Publications by Katherine Blundell

Abell 2256 - Observing a Mpc(3) nonthermal laboratory

ASTR SOC P 250 (2002) 428-431

TE Clarke, TA Ensslin

The galaxy cluster Abell 2256 is in the violent stage of a giant cluster merger event. Evidence of this merger is seen in the X-ray substructure of ROSAT images (Briel et al. 1991). Radio images reveal the complex nature of this cluster: a synchrotron halo, several head-tail radio galaxies, and two extended irregular and sharp-edged regions of diffuse radio emission - so called radio relics (Fig. 1). The relics are believed to be powered by the energy input of the merger shock into an old (relic) relativistic electron population. This theory is supported by the observed radio polarization properties of the relics which closely match the polarization predicted by Ensslin et al. (1998) to result from synchrotron emission in shock-compressed magnetic fields. We report on an on-going project designed to obtain detailed Faraday rotation measure maps of these radio relics. The differential "Faraday screen" effect over extended radio sources allows us to study the magnetic field distribution inside these sources and in the foreground Faraday rotating medium.

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.

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.

Evidence for helical B-fields in the jets of BL Lac objects

ASTR SOC P 250 (2002) 180-183

DC Gabuzda, AB Pushkarev

The parsec-scale radio jets of BL Lacertae objects imaged with Very Long Baseline Interferometry (VLBI) show a number of distinctive features. The most striking of these is the dominance of transverse magnetic fields on the wide range of scales probed by VLBI observations at frequencies from 43 to 5 GHz. Some compact VLBI components with transverse fields axe undoubtedly relativistic shocks, in which the transverse field has been enhanced by compression. However, there is considerable evidence that we axe also detecting the toroidal component of an underlying helical magnetic field associated with the VLBI jet.

A high-resolution multi-wavelength study of the jet in 3C 273

ASTR SOC P 250 (2002) 243-247

S Jester, HJ Roser, K Meisenheimer, R Perley, S Garrington

We present HST images of the jet in 3C 273 at 622 nm and 300 nm and determine the variation of optical spectral index at 0.2 arcsec along the jet. We find no evidence for localized acceleration or loss sites: only slight changes in the spectral shape are observed throughout the jet. We consider this further evidence in favour of a distributed acceleration process.

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.

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.

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.

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.

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.

Simulations of relativistic jet formation in radio sources

ASTR SOC P 250 (2002) 10-21

DL Meier

Radio galaxies and quasars produce collimated, relativistic flows with. Lorentz factors of at least 15. It is generally believed that such flow Velocities indicate that jet acceleration and collimation occurs in the relativistic environment of a supermassive black hole. Recently, several groups around the world have begun to test theories of jet formation using magnetohydrodynamic (MHD) simulations of magnetized gas flow around black holes. This paper reviews the field of simulations of MHD jet formation, with an emphasis on producing the observed jet speeds and on the role that black hole angular momentum might play. Jet speeds are expected to be of order the escape velocity in the jet-formation region (bulk Lorentz factor similar to 2.4 if in the ergosphere of a rotating black hole), but could be potentially much higher if rapid acceleration can occur near the hole in less than a dynamical time. Transient simulations often produce tightly-collimated jets, but in a steady state jet collimation is generally quite slow and broad. Regardless of the source of the rotational energy powering the jet (accretion disc or black hole spin), the total jet power should be proportional to the black hole mass and the accretion rate. When the type of accretion disc is taken into account, it is shown that the most powerful jets should occur when the black hole is rotating rapidly and when the accretion disc is geometrically. thick and hot. The implications of this modified spin paradigm for explaining phenomenological properties of both supermassive and stellar mass black hole systems is discussed.

The origin of ultra high energy cosmic rays: where we are now and what the future holds

ASTR SOC P 250 (2002) 117-122

AA Watson

The observational picture for cosmic rays above 10(19) eV is described and the enigma that these results pose is discussed. The existence of particles above 10(20) eV may have an impact on our understanding of magnetic fields in intergalactic space and in possible sources.

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.