Publications by Katherine Blundell

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.

Acceleration by relativistic shock fronts

ASTR SOC P 250 (2002) 41-47

JG Kirk

Both a semi-analytic treatment and Monte-Carlo simulations of the problem of test particle acceleration at an ultra-relativistic shock predict a power-law spectrum of index d ln f/d ln p approximate to -4.2, where f is the phase space density and p the particle momentum. A brief review is given of this result, together with a discussion of its robustness and relevance to observations.

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.

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.

3-D general relativistic MHD simulations of generating jets

ASTR SOC P 250 (2002) 22-26

KI Nishikawa, S Koide, K Shibata, T Kudoh, H Sol

We have performed the first full 3-D GRMHD simulation of a Schwarzschild black hole with a freely falling corona. The preliminary simulation results show that the accretion disc is falling with the corona and that the proper pressure increases near the black hole, as in the previous axisymmetric simulations. We expect that in this case an instability around the black hole will occur as a result of the steep pressure gradient and the twisted magnetic fields. We plan to investigate how the instability affects jet formation.

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.

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.

Information on particles and fields from parsec and sub-parsec scale jets

ASTR SOC P 250 (2002) 80-92

GV Bicknell, SJ Wagner, BA Groves

With particular reference to the BL Lac object, Markarian 501, we emphasize the importance of X-ray and gamma-ray observations for elucidating the parameters of parsec scale and sub-parsec.scale jets. In particular the magnetic field, particle energy density and Doppler factor can be inferred from simultaneous observations. The energy flux in the Mkn 501 jet is particle dominated. If the composition is electron-proton then the minimum electron Lorentz factor, gamma(1) similar to 100. Consideration of a sample of quasar and BL Lac jets, shows that these jets may consist of electron-positron pairs, also if gamma(1) similar to 100. Seyfert jets may be examples of jets in which the plasma composition is predominantly electron-proton, suggesting that they originate at larger radii (with respect to the gravitational radius) than jets in radio-loud galaxies.

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.

Models of decelerating relativistic jets in 3C 31

ASTR SOC P 250 (2002) 269-275

RA Laing, AH Bridle

We model the brightness and polarization structure of the inner jets in the low-luminosity radio galaxy 3C 31 on the assumption that they are intrinsically symmetrical, axisymmetric relativistic jets. Our approach is to make parametrized models of velocity, emissivity and field ordering, to predict the radio emission and to optimize the parameters by fitting to deep, high-resolution VLA observations of Stokes I, Q and U. Our models axe in excellent agreement with the observations for an angle to the line of sight of approximate to 50degrees. The jets decelerate from v/c approximate to 0.9 to sub-relativistic speeds on several-kiloparsec scales. We use this velocity variation, together with the formulation of conservation of particle number, energy and momentum given by Bicknell (1994), to calculate the physical parameters of the flow. ROSAT observations constrain the external pressure distribution, allowing us to derive unique solutions for pressure, density, Mach number and mass flux as functions of distance from the nucleus. Both stellar mass injection and entrainment of the surrounding IGM axe likely to contribute to jet deceleration.

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.

Long term monitoring of the extreme intraday variable quasar PKS 0405-385

ASTR SOC P 250 (2002) 128-132

L Kedziora-Chudczer, DL Jauncey, JEJ Lovell, MA Walker, JP Macquart, MH Wieringa, AK Tzioumis, RA Perley, JE Reynolds

The quasar PKS 0405-385 exhibits episode's of unusually strong hourly variability at 8.6, 4.8, 2.4 and 1.4 GHz. These variations, seen first in June 1996, ceased after several weeks, but reappeared in November 1998 and were monitored closely at the. ATCA. We argue that our data are in agreement with interstellar scintillation of a source component which is a few micro-arcseconds in size. This claim is further supported by our recent finding of the time delay in the variability pattern measured between the ATCA and the VLA.

The early days of a radio source

ASTR SOC P 250 (2002) 404-407

D Dallacasa, C Stanghellini

We present a sample of bright compact sources with peaked radio spectra. The sample is likely to contain a mixture of a number of very young radio sources and some beamed objects whose radio emission is dominated by a single component. Flux density Variability is common among these sources, which we call "High Frequency Peakers" (HFPs).

Spectral ageing: a new age perspective

ASTR SOC P 250 (2002) 363-371

KM Blundell, S Rawlings

We present an up-to-date critique of the physical basis for the spectral ageing method. We find that the number of cases where this method may be meaningfully applied to deduce the ages of classical double radio sources is small indeed. This critique is much more than merely a re-expression of anxieties about the calibration of spectral ageing (which have been articulated by others in the past).

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.

Too hot, too fast or forever young?

ASTR SOC P 250 (2002) 133-136

J Dennett-Thorpe, AG de Bruyn

We present the results of a year-long WSRT monitoring campaign for the quasar J 1819+3845. The extreme variations (regularly 10% per minute) axe explained by interstellar scintillation of a source which is no more that 30 microarrseconds in diameter, with a corresponding brightness temperature of similar to 10(13) K.We use the results of this WSRT campaign to infer critical source parameters (size, structure and lifetime). To first order, the changes in the observed scintillation behaviour over the year are interpreted as being due to a peculiar velocity of the scattering plasma (similar to 20 pc distant), and not due to any changes within the source itself. We discus's the source structure on a size of tens of microarcseconds, and illustrate how such a monitoring campaign can yield such, information.

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.

Decelerating relativistic radio jets in B2 0755+379

ASTR SOC P 250 (2002) 276-280

M Bondi, P Parma, H de Ruiter, R Fanti, RA Laing

We apply a model for an adiabatically expanding relativistic jet to the radio galaxy B2 0755+379 using the observed surface brightness and jet width obtained from VLA and MERLIN radio images. We derive velocity profiles along the main jet for various assumed starting conditions, and show that these profiles axe consistent with the observed jet/counter-jet brightness ratio provided that the angle to the line of sight of the jet is theta similar or equal to 27degrees and the starting velocity of the jet (actually. the velocity at 0.5 kpc from the nucleus) is similar or equal to 0.9c.