arXiv subjects

M. J. Hardcastle

Publications and source records attributed to M. J. Hardcastle.

306 records · Page 17Linked to original sources

arXiv2000

The environments of FRII radio sources

Using ROSAT observations, we estimate gas pressures in the X-ray-emitting medium surrounding 63 FRII radio galaxies and quasars. We compare these pressures with the internal pressures of the radio-emitting plasma estimated by assuming minimum energy or equipartition. In the majority of cases (including 12/13 sources with modelled, spatially resolved X-ray emission) radio sources appear to be underpressured with respect to the external medium, suggesting that simple minimum-energy arguments underestimate the sources' internal energy density. We discuss possible departures from the minimum energy condition and the consequences of our result for models of the dynamics of radio galaxies, in particular self-similar models (Kaiser & Alexander 1997).

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arXiv2000

The Infrared Jet In 3C66B

We present images of infrared emission from the radio jet in 3C66B. Data at three wavelengths (4.5, 6.75 and 14.5 microns) were obtained using the Infrared Space Observatory. The 6.75 micron image clearly shows an extension aligned with the radio structure. The jet was also detected in the 14.5 micron image, but not at 4.5 micron. The radio-infrared-optical spectrum of the jet can be interpreted as synchrotron emission from a population of electrons with a high-energy break of 4e11 eV. We place upper limits on the IR flux from the radio counter-jet. A symmetrical, relativistically beamed twin-jet structure is consistent with our results if the jets consist of multiple components.

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arXiv2000

Determining the reality of X-ray filaments

A number of authors have reported filaments connecting bright structures in high-resolution X-ray images, and in some cases these have been taken as evidence for a physical connection between the structures, which might be thought to provide support for a model with non-cosmological redshifts. In this paper I point out two problems which are inherent in the interpretation of smoothed photon-limited data of this kind, and develop some simple techniques for the assessment of the reality of X-ray filaments, which can be applied to either simply smoothed or adaptively smoothed data. To illustrate the usefulness of these techniques, I apply them to archival ROSAT observations of galaxies and quasars previously analysed by others. I show that several reported filamentary structures connecting X-ray sources are not in fact significantly detected.

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arXiv1999

Radio, optical and X-ray nuclei in nearby 3CRR radio galaxies

HST observations have shown that low-redshift 3CR radio galaxies often exhibit a point-like optical component positionally coincident with the GHz-frequency radio core. In this paper we discuss the correlation between the luminosities of the radio, optical and X-ray cores in these objects, and argue that all three components have a common origin at the base of the relativistic jets. In unified models, FRI radio galaxies should appear as dimmed, redshifted versions of BL Lac objects. We show that such models are consistent with the spectral energy distributions of the radio galaxies only if the nuclear X-ray emission in radio galaxies is inverse-Compton in origin.

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arXiv1999

Sub-Arcsecond Imaging of 3C123:108-GHz Continuum Observations of the Radio Hotspots

We present the results of sub-arcsecond 108 GHz continuum interferometric observations toward the radio luminous galaxy 3C123. Using multi-array observations, we utilize the high u,v dynamic range of the BIMA millimeter array to sample fully spatial scales ranging from 0.5" to 50". This allows us to make one-to-one comparisons of millimeter-wavelength emission in the radio lobes and hotspots to VLA centimeter observations at 1.4, 4.9, 8.4, and 15 GHz. At 108 GHz, the bright, eastern double hotspot in the southern lobe is resolved. This is only the second time that a multiple hotspot region has been resolved in the millimeter regime. We model the synchrotron spectra of the hotspots and radio lobes using simple broken power-law models with high energy cutoffs, and discuss the hotspot spectra and their implications for models of multiple hotspot formation.

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arXiv1999

Chandra X-Ray Detection of the Radio Hotspots of 3C295

An observation of the radio galaxy 3C295 during the calibration phase of the Chandra X-ray Observatory reveals X-ray emission from the core of the galaxy, from each of the two prominent radio hotspots, and from the previously known cluster gas. We discuss the possible emission processes for the hotspots and argue that a synchrotron self-Compton model is preferred for most or all of the observed X-ray emission. SSC models with near equipartition fields thus explain the X-ray emission from the hotspots in the two highest surface brightness FRII radio galaxies, Cygnus A and 3C295. This lends weight to the assumption of equipartition and suggests that relativistic protons do not dominate the particle energy density.

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arXiv1999

The complex radio spectrum of 3C130

I present four-frequency radio observations of the wide-angle tail radio galaxy 3C130. By a technique of simulated observations, I assess the systematic errors in the data due to the varying uv plane coverage in the different observations used. Using spectral tomography I show that, at least in the southern plume, the source can be represented by a two-component model consisting of a flat-spectrum core and steep-spectrum sheath, as recently found in other FRI sources. In addition, there is a strong difference in the high-frequency spectra of the northern and southern plumes. I discuss the implications of these observations for the source's jet dynamics and particle acceleration, and discuss models of the class of WATs as a whole.

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arXiv1999

ROSAT X-ray observations of 3CRR radio sources

Over half the 3CRR sample of radio galaxies and quasars has been observed in X-rays with ROSAT pointed observations, and we present results from these observations, discussing many of the sources in detail. The improved spatial resolution of ROSAT over earlier missions allows a better separation of the nuclear and extended components of the X-ray emission. We investigate the relationship between nuclear X-ray and core radio luminosity and show that our results support a model in which every radio galaxy and quasar has a beamed nuclear soft X-ray component directly related to the radio core. We report evidence for rich cluster environments around several powerful quasars. These X-ray environments are comparable to those of high-redshift radio galaxies.

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arXiv1999

X-ray observations of low-power radio galaxies from the B2 catalogue

We present an analysis of X-ray data, taken with ROSAT, for a well defined sample of low-power radio galaxies from the Bologna B2 catalogue. Where possible, the HRI has been used in order to take advantage of the higher spatial resolution provided by this instrument. A variety of models are fitted to radial profiles in order to separate the resolved and unresolved X-ray emission from the galaxies. We demonstrate a strong, approximately linear, correlation between the luminosities of the unresolved X-ray components and the 5-GHz luminosities of the radio cores in this sample. This suggests a physical relationship between the soft X-ray emission of radio galaxies and the jet-generated radio core emission. We infer a nuclear jet-related origin for at least some of the X-ray emission.

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arXiv1998

Extended X-ray emission from the BL Lac object PKS 0521-365

Models that seek to unify BL Lacs and low-power radio galaxies predict that the two types of object should show similar isotropically emitted X-ray emission. Testing this is usually limited by difficulties in separating strong X-ray emission from a BL Lac nucleus and surrounding low-surface brightness emission. In this paper we report ROSAT HRI observations of the z=0.055 BL Lac object PKS 0521-365. We are able to separate a luminous extended X-ray component from the bright nucleus. Using a new radio map, we show that it is unlikely that the extended emission is due to inverse-Compton scattering of photons from the active nucleus, and instead interpret it as thermal emission from dense, rapidly cooling gas. This is a more extreme environment than is found in typical FRI radio galaxies, and may pose a problem for unified models.

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arXiv1998

FRII radio galaxies with z<0.3 -- II. Beaming and unification

In a previous paper we presented measurements of the properties of jets and cores in a large sample of FRII radio galaxies with z<0.3. Here we test, by means of Monte Carlo simulations, the consistency of those data with models in which the prominences of cores and jets are determined by relativistic beaming. We conclude that relativistic beaming is needed to explain the relationships between core and jet prominences, and that speeds between 0.5 and 0.7c on kpc scales provide the best fits to the data.

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arXiv1998

Extended and compact X-ray emission from the powerful radio galaxy 3C220.1

We report on ROSAT HRI observations of the z=0.61 radio galaxy 3C220.1. The X-ray emission from this object consists of an extended component, which we attribute to luminous cluster emission, and a compact central source. The compact component is too bright to be modelled as a cooling flow under some plausible assumptions for the hot gas temperature and distribution; we suggest instead that it is directly related to the core of the radio source. The X-ray flux of the compact component is consistent with the prediction of Worrall et al (1994) that all powerful radio galaxies should have a central jet-related X-ray emission component that is proportional in strength to the radio core flux density. Other observations of distant 3CR radio sources are consistent with this model.

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arXiv1998

Jets, plumes and hot spots in the wide-angle tail source 3C130

I present 1.5- and 8.4-GHz observations with all configurations of the NRAO VLA of the wide-angle tail source 3C130. The source has a pair of relatively symmetrical, well-collimated inner jets, one of which terminates in a compact hot spot. Archival ROSAT PSPC data confirm that 3C130's environment is a luminous cluster with little sign of sub-structure in the X-ray-emitting plasma. I compare the source to other wide-angle tail objects and discuss the properties of the class as a whole. None of the currently popular models is entirely satisfactory in accounting for the disruption of the jets in 3C130.

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arXiv1998

Dynamics of the radio galaxy 3C449

We present ROSAT PSPC observations of the twin-jet radio galaxy 3C449. The soft X-ray emission from this object is dominated by an extended halo with a scale comparable to that of the radio source. The asymmetry of the X-ray emission is reflected in that of the radio lobes, providing evidence that the behaviour of the jets is strongly influenced by the external medium. A region of reduced X-ray surface brightness coincident with the southern radio lobe of 3C449 suggests that the radio source has displaced thermal plasma from the X-ray-emitting halo. However, the minimum pressure in the radio lobe is considerably lower than our estimates of the pressure in the external medium. We discuss the implications for the dynamics of the radio source.

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arXiv1998

Intensive monitoring of the strongly variable BL Lac S5 0716+714

The BL Lac object S5 0716+714 was monitored during a multifrequency campaign in 1996. Preliminary analysis of the optical, ROSAT and RXTE data are presented. Strong variability on short time scales was observed. The data suggest an interpretation within a multi-component model.

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arXiv1998

FR II radio galaxies with z<0.3 -- I. Properties of jets, cores and hot spots

In previous papers we have discussed high-resolution observations of a large sample of powerful radio galaxies with z<0.3. Jets are detected in up to 80 per cent of the sample, and radio cores in nearly all the objects; in addition, we are able to resolve the hot spots in most sources. In this paper we present measurements of the radio properties of these components. The prominences of the jets detected do not appear to be a function of radio luminosity, providing the clearest evidence yet that the reported low detection rate of jets in radio galaxies has been an artefact of low-sensitivity observations. We find a positive correlation between the total source length and core prominence in the narrow-line radio galaxies. We have found evidence for a relationship between hot spot size and total source size, but few other significant relationships between hot spot properties and those of the jets or lobes. We compare our measurements to those of Bridle et al (1994), based on observations of a sample of quasars, and argue that the results are consistent with a modification of the unified model in which the broad-line radio galaxies are the low-luminosity counterparts of quasars, although the situation is complicated by contamination with low-excitation radio galaxies which appear to have radio properties different from the high-excitation objects. We discuss the classes of empirical model that can be fitted to the dataset.

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arXiv1997

Magnetic field strengths in the hot spots of 3C33 and 3C111

We report on ROSAT HRI observations of the nearby powerful radio galaxies 3C33 and 3C111, which both have detected optical hot spots. We find nuclear X-ray sources in both objects, but no X-ray emission from the hot spots. This confirms the presence of a high-energy cutoff in the spectrum of synchrotron-emitting electrons. Since these electrons necessarily scatter the synchrotron photons by the inverse-Compton process, our upper limits on the X-ray fluxes of the hot spots allow us to set lower limits of a few nanotesla on their magnetic flux density, close to or greater than the fields implied by equipartition of energy between radiating particles and magnetic field.

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arXiv1997

The jets in 3C 296

We present observations made with the VLA at 1.5 and 8.4 GHz of the nearby FRI radio galaxy 3C296. The most recent models of FRI radio galaxies suggest that substantial deceleration must take place in their jets, with strongly relativistic velocities on parsec scales giving place to at most mildly relativistic velocities on scales of tens of kiloparsecs. The region over which this deceleration takes place is therefore of considerable interest. By considering the side-to-side asymmetries of the jets of 3C296, we constrain the region of strong deceleration in the source. Our observations show evidence that the jets have slow edges surrounding faster central spines. We discuss the implications of our observations for models of the magnetic field structure in these objects.

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