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.

On the origin of the Fanaroff-Riley dichotomy

ASTR SOC P 250 (2002) 290-293

Gopal-Krishna, PJ Wiita

A small fraction of double radio sources show a peculiar and striking hybrid morphology; they have a distinctly FRI structure on one side of the nucleus, and a FRII structure on the other. We argue that the mere existence of these HYMORS is quite incompatible with the theoretical explanations for the Fanaxoff-Riley dichotomy that are based upon the nature of the jet plasma, or those invoking an intrinsic property of the central engine. Rather, these HYMORS strongly support models that explain the difference between FRI and FRII sources in terms of asymmetry of interaction of the jets with the external environments. We further show that a model for radio source dynamics we had earlier proposed can neatly reproduce the observed dependence of the radio power dividing the two FR classes on the optical luminosity of the host galaxy, as found by Owen & White and Ledlow Owen.

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.

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.

The hydrodynamics of radio galaxy cocoons

ASTR SOC P 250 (2002) 449-453

CS Reynolds

We use numerical simulations to discuss some aspects of the hydrodynamics of radio galaxies, and the affect that hydrodynamical considerations have on the long term evolution of such sources. Using these simulations, we can start to assess the impact that a radio galaxy may have on the energy and entropy budget of its host galaxy or cluster. A full hydrodynamic treatment of radio galaxy expansion is also necessary to interpret correctly the X-ray shells and cavities that, are seen in some sources by ROSAT and Chandra.

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.

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.

Jet formation: magnetic fields and accretion discs

ASTR SOC P 250 (2002) 27-31

F Casse, J Ferreira

We present new results on magnetized accretion discs launching bipolar, self-collimated jets. We show that the jet mass-loading depends critically on the disc vertical equilibrium. In particular, it is strongly influenced by any entropy generation occurring at the disc surface. Discs around supermassive black holes and close to a magnetohydrostatic balance have the potential to produce mildly relativistic jets.

The environments of FR II radio sources

ASTR SOC P 250 (2002) 358-362

MJ Hardcastle, DM Worrall

Using ROSA T observations, we estimate gas pressures in the X-ray-emitting media surrounding 63 FR II radio galaxies and quasars. We compare these pressures with the internal pressures of the radio-emitting plasma estimated by assuming minimum energy or equipartition. The majority of the radio sources (including 12/13 sources with modelled, spatially resolved X-ray emission) Appear to be underpressured with respect to the external medium, suggesting that simple minimum-energy arguments underestimate the sources' internal energy densities. Some consequences of this result are discussed.

MHD disc-wind solutions crossing all the singularities

ASTR SOC P 250 (2002) 32-35

E Trussoni, N Vlahakis, K Tsinganos, C Sauty

We extend the model of Blandford & Payne (1982) for magnetocentrifugally driven winds by presenting solutions that satisfy the singularity conditions at all critical surfaces. In these solutions, the asymptotic supercritical zone is causally disconnected from the upstream region,of the outflow, unlike the,situation in previous studies.

NGC 6251 at multiple scales and wavelengths

ASTR SOC P 250 (2002) 294-297

PN Werner, DM Worrall, M Birkinshaw

We have studied the FRI radio galaxy NGC 6251 and its environment at several wavelengths and scale lengths. On the large scale, we have probed the gravity field by measuring the velocity dispersion of the cluster members associated with NGC 6251 and relating this to, the cluster's X-ray emission. On the small scale, the gravitational information is provided by cold HI near the nucleus and the distribution of stars and gas near the centre of the galaxy. The cold HI gas which we have measured explains the absorption of the central X-ray emission and is consistent with the extinction through the recently discovered HST gas disc of NGC 6251.

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.

The interaction of radio sources and cooling flows

ASTR SOC P 250 (2002) 471-480

AC Fabian

The X-ray emission in many clusters of galaxies shows a central peak in surface brightness coincident with a drop in temperature. These characterize a cooling flow. There is also often a radio source at the centre of such a region. Data from Chandra now enable us to map the interaction between the radio source and the intracluster medium. Preliminary work shows no sign of heating of the gas beyond the radio lobes, which are often devoid of cooler gas and so appear as holes. In the case of the Perseus cluster around 3C 84, the coolest X-ray emitting gas occurs immediately around the inner radio lobes.

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.

Face-on dust discs in galaxies with optical jets

ASTR SOC P 250 (2002) 254-258

WB Sparks, SA Baum, J Biretta, FD Macchetto, A Martel

The presence of optical synchrotron jets in radio galaxies is relatively unusual. We show that of the nearest five FRI 3CR radio galaxies showing optical jets, four display evidence for almost circular, presumably face-on, dust discs. None of the other twenty nearby FRIs in our sample show circular dust discs, although dust is found in 19/20 cases. This is strong support for the two-fold idea that (1) jets emerge close to perpendicular to inner gas discs and (2) optical non-thermal synchrotron emission is seen only when the jet points towards the observer. The implied critical angle to the line-of-sight is approximately 30-40 degrees, i.e. if the angle of the jet to the line-of-sight is less than about 35 degrees we see an optical jet. The corresponding Lorentz factor is gamma approximate to 1.5, which is consistent with current observations of jet proper motion that show an apparent velocity range from; approximate to 0.6c to 6c for M 87 in the optical (Biretta, Sparks & Macchetto 1999). The relatively low speeds implied by the dust discs may be due to a global deceleration of the jet as in unified theories, or else to stratification within the jet. Unresolved nuclei are common in the optical. Their luminosities are also consistent with the beaming concept when compared to inclination inferred from the dust lanes. The disc sizes are typically several hundred parsecs, to kiloparsec size. The galaxy with an optical jet that does not show a face-on disc, M 87, instead has more complex radial dust and ionized gas filaments.

The evolution of classical double radio galaxies

LECT NOTES PHYS 589 (2002) 71-87

KM Blundell

Being immensely powerful, and hence detectable out to great distances, classical double radio galaxies have long been recognised as cosmological probes of great potential. Before this potential can be realised, it is necessary to understand the physical mechanisms by which these objects evolve and change with time. This chapter describes how to deduce from classical double radio source observables (luminosity, spectral index, redshift and linear size) the essential nature of how these objects evolve and the true relationships between the underlying physical parameters (jet-power, age etc). I discuss the key role played by hotspots in governing the energy distribution of the lobes they feed, and subsequent spectral evolution.