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.

Two types of radio galaxies: a new approach

ASTR SOC P 250 (2002) 281-289

J Eilek

We do not fully understand the dynamics and evolution of a radio galaxy. Models of classical double (FR II; Fanaroff & Riley 1974) sources are in a reasonable state, but these objects are rare. Non-FRII sources (generically called FRI) axe far more common, but much less well understood. In this paper I use the data to suggest possible new models for FRI sources, and discuss, the physical questions which these new models raise.

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.

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.

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.

Chandra X-ray observations of Cygnus A and Pictor A

ASTR SOC P 250 (2002) 213-223

AS Wilson, AJ Young, PL Shopbell

Results from Chandra observations of the two nearest, powerful radio galaxies are summarised.

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.

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.

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 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.

Numerical simulation of test particle diffusive shock acceleration in relativistic shocks with application to AGN 'central engines'

ASTR SOC P 250 (2002) 48-52

JJ Quenby, A Meli

Numerical modelling is performed for extreme relativistic parallel shocks with upstream Lorentz factors Gamma = 50. Assuming that the scattering is either large-angle or over pitch angles > Gamma(-1), spectral flattening and shock acceleration speed-up is found. The energy gain per cycle is similar to Gamma(2). The likely output from relativistic shocks due to the infall from the accretion disc to the AGN black hole is computed. Neutrinos from proton-gamma interactions may be detectable with planned neutrino telescopes but the gamma-ray output may contribute only 1/100th of the observed flux of 3C 273.

Equipartition in the hot-spots of 3C 123

ASTR SOC P 250 (2002) 234-237

MJ Hardcastle, M Birkinshaw, DM Worrall

Chandra X-ray Observatory observations of the powerful, peculiar radio galaxy 3C 123 have resulted in an X-ray detection of the bright eastern hot-spot, with a 1-keV flux density of similar to 5 nJy. The X-ray flux and spectrum of the hot-spot axe consistent with the X-rays being inverse-Compton emission from the population of electrons responsible for the radio emission, if the magnetic fields in the hot-spot are close to their equipartition values. 3C 123 is thus the third radio galaxy to show direct evidence for equipartition in hot-spots.

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).

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.

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.

Optical structure and physics of the M 87 jet

ASTR SOC P 250 (2002) 248-253

ES Perlman, WB Sparks, J Biretta, D Macchetto, JP Leahy

We summarize HST observations of the M 87 jet, concentrating on polarimetry and spectral index maps, and compare its optical and radio structures. The evidence now supports a stratified model for the structure of the jet, in which high-energy electrons emitting optical synchrotron radiation and their lower-energy, radio-emitting counterparts occupy separate regions of the jet, with different magnetic-field configurations. The higher-energy paxticles are closer to the jet axis, where the shocks that produce the knots in the inner jet appear to originate. Knot regions have optical spectra Which axe much flatter than the average for the jet, with the flattest-spectrum regions coinciding with flux maxima of the knots. These knots are preceded by regions where perpendicular apparent magnetic fields are seen. Thus not only do we see all the necessary ingredients for in situ particle acceleration in the knots, but there is now fairly direct evidence for it as well. By tracking the changes in radio-optical and optical spectral indices in the knot regions, we can estimate the relative acceleration. and cooling time-scales in the knots.

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.

Equipartition, inverse-Compton radiation, and PKS 0637-752

ASTR SOC P 250 (2002) 230-233

M Birkinshaw

The Chandra measurement of X-rays from the radio jet in PKS 0637-752 has implications for the source kinematics and energetics. This contribution discusses several possibilities for explaining the high X-ray output from the jet, emphasizing the impact on the assumption of equipartition between relativistic particles and fields in the jet plasma.