Publications by Katherine Blundell

Anisotropic inverse Compton scattering in radio galaxies and the particle energy distribution

ASTR SOC P 250 (2002) 384-388

G Brunetti, G Setti

We briefly review the inverse-Compton (IC) scattering of nuclear photons in the lobes of radio galaxies and quasars. We concentrate on the possibility of testing this model with the Chandra observatory and of constraining the electron energy distribution by measuring the Xray fluxes produced by this effect. We also discuss the evidence for IC scattering of nuclear photons in powerful radio galaxies.

Jet evolution in nearby FRI radio galaxies

ASTR SOC P 250 (2002) 175-179

CP O'Dea, SA Baum, C Xu, JM Wrobel, JJ Condon

We have observed 17 sources in a complete sample of FRIs in nearby UGC galaxies with the VLBA at 1.67 GHz. We detect core-jets in 60% and twin-jets in 10%. The data (alignment, sidedness ratios, detection statistics) are consistent with relativistic Doppler boosting effects. The luminosity per unit length of the jets fades with distance l from the core as L-j proportional to l(-2). The fading is not consistent with either deceleration of the jets (with subsequent reduction of Doppler boosting) or synchrotron losses. However, the fading can be explained by adiabatic losses in a constant velocity jet with constant opening angle and magnetic field perpendicular to the jet axis. If confirmed by VLBA polarimetry, this would imply that the jets must decelerate on scales larger than tens of parsec.

Synchrotron and SSC emission models for the hot-spots of powerful radio galaxies

ASTR SOC P 250 (2002) 238-242

G Brunetti

We present some preliminary results from synchrotron and synchrotron-self-Compton (SSC) emission models for the hot-spots of powerful radio galaxies. In addition to well known radiative models already reported in the literature, we include the computation of the spectrum of the emitting electrons as a function of the distance from the hot-spot centre. The radial evolution of the spectrum takes into account possible particle reacceleration, synchrotron, SSC and adiabatic losses. We concentrate on the case of the northern hot-spot of the radio galaxy 3C 295 where radio, optical and X-ray data are available.

What triggers radio galaxies?

ESO ASTROPHY SYMP (2002) 125-127

MJ Cruz, KM Blundell

We are investigating possible triggering scenarios which may give rise to powerful jetted active galaxies such as classical double radio galaxies. Important clues come from studying those objects which have most recently been triggered. The newly discovered 'Youth-Redshift Degeneracy' for classical double radio galaxies means that the highest redshift objects in a flux-limited survey will be observed to be significantly younger (more recently triggered) than the more nearby objects. We are pursuing this investigation with a new sample specially filtered to favour the detection of high-redshift radio galaxies. We will present some preliminary results of a near-IR imaging campaign using the UIFTI on the UKIRT.

Estimating jet power in proton blazar models

ASTR SOC P 250 (2002) 113-116

RJ Protheroe, A Mucke

We discuss the various contributions to the jet luminosity in proton blazar models of active galactic nuclei and describe a method of estimating the jet luminosity from the observed spectral energy distribution (SED) and the fitted model parameters. We apply this to a synchrotron proton blazar (SPB) model for Markarian 501.

3D MHD simulations of radio galaxies including non-thermal electron transport

ASTR SOC P 250 (2002) 324-335

TW Jones, IL Tregillis, D Ryu

We report on an effort to study the connections between dynamics in simulated radio galaxy plasma flows and the properties of non-thermal electron populations carried in those flows. To do this we have introduced a new numerical scheme for electron transport that allows a much more detailed look at this problem than has been possible before. Especially when the dynamics axe fully three dimensional the flows are generally chaotic in the cocoon, and the jet itself can flail about violently. The bending jet can pinch itself off and redirect itself to enhance its penetration of the ambient medium. These behaviours often eliminate the presence of a strong jet termination shock, which is assumed present in all modern cartoon models of the radio galaxy phenomenon. Instead a much more complex "shock web" forms near the end of the jet that leads to a far less predictable pattern of particle acceleration. Similarly, the magnetic fields in these flows are highly filamented, as well as spatially and temporally intermittent. This leads to a very localized and complex pattern of synchrotron aging for relativistic electron populations, which makes it difficult to use properties of the electron spectrum to infer the local rate of aging.

Information on particles and fields from parsec and sub-parsec scale jets

ASTR SOC P 250 (2002) 80-92

GV Bicknell, SJ Wagner, BA Groves

With particular reference to the BL Lac object, Markarian 501, we emphasize the importance of X-ray and gamma-ray observations for elucidating the parameters of parsec scale and sub-parsec.scale jets. In particular the magnetic field, particle energy density and Doppler factor can be inferred from simultaneous observations. The energy flux in the Mkn 501 jet is particle dominated. If the composition is electron-proton then the minimum electron Lorentz factor, gamma(1) similar to 100. Consideration of a sample of quasar and BL Lac jets, shows that these jets may consist of electron-positron pairs, also if gamma(1) similar to 100. Seyfert jets may be examples of jets in which the plasma composition is predominantly electron-proton, suggesting that they originate at larger radii (with respect to the gravitational radius) than jets in radio-loud galaxies.

Polarization observations of the hot-spot Pictor A West: shocks in backflows?

ASTR SOC P 250 (2002) 259-263

SJ Wagner, G Bicknell, T Szeifert

We present optical polarization maps of the western hot-spot of the radio galaxy Pictor A. We confirm the presence of optical emission in a bar-shaped structure extending over 24 arcsec perpendicular to the jet direction upstream of the hot-spot. We find its optical emission to be highly polarized with magnetic vectors being aligned perpendicular to the jet axis. From the high degree of polaxization we infer that the extended optical emission is of synchrotron origin. Radiative lifetimes of electrons emitting synchrotron emission at frequencies as high as 10(14) Hz are much shorter than diffusion time scales of particles accelerated within a narrow jet. Among different ways. to account for local acceleration of these electrons, we favour a scenario explaining the extended bar-shaped region as a shock-front in the back-flow of the radio-jet.

A high-frequency and multi-epoch VLBI study of 3C 273

ASTR SOC P 250 (2002) 184-190

TP Krichbaum, DA Graham, A Witzel, JA Zensus, A Greve, M Grewing, A Marscher, AJ Beasley

We show results from a 7 year VLBI monitoring programme of 3C 273 at millimetre wavelengths. We find evidence for component acceleration, motion or rotation of fluid dynamical patterns, and an outburst-ejection relation between gamma-ray flares and, new jet components.

Size-luminosity scaling and blazar unification

ASTR SOC P 250 (2002) 104-108

M Georganopoulos, JG Kirk, A Mastichiadis

Reverberation mapping of radio-quiet active galactic nuclei (AGN) has shown that the size of the broad line region (BLR) scales with the power of the source. We assume that this scaling also applies to blazars. We demonstrate that, according to this scaling, the inverse Compton (IC) losses are dominated by synchrotron - self Compton scattering (SSC) in weak sources like Mkn 421, while in powerful sources, like 3C 279, they are dominated by external Compton (EC) scattering of BLR photons. We also show that this scaling is in good agreement with the observed sequence of blazar, properties with luminosity.

FRIIb radio sources, ambient gas densities, and clusters of galaxies

ASTR SOC P 250 (2002) 412-416

RA Daly, EJ Guerra, MM Chester

FRIIb radio sources axe a special subset of all classical double radio sources with particularly simple radio bridge structures. At low radio frequencies, the surface brightness across the ridge-line of the source is accurately reproduced by accounting for adiabatic expansion in the lateral direction by an amount indicated by the observed change in the width of the radio bridge as a function of position. Thus, the physics of FRIIb sources is quite simple. The radio properties of an FRIIb source may provide a useful probe of the pressure, density, and temperature of gas surrounding the radio source. Estimates of the ambient gas density obtained from considerations of the rain-pressure confinement of the forward region of FRIIb radio sources axe discussed here. Typical ambient gas densities and the composite density profile indicate that the sources lie in gaseous environments very much like the cores of galaxy clusters. Some evolution of the core gas density and radius with redshift axe suggested by the data.

Long term monitoring of the extreme intraday variable quasar PKS 0405-385

ASTR SOC P 250 (2002) 128-132

L Kedziora-Chudczer, DL Jauncey, JEJ Lovell, MA Walker, JP Macquart, MH Wieringa, AK Tzioumis, RA Perley, JE Reynolds

The quasar PKS 0405-385 exhibits episode's of unusually strong hourly variability at 8.6, 4.8, 2.4 and 1.4 GHz. These variations, seen first in June 1996, ceased after several weeks, but reappeared in November 1998 and were monitored closely at the. ATCA. We argue that our data are in agreement with interstellar scintillation of a source component which is a few micro-arcseconds in size. This claim is further supported by our recent finding of the time delay in the variability pattern measured between the ATCA and the VLA.

Nonthermal emission in radio galaxies from simulated relativistic electron transport in 3D MHD flows

ASTR SOC P 250 (2002) 336-339

IL Tregillis, TW Jones, D Ryu

We perform a series of so-called "synthetic observations" on a set of 3D MHD jet simulations which explicitly include energy-dependent transport of relativistic electrons, as described in the companion paper by Jones, Tregillis & Ryu. Analyzing them in light of the complex source dynamics and energetic particle distributions described in that paper, we find that the standard model for radiative aging in radio galaxies does not always adequately reflect the detailed source structure.

Simulations and observations of cocoon morphologies

ASTR SOC P 250 (2002) 315-318

G Bodo, P Rossi, A Capetti, S Massaglia, A Ferrari

Numerical simulations suggest that, depending on, the jet parameters and on the properties of the external medium, two radically different morphologies for the lobes of extended radio sources can be expected. Analysis of maps of FRII radio sources shows that indeed such a dichotomy is reproduced in real radio sources: their lobes display either a "spearhead" or a "fat" morphology, characterised by the lobe aspect ratio. It therefore appears possible to derive information on the jet physical parameters from the radio lobe morphology.

Particles and fields in radio galaxies: a summary

ASTR SOC P 250 (2002) 487-498

RD Blandford

A summary is presented of a meeting on Particles and Fields in Radio Galaxies held in Oxford in August 2000. Recent detailed studies of radio maps and the first X-ray images from the Chandra X-ray Observatory, together with new capabilities in simulating hydromagnetic flows, are transforming our understanding of how jets are formed by accretion disks orbiting massive black holes, how these jets dissipate and how they inflate the giant radio-emitting lobes by which they were first identified. As they can be imaged in such detail, extragalactic radio sources remain central to the study of jet outflows in general although there is a strong convergence with corresponding studies of Galactic superluminal sources, gamma ray bursts and young stellar objects..

Hotspot spectral indices

ASTR SOC P 250 (2002) 298-302

M Sandell, JP Leahy

In this contribution we present some interim results concerning the spectral indices of the hot-spots of high redshift radio galaxies and quasars. This work forms a part of the Distant DRAGNs Survey, a project to map at sub-kpc resolution a sample of z > 1.5 radio galaxies and quasars drawn from multiple flux-density-limited samples.

A multi-frequency study of the radio galaxy NGC 326

ASTR SOC P 250 (2002) 380-383

M Murgia, P Parma, R Fanti, HR de Ruiter, RD Ekers, EB Fomalont

We present preliminary results of a multi-frequency study of the inversion-symmetric radio galaxy NGC 326 based on VLA observations at 1.4, 1.6, 4.8, 8.5, and 14.9 GHz. These data allow us to investigate in detail the morphological, spectral and polarization properties of this peculiar object at different spatial resolutions.

Too hot, too fast or forever young?

ASTR SOC P 250 (2002) 133-136

J Dennett-Thorpe, AG de Bruyn

We present the results of a year-long WSRT monitoring campaign for the quasar J 1819+3845. The extreme variations (regularly 10% per minute) axe explained by interstellar scintillation of a source which is no more that 30 microarrseconds in diameter, with a corresponding brightness temperature of similar to 10(13) K.We use the results of this WSRT campaign to infer critical source parameters (size, structure and lifetime). To first order, the changes in the observed scintillation behaviour over the year are interpreted as being due to a peculiar velocity of the scattering plasma (similar to 20 pc distant), and not due to any changes within the source itself. We discus's the source structure on a size of tens of microarcseconds, and illustrate how such a monitoring campaign can yield such, information.

Decelerating relativistic radio jets in B2 0755+379

ASTR SOC P 250 (2002) 276-280

M Bondi, P Parma, H de Ruiter, R Fanti, RA Laing

We apply a model for an adiabatically expanding relativistic jet to the radio galaxy B2 0755+379 using the observed surface brightness and jet width obtained from VLA and MERLIN radio images. We derive velocity profiles along the main jet for various assumed starting conditions, and show that these profiles axe consistent with the observed jet/counter-jet brightness ratio provided that the angle to the line of sight of the jet is theta similar or equal to 27degrees and the starting velocity of the jet (actually. the velocity at 0.5 kpc from the nucleus) is similar or equal to 0.9c.

Current problems for X-ray emission from radio jets

ASTR SOC P 250 (2002) 204-212

DE Harris

A list is presented of known extragalactic radio jets which also have associated X-ray emission. The canonical emission processes for the production of X-rays are reviewed and the sources axe categorized on the basis of our current understanding. Although it seems clear that the X-ray emission is non-thermal, the two possible processes, synchrotron and inverse Compton emission, arise from extremely high energy (synchrotron) or extremely low energy (beaming models with IC emission), relativistic electrons. Only synchrotron self-Compton emission from a few hot-spots provides information on the 'normal' energy range of the electrons responsible for the observed radio emission.