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 interface between radio jets and ionized gas clouds

ASTR SOC P 250 (2002) 467-470

SM Simkin

Optical spectra of the narrow, disturbed, interface regions between radio jets and gas clouds in the interstellar medium of the host galaxies of two nearby radio sources have diagnostic (BPT) diagrams for line ratios of [NII], [OIII], [SII], Halpha and Hbeta which are inconsistent with published models for shock heating, as well as those for photoionization/excitation by either a "hard" continuum or flux from ordinary hot stars. This may indicate the need for additional work on these models or it may indicate that the ionized gas in the jet/cloud interface has an energy distribution which is partly characterised by that in the radio jet plasma itself If the latter is correct, then careful measurements of these interface regions may allow us to distinguish between beams of particles composed of electrons and positrons and those composed of electrons and protons.

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.

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.

Models of polarized and variable radio emission in the IDV source 0917+624

ASTR SOC P 250 (2002) 147-151

T Beckert, A Kraus, TP Krichbaum, A Witzel, JA Zensus

We examine the power spectrum of intra-day variability and describe the new information which can be derived from wavelet analysis of light curves of the quasar 0917+624. Results for total and polarized flux at 11 cm are shown. Both interstellar scattering and intrinsic models have difficulties in explaining the similar to 1 day period variations. A theoretical model for the time-averaged emission is presented. This provides a basis for the analysis of possible variations.

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.

ASCA observations of inverse-Compton X-rays from radio lobes and the field-particle energy densities

ASTR SOC P 250 (2002) 389-393

M Tashiro

ASCA observations of inverse-Compton (IC) X-rays from radio lobes are presented. By comparing the IC X-ray flux with the synchrotron radio flux, the electron energy density and magnetic field intensity in the radio lobes have been determined separately without a minimum energy assumption (Tashiro et al. 1998; Kaneda et al. 1995; Feigelson et al. 1995). X-ray images of Fornax A and PKS B2356-611 obtained recently with ASCA reveal positional discrepancies between Xray and radio brightness distributions. Spatial distributions of field and particle energy density suggested by the results are discussed.

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).

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.

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.

Jet speeds in FRI and FRII radio sources

ASTR SOC P 250 (2002) 264-268

TG Arshakian

Analytical relations between the mean jet speed and the mean jet flux asymmetry and between their variances are derived in the context of a unified scheme. The mean jet speeds of classical double radio sources axe estimated by using the asymmetry of jet-counter-Jet flux densities taken from the 3CRR and B2 samples. For FRI radio sources the mean jet speed is similar to 0.54 +/- 0.03c, while for FRII low-redshift radio galaxies and intermediate-redshift quasars the values found are similar to 0.4c +/- 0.06c and greater than or equal to 0.6c respectively.

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.

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.

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.

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.