Publications by Katherine Blundell

Intra-Day Variability, origins and implications

ASTR SOC P 250 (2002) 123-127

DL Jauncey, L Kedziora-Chudczer, JEJ Lovell, JP Macquart, GD Nicolson, RA Perley, JE Reynolds, AK Tzioumis, MA Wieringa, HE Bignall

Considerable evidence has accumulated that strongly favours interstellar scintillation (ISS) as the principal mechanism causing intra-day variability (IDV) at cm wavelengths. This includes the observed frequency dependence of IDV, the measured time-delay in the IDV pattern as seen at two widely spaced radio telescopes, and the presence of an annual time-signature in the behaviour of the long-term IDV variability. While ISS reduces the implied brightness temperatures, they remain uncomfortably high. There appear to be several obstacles to explaining the observations with synchrotron theory: first, the brightness temperatures appear to be up to two orders of magnitude above the inverse Compton limit, second, the observed IDV lifetimes of more than a decade are much longer than expected, and third, there is the presence of strong and variable circular polarization in several sources.

The cluster environment of Abell 3667

ASTR SOC P 250 (2002) 432-436

M Johnston-Hollitt, RD Ekers, RW Hunstead

Abell 3667 is a rich X-ray luminous southern cluster of galaxies at a redshift of 0.055. The properties of this cluster suggest that it is probably in a post-merger state. Recent radio observations taken with the Australia Telescope Compact Array (ATCA) have allowed a detailed investigation of the environment in this cluster. We present here a synopsis of these observations.

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.

Confronting hydrodynamic simulations of relativistic jets with data: what do we learn about particles & fields?

ASTR SOC P 250 (2002) 303-314

PA Hughes

We review recent relativistic hydrodynamic simulations of jets, and their interpretation in terms of the results from linear stability analysis. These studies show that, interpreted naively, the distribution of synchrotron intensity will in general be a poor guide to the physical state (density and pressure) of the underlying flow, and that even if the physical state can be inferred, it, in turn, may prove to be a poor guide to the source dynamics, in terms of the transport of energy and momentum from the central engine. However, we demonstrate that an interplay of simulation and linear stability analysis provides a powerful tool for elucidating the nature and character of structures that jets may sustain. From such studies we can explain the complex behaviour of observed jets, which manifest both stationary and propagating structures, without recourse to ad hoc macroscopic disturbances. This provides a framework for the interpretation of multi-epoch total intensity data wherein an understanding of the character of individual flow features will allow the effects of physical state and dynamics to be deconvolved.

Radio galaxies and energetics of the intracluster medium

ASTR SOC P 250 (2002) 443-448

MC Begelman

The time- and ensemble-averaged mechanical energy outputs of radio galaxies may be large enough to offset much of the cooling inferred from X-ray observations of galaxy clusters. But does this heating actually counterbalance the cooling, diminishing cooling flows or quenching them altogether? I will argue that energy injection by radio galaxies may be important even in clusters where no active source is present, due to the likely intermittency of the jets. If the energy injected by radio galaxies percolates through the intracluster medium without excessive mixing, it could stabilize the atomic cooling responsible for X-ray emission.

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.

Circular polarization in intraday variable blazars

ASTR SOC P 250 (2002) 142-146

SJ Wagner, K Mannheim

We have measured Stokes IUQV of several rapidly variable and gamma-bright blazars and detected variable optical circular polarization, occasionally exceeding 1%, in 3C 279. We discuss possible origins for significant amounts of circular polarization (CP) in blazars and suggest direct CP from particles radiating in strong magnetic fields or anisotropies in the particle distribution function as the most plausible explanations.

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.

The parsec-scale central components of FRI radio galaxies

ASTR SOC P 250 (2002) 100-103

P Kharb, P Shastri

A majority of a complete sample of 3CR FR I radio galaxies show unresolved optical nuclear sources on the scales of 0.1 aresec. About half of the 3CR FR II radio galaxies observed with the HST also show Compact Central Cores (CCC). These CCCs have been interpreted as the optical counterparts of the non-thermal radio cores in these radio galaxies (Chiaberge, Capetti & Celotti 1999). We show that the optical flux density of the CCCs in FR Is is correlated with the radio core prominence. This correlation supports the argument of Chiaberge et al. that the CCC radiation is of a non-thermal synchrotron origin, which is relativistically beamed along with the radio emission.

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.

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.

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.

Shocks, plasma instabilities and undisturbed flows in parsec-scale jets

ASTR SOC P 250 (2002) 195-199

AP Lobanov, J Roland

We discuss the relative contributions made by shocks, plasma instabilities and undisturbed flows to the emission and dynamics of compact, extragalactic jets. We summarize recent, in-depth studies of the jets in 3C 273 and 3C 345 and show that interpretation of these result calls for a new paradigm for the parsec-scale jets. Contrary to the commonly accepted views, the observed properties of the jets in these objects indicate that extragalactic jets remain relatively undisturbed on scales of similar to1-10 parsec. Farther on, on scales of similar to10-100 parsec, Kelvin-Helmholtz instabilities become the main contributor to the observed emission and kinematic changes. Relativistic shocks appear to be gaining prominence only on larger, sub-kiloparsec scales.

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.

Maser radiation from astrophysical jets

ASTR SOC P 250 (2002) 164-169

R Bingham, RA Cairns, JM Dawson, J Tonge, JT Mendonca

Electromagnetic radiation from jets is normally considered to be incoherent synchrotron emission. Some aspects of radiation from both stellar and extragalactic jets could, however, be due to coherent emission mechanisms. Coherent radiation mechanisms have the advantage of being able to account for the high brightness temperatures inferred from some observations. One of the most popular coherent radio emission mechanisms is the electron cyclotron maser instability. In this article we discuss electron cyclotron maser emission associated with particular types of charged particle acceleration commonly inferred in astrophysical jets, including turbulence and shocks.

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.