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.

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.

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.

Deflection of jets induced by jet-cloud interactions

ASTR SOC P 250 (2002) 462-466

S Mendoza, MS Longair

Non-relativistic and relativistic models in which astrophysical jets are deflected on passing through an isothermal high density region are analysed. The criteria for the stability of jets to be affected by the formation of internal shocks are discussed.

The large-scale structure of 3C 31

ASTR SOC P 250 (2002) 437-442

RA Laing, P Parma, M Murgia, L Feretti, G Giovannini, AH Bridle, RA Perley

The results of a multifrequency VLA imaging study of the nearby radio galaxy 3C 31 are briefly summarized. The transition between jets and lobes is much more complex than was apparent from earlier observations, and is associated with significant variations in spectral index. We demonstrate that the known depolarization asymmetry in 3C 31 is caused by foreground Faraday rotation in the halo of the host galaxy, but the details of the associated field and density structure are not yet clear.

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.

AGN and cooling flows

ASTR SOC P 250 (2002) 481-486

J Binney

For two decades the steady-state cooling-flow model has dominated-the literature of cluster and elliptical-galaxy X-ray sources. For ten years this model has been in severe difficulty from a theoretical point of view, and it is now coming under increasing pressure observationally A small number of enthusiasts have argued for a radically different interpretation of the data, but had little impacton prevailing opinion be-causeAhe unsteady heating picture that they-advocate is extremely hard to work out in detail. Here I explain why it is difficult to extract robust observational predictions from the heating picture. Major problems include the variability of the sources, the different ways in which a bi-polar flow can impact on X-ray emission, the weakness of synchrotron emission from sub-relativistic flows, and the sensitivity of synchrotron emission to a magnetic field that is probably highly localized.

Double-double radio galaxies: probing duty cycles in AGN and the cocoons of powerful radio sources

ASTR SOC P 250 (2002) 408-411

A Schoenmakers

Recently, we have identified a small number of extended radio sources that consist of two double-lobed radio sources, which axe well aligned and centered on a common nucleus. We have called these 'Double-double radio galaxies' (DDRGs). The observed structures strongly suggest interrupted central activity as the origin of these sources. We have developed a model based on this assumption with which we are able to explain several observed properties. Since the inner structures of the DDRGs advance inside the cocoon originally formed by the outer lobes, these allow us to probe the cocoon medium. Our results indicate that the cocoon density must be higher than has been assumed in earlier models.

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.

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.

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.

MHD mechanisms for jet formation

ASTR SOC P 250 (2002) 1-9

MC Begelman

I will discuss several issues related to the acceleration and collimation of jets from AGNs. Hydromagnetic stresses provide the best bet both for accelerating relativistic flows And for providing a certain amount of initial collimation. However, there are limits to how much self-collimation can be achieved without the help of an external pressurised medium. Moreover, existing models which postulate highly organized poloidal magnetic flux over much of the flow may be unrealistic. Instead, a large fraction of the magnetic energy may reside in a highly disorganized "chaotic" field. Such a field can also accelerate the flow to relativistic speeds, in some cases with greater efficiency than highly organized fields, but at the expense of self-collimation. In any case, acceleration to highly relativistic speeds may be a gradual process, occurring over several decades in radius.

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.

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.

VLA images of Virgo A and their implications

ASTR SOC P 250 (2002) 422-427

F Owen, J Eilek

Results of recent imaging observations of Virgo A (M 87) are presented. The flow seen in the inner region as the M 87 jet appears to continue out onto much large scales, up to 35 kpc in projection from the cluster core. The energy flux, which supposedly originates in the black hole in the M 87 core, appears. ultimately to be deposited in the hot X-ray gas in the cluster core in the form of heat and work in blowing up two large bubbles. The input energy from the jet currently exceeds the radiative cooling by about a factor of ten or more. This probably transient phenomenon may play an important role in the life history of a cluster core but requires the study of many more examples to understand fully.