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.

Evidence for interaction with a surrounding medium in several BL lacertae objects.

ASTR SOC P 250 (2002) 200-203

AB Pushkarev, DC Gabuzda

In February 1997, we obtained multi-frequency polarization VLBA observations of 23 sources from a complete sample of northern BL Lac objects. Total intensity and linear polarization images of two sources (0745+241 and 0820+225) showing evidence for the presence of sheaths of longitudinal magnetic field surrounding their jets are presented. These results suggest that appreciable amounts of thermal plasma are often present on parsec scales, and that the jet magnetic field can be influenced by interaction with this surrounding medium.

Cosmic ray injection at SNR shocks: a laboratory for radio galaxy plasma physics?

ASTR SOC P 250 (2002) 53-74

RO Dendy, KG McClements, RA Laing, M Dieckmann

The standard paradigm for extragalactic radio sources assumes that a description in terms of single-fluid MHD, adiabatic work, and radiation losses should provide an adequate model for the observed radiation and morphology. However this approach omits several physical elements that are now standard in other branches of laboratory, space, and astrophysical plasma physics, and we examine whether current observations of extragalactic radio sources motivate extensions to the standard paradigm. Supernova remnants are analogous to extragalactic radio sources in producing synchrotron radiation from radio to X-ray wavelengths, and electron energies as high as 10(14) eV axe required in some cases. Diffusive shock acceleration can generate these electrons from a mildly relativistic population, but the 'injection' problem of prior acceleration of background electrons to mildly relativistic energies has remained. Here we describe the 'bootstrap' solution of the electron injection problem, by which a shock-reflected proton population drives collective plasma instabilities, which in turn excite waves in the plasma which accelerate thermal electrons to mildly relativistic energies. The physical processes incorporated in this solution lie outside the standard paradigm for extragalactic radio sources, and we discuss their potential relevance.

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.

Are cluster radio relics revived fossil radio cocoons?

ASTR SOC P 250 (2002) 454-457

TA Ensslin, Gopal-Krishna

A new model for the so called cluster radio relics is presented (see Ensslin & Gopal-Krishna 2001 for more details). Fossil radio cocoons, resulting from the former activity of radio galaxies, should contain a low energy relativistic electron population and magnetic fields. Even electrons with ages as high as 2 Gyr can be re-accelerated adiabatically to radio- emitting energies, if the fossil radio plasma gets compressed in an environmental shock wave. Such a wave can be caused by merging events in galaxy clusters, or by accretion onto clusters. An implication of this model is the existence of a population of diffuse, ultra-steep spectrum, very low-frequency radio sources located inside and possibly outside of clusters of galaxies, tracing the revival of aged fossil radio plasma by the shock waves associated with large-scale structure formation.

What shape are your spectra in?

ASTR SOC P 250 (2002) 372-379

L Rudnick

The shapes of synchrotron spectra contain important information about the origin and evolution of the relativistic electron distributions. The analyses of spectral shapes that appear in the literature are inadequate, at best, to address the important scientific issues. A proper analysis of spectra can illuminate the energy distribution of particles as they are "injected" into diffuse regions, the acceleration and loss processes, the bulk and microscopic transport of electrons and the role of inhomogeneities in the magnetic field. I describe some observational difficulties we face in isolating the actual particle distributions, highlight some of the interpretational "sins" we have committed, and make some modest recommendations.

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.

X-ray constraints on jet composition

ASTR SOC P 250 (2002) 345-357

JP Leahy, N Gizani, D Tsakiris

We review several ways in which X-ray observations are helping to tie down the physical parameters in radio lobes and jets, resolving most of the notorious uncertainties which afflict estimates from synchrotron emission alone. Together, the new results imply that the energy density of jet plasma is dominated by particles which do not contribute to the observed radiation, such as relativistic protons or mildly relativistic electrons, These components have long been hypothesised but their implications have often been ignored. Their presence substantially increases estimates of the energy budget for jet activity, and implies that jets may be an important heat source for the intracluster gas.

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.

The nature of jets: evidence from circular polarization observations

ASTR SOC P 250 (2002) 152-163

JFC Wardle, DC Homan

We review recent observations of circularly polarized radiation from AGN made with the VLBA and with the ATCA. We also discuss briefly the detections of the Galactic sources Sgr A* and SS 433. The origin of the circular polarization is still an open question in most cases, and we discuss four possible mechanisms. Detectable circular polarization is a common property of quasars, but not of radio galaxies, and is always associated with the compact core. There is growing evidence that the sign of the circular polarization stays the same over at least 20 - 30 years, suggesting that it is a fundamental property of the jet.

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.

Beam powers, active lifetimes, and total energies of FRIIb radio galaxies

ASTR SOC P 250 (2002) 417-421

EJ Guerra, RA Daly

Multi-frequency radio observations of a powerful classical double radio galaxies can be used to determine the beam power of the jets emanating from the AGN, the total time the source will actively produce jets, and the total energy available to power the jets during their lifetime. Empirical determinations of these quantities are presented for 20 classical double radio galaxies. The model assumptions, trends with redshift, and implications for energy extracted from central engines are discussed.

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.

VLBI observations of a complete sample of radio galaxies - 10 years later

ASTR SOC P 250 (2002) 170-174

G Giovannini, L Feretti, T Venturi, WD Cotton, L Lara

A complete sample of 27 radio galaxies was selected from the B2 and 3CR catalogues, in order to study their properties on milliarcsecond scales. In this work we discuss the evidence for high velocity jets in low power radio galaxies and the possible existence of a two velocity jet. structure (fast spine and lower velocity external shear layer).

Magnetic field amplification in FR II radio sources

ASTR SOC P 250 (2002) 75-79

DS De Young

If magnetic fields are near the energy equipartition value in the lobes of FR II radio sources, it can be shown that it is very unlikely that such fields are passively advected outward with the jet that creates the source. This has led to the idea that the fields are amplified within the lobes; and a likely site for this is the complex and turbulent region inside the hot-spots, as suggested by Blundell, Rawlings & Willott (1999). if this process occurs, a natural question is the subsequent evolution of the field strength and geometry as the field and fluid leave the hot-spot and fill the lobe. This evolution is followed via a time dependent turbulent MHD calculation under a variety of conditions. While the small scale field can decay away rather quickly, the large scale field structures remain surprisingly robust.

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.

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.