deceleration is taking place. We also present X-ray spectra and luminosities of the galaxies in the Arp 331 chain of which NGC 383 is a member. The spectrum and spatial properties of the nearby bright X-ray source 1E 0104+3153 are used to argue that the soft X-ray emission is mostly due to a foreground group of galaxies rather than to the background broad absorption-line quasar., Comment: 11 pages, 10 figures. Accepted for MNRAS: two companion papers to follow

Winged, or X-shaped, radio sources form a small class of morphologically peculiar extragalactic sources. We present multi-frequency radio observations of two such sources. We derive maximum ages since any re-injection of fresh particles of 34 and 17Myr for the wings of 3C223.1 and 3C403, respectively, based on the lack of synchrotron and inverse Compton losses. On morphological grounds we favour an explanation in terms of a fast realignment of the jet axis which occurred within a few Myr. There is no evidence for merger activity, and the host galaxies are found to reside in no more than poor cluster environments. A number of puzzling questions remain about those sources: in particular, although the black hole could realign on sufficiently short timescales, the origin of the realignment is unknown., Comment: Accepted for publication in MNRAS

We present multi-frequency VLA, multicolor CCD imaging, optical spectroscopy, and ROSAT HRI observations of the giant FR II radio galaxy NVSS 2146+82. This galaxy, which was discovered by the NRAO VLA Sky Survey (NVSS), has an angular extent of nearly 20' from lobe to lobe. The radio structure is normal for an FR II source except for its large size and regions in the lobes with unusually flat radio spectra. Our spectroscopy indicates that the optical counterpart of the radio core is at a redshift of z=0.145, so the linear size of the radio structure is ~4 h_50^-1 Mpc. This object is therefore the second largest FR II known (3C 236 is ~6 h_50^-1 Mpc). Optical imaging of the field surrounding the host galaxy reveals an excess number of candidate galaxy cluster members above the number typically found in the field surrounding a giant radio galaxy. WIYN HYDRA spectra of a sample of the candidate cluster members reveal that six share the same redshift as NVSS 2146+82, indicating the presence of at least a ``rich group'' containing the FR II host galaxy. ROSAT HRI observations of NVSS 2146+82 place upper limits on the X-ray flux of 1.33 x 10^-13 ergs cm^-2 s^-1 for any hot IGM and 3.52 x 10^-14 ergs cm^-2 s^-1 for an X-ray AGN, thereby limiting any X-ray emission at the distance of the radio galaxy to that typical of a poor group or weak AGN. Several other giant radio galaxies have been found in regions with overdensities of nearby galaxies, and a separate study has shown that groups containing FR IIs are underluminous in X-rays compared to groups without radio sources. We speculate that the presence of the host galaxy in an optically rich group of galaxies that is underluminous in X-rays may be related to the giant radio galaxy phenomenon., Comment: 46 pages, 15 figures, AASTeX aaspp4 style, accepted for publication in AJ

We investigate the correlations between spectral index, jet side and extent of the radio lobes for a sample of nearby FRII radio galaxies. In Dennett-Thorpe et al. (1997) we studied a sample of quasars and found that the high surface brightness regions had flatter spectra on the jet side (explicable as a result of Doppler beaming) whilst the extended regions had spectral asymmetries dependent on lobe length. Unified schemes predict that asymmetries due to beaming will be much smaller in narrow-line radio galaxies than in quasars: we therefore investigate in a similar manner, a sample of radio galaxies with detected jets. We find that spectral asymmetries in these objects are uncorrelated with jet sidedness at all brightness levels, but depend on relative lobe volume. Our results are not in conflict with unified schemes, but suggest that the differences between the two samples are due primarily to power or redshift, rather than to orientation. We also show directly that hotspot spectra steepen as a function of radio power or redshift. Whilst a shift in observed frequency due to the redshift may account for some of the steepening, it cannot account for all of it, and a dependence on radio power is required., Comment: accepted for publication in MNRAS, 10 pages; typos/minor correction

The less depolarized lobe of a radio source is generally the lobe containing the jet (Laing-Garrington correlation) but the less depolarized lobe is also generally that with the flatter radio spectrum (Liu-Pooley correlation). Both effects are strong; taken together they would imply a correlation between jet side and lobe spectral index, i.e. between an orientation-dependent feature and one which is intrinsic. We test this prediction using detailed spectral imaging of a sample of quasars with well-defined jets and investigate whether the result can be reconciled with the standard interpretation of one-sided jets in terms of relativistic aberration. Our central finding is that the spectrum of high surface brightness regions is indeed flatter on the jet side, but that the spectrum of low surface brightness regions is flatter on the side with the longer lobe. We discuss possible causes for these correlations and favour explanations in terms of relativistic bulk motion in the high surface brightness regions and differential synchrotron ageing in the extended lobe material., Comment: to be published in MNRAS. 12 pages, uses mn.sty. Replacement corrects for error in postscript (fig. 1)

A representative sample of 12 extended quasars from the 3CR catalog has been imaged at 4.9 GHz using the Very Large Array (VLA). These full synthesis observations typically achieve an rms noise of 20 micro-Jy per beam. Jets are detected on at least one side of every source. The jets are well collimated compared with those in less powerfull sources, but spreading is detected in most of them. The opening angles of several jets are not constant, but show recollimation after an initial regime of rapid spreading. Many of the jets contain quasiperiodic strings of knots, of which the knot closest to the central feature is usually the brightest (until the jet nears its hot spot). The degrees of linear polarization at the jet knots range from less than 5% to approx. 50%, but show no common trend with distance along the jets. In knots that are elongated in directions close to that of the jet, the E vectors tend to be orthogonal to the jet axis. The prominence of the inner, straighter jet segments relative to the extended lobes correlates significantly with the prominence of the milliarcsecond-scale central features, but the prominence of the more bent jet segments does not. Candidates for counterjet emission are detected in seven sources, but there is no unambiguous, continuous counterjet in any of them. Estimates of the flux density ratios between the straighter jet segments and the counterjets based on these tentative detections range from 1.2:1 to greater than 175:1. There is no evidence in this sample that counterjet detectability correlates with such putative inclination indicators as central feature prominence or projected linear size. There is also no evidence that the prominence of the counterjets anticorrelates with that of the jets as predicted by simple relativistic-beaming models for the jet/counterjet asymmetry. There is, however, strong evidence that large bends in the main jet favor counterjet detection, and there are no counterjet candidates opposite long, uninterrupted straight segments of the main jets. The detectability of the counterjets in these quasars may therefore be strongly influenced by interactions between the underlying beams and inhomogeneities in the surrounding material. We offer a new empirical definition of the term 'hot spot' that is intended to improve the distinction between such features and 'jet knots.' Both the compactness of hot spots and their position in the lobe are affected by whether they are fed by a detectable jet. When the hot spots differ significantly in compactness, the more compact one is always on the jetted side. Jetted hot spots are also more likely to be recessed deeply from the outer edge of their lobes than are their counterjetted counterparts. The jetted hot spot is less prominent relative to other extended emission if the jet bends through a large angle, particularly if a large bend occurs abruptly. The counterjetted hot spot is also less well defined if the jet is more bent. The lobes of several sources show considerable inhomogeneity, including filamentation.