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.

Circular polarization in scintillating sources

ASTR SOC P 250 (2002) 137-141

JP Macquart, L Kedziora-Chudczer, DL Jauncey, R Sault, DP Rayner

We discuss the detection of variable circular. polarization (CP) in several scintillating radio sources: PKS 1519-273, PKS 0405-385 and Sgr A*. The CP in PKS 1519-273 is strong, and varies on a timescale of hours to days at frequencies between 1.4 and 8.6 GHz. We argue that the variability is due to scintillation of a compact (15 - 35 muas) component of the source with -3.8 +/- 0.4% circular polarization at 4.8 GHz. We find that no simple model can account for the magnitude and spectrum of the circular polarization in this source. We also interpret the variable CP observed in PKS 0405-385 in terms of scintillation, although the behaviour of the CP is more complicated, and changes character between scintillation epochs. We also discuss CP in Sgr A*, which is found to be variable at 2.5 and 1.4 GHz on a timescale similar to 7 days. The fractional degree of variability in circular polarization greatly exceeds that in total intensity in all three sources.

High energy emission in blazars

ASTR SOC P 250 (2002) 93-99

A Celotti

Different scenarios which have been proposed for the production of high energy radiation in blazars will be briefly summarized. Recent results on the energy spectral distribution of high redshift radio-loud quasars are then considered in the context of the blazar phenomenology and their consequences for our understanding of jets are discussed. Finally, a possible scenario for the origin of X-ray emission from large scale jets, as detected by Chandra, is presented.

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.

What triggers radio galaxies?

ESO ASTROPHY SYMP (2002) 125-127

MJ Cruz, KM Blundell

We are investigating possible triggering scenarios which may give rise to powerful jetted active galaxies such as classical double radio galaxies. Important clues come from studying those objects which have most recently been triggered. The newly discovered 'Youth-Redshift Degeneracy' for classical double radio galaxies means that the highest redshift objects in a flux-limited survey will be observed to be significantly younger (more recently triggered) than the more nearby objects. We are pursuing this investigation with a new sample specially filtered to favour the detection of high-redshift radio galaxies. We will present some preliminary results of a near-IR imaging campaign using the UIFTI on the UKIRT.

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.

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.

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.

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.

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 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.

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.

Testing self-similar models of powerful radio sources

ASTR SOC P 250 (2002) 319-323

JC Carvalho, CP O'Dea, SA Baum

We have carried out 2-D axisymmetric numerical simulations of light, supersonic jets. These simulations are used to test the hypothesis of self-similarity for the propagation of jets in powerful radio galaxies. A large range of jet parameters has been explored and we find that the jet flow is not always self-similar. Instead, it undergoes up to three phases which, individually, can be considered to be scale-free although, the jet does not exhibit self-similar behaviour during the transition. The number of phases that a jet goes through and their duration seem to depend upon the initial Mach number.

Particles and fields in radio galaxies: a summary

ASTR SOC P 250 (2002) 487-498

RD Blandford

A summary is presented of a meeting on Particles and Fields in Radio Galaxies held in Oxford in August 2000. Recent detailed studies of radio maps and the first X-ray images from the Chandra X-ray Observatory, together with new capabilities in simulating hydromagnetic flows, are transforming our understanding of how jets are formed by accretion disks orbiting massive black holes, how these jets dissipate and how they inflate the giant radio-emitting lobes by which they were first identified. As they can be imaged in such detail, extragalactic radio sources remain central to the study of jet outflows in general although there is a strong convergence with corresponding studies of Galactic superluminal sources, gamma ray bursts and young stellar objects..

Gamma-ray bursts from black hole winds

ASTR SOC P 250 (2002) 36-40

M van Putten

Cosmological gamma-ray bursts are probably powered by high-angular momentum black hole-torus systems, formed by black hole-neutron star coalescence or magnetized collapsars. Rapidly rotating black holes surrounded by torus magnetospheres are magnetized in their ground state. In this state, the black hole is connected to infinity by,an open flux-tube of magnetic field-lines. Differential. frame-dragging permits in-situ pair-production along these flux-tubes. HETE-II may bring our understanding of short bursts on pax with that of long bursts.

Radio galaxy spectra

ASTR SOC P 250 (2002) 400-403

CA Jackson, JV Wall

Radio spectra of radio galaxies are often ascribed a simple power law form (S-v proportional to v(alpha)) with the spectral index, alpha, being approximately -0.7. However, all radio galaxies deviate from this simple power law behaviour. In this paper we derive simple expressions for the average rest-frame spectra of FRI and FRII radio galaxies. These will be used to describe the spectral curvature of the parent (FRI and FRII) populations in models of radio source evolution.

Parsec-scale radio morphology in Seyfert galaxies

ASTR SOC P 250 (2002) 191-194

DV Lal, P Shastri, DC Gabuzda

We have observed a sample of Seyfert galaxies with global VLBI in order to test the predictions of the Unified Scheme for Seyferts, which hypothesises that Seyferts of type 1 and of type 2 differ only in the orientation of the axisymmetric active nucleus with respect to the observer. In this case, the parsec-scale radio structures of the two types should be similar. The 10 Seyfert 1s and 10 Seyfert 2s in our sample have been selected to have similar distributions of redshift and properties of their host galaxies: [OIII] luminosity, galaxy bulge luminosity etc. In this way, we ensure that the two subsamples of Seyferts are matched with respect to properties that axe expected to be orientation-independent. We are thus able to test the Unified Scheme rigorously. We detect all the objects that we observed and discuss the results. Our results are consistent with the prediction of the simple Unified Scheme.

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.

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.