arXiv subjects

M. J. Hardcastle

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

At least 271 records · Page 15Linked to original sources

arXiv2005

A Chandra and XMM-Newton study of the wide-angle tail radio galaxy 3C465

We have observed the prototypical wide-angle tail radio galaxy 3C465 with Chandra and XMM-Newton. X-ray emission is detected from the active nucleus and the inner radio jet, as well as a small-scale, cool component of thermal emission, a number of the individual galaxies of the host cluster (Abell 2634), and the hotter thermal emission from the cluster itself. The X-ray detection of the jet allows us to argue that synchrotron emission may be an important mechanism in other well-collimated, fast jets, including those of classical double radio sources. The bases of the radio plumes are not detected in the X-ray, which supports the model in which these plumes are physically different from the twin jets of lower-power radio galaxies. The plumes are in fact spatially coincident with deficits of X-ray emission on large scales, which argues that they contain little thermal material at the cluster temperature, although the minimum pressures throughout the source are lower than the external pressures estimated from the observed thermal emission. Our observations confirm both spatially and spectrally that a component of dense, cool gas with a short cooling time is associated with the central galaxy. However, there is no evidence for the kind of discontinuity in external properties that would be required in many models of the jet-plume transition in WATs. Although the WAT jet-plume transition appears likely to be related to the interface between this central cool component and the hotter intra-cluster medium, the mechanism for WAT formation remains unclear. We revisit the question of the bending of WAT plumes, and show that the plumes can be bent by plausible bulk motions of the intra-cluster medium, or by motion of the host galaxy with respect to the cluster, as long as the plumes are light.

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arXiv2005

Interactions of Radio Galaxies and the Intra-Cluster Medium in Abell 160 and Abell 2462

We present Chandra and VLA observations of two galaxy clusters, Abell 160 and Abell 2462, whose brightest cluster galaxies (BCGs) host wide angle tailed radio galaxies (WATs). We search for evidence of interactions between the radio emission and the hot, X-ray emitting gas, and test various jet termination models. We find that both clusters have cool BCGs at the cluster centre, and that the scale of these cores ~30-40kpc for both sources) is of approximately the same scale as the length of the radio jets. For both sources, the jet flaring point is coincident with a steepening in the host cluster's temperature gradient, and similar results are found for 3C465 and Hydra A. However, none of the published models of WAT formation offer a satisfactory explanation as to why this may be the case. Therefore it is unclear what causes the sudden transition between the jet and the plume. Without accurate modelling, we cannot ascertain whether the steepening of the temperature gradient is the main cause of the transition, or merely a tracer of an underlying process.

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arXiv2005

Low-frequency constraints on the spectra of the lobes of the microquasar GRS 1758-258

I present low-frequency observations of the lobes of the microquasar GRS 1758-258, which, together with earlier radio observations, provide the best constraints yet on the lobe spectra, and show that they are consistent with the electron energy indices expected from standard shock acceleration models. There is no statistically significant evidence for synchrotron ageing along the length of the lobes, which is consistent with expectation if the magnetic field strength is close to the minimum-energy value. If the environment of the lobes has typical ISM properties, the lobes will dissipate on timescales of thousands of years or less, long before synchrotron ageing can become significant.

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arXiv2005

A Chandra Study of the Multi-Component X-ray Emission from the X-shaped Radio Galaxy 3C 403

We present results from a 49.4 ks Chandra/ACIS-S observation of the nearby ($z$=0.059) X-shaped FRII radio galaxy 3C~403. This is the first Chandra observation of an X-shaped radio galaxy, and one of the goals of this pioneering study is to determine the relationship between the X-ray emitting gas and the X-shaped radio morphology. The X-ray isophotes of the hot gas within $\sim3.5''$ of the central galaxy are highly elliptical (eccentricity$\sim$0.57) and co-aligned with the elliptical optical isophotes. This supports the hypothesis that X-shaped radio sources are created by propagation of jets through asymmetric density distributions. Within large uncertainties, there is no evidence that the lobes or wings are overpressurized relative to the ISM. We have detected X-ray emission from several of the radio knots to the E of the active nucleus, and diffuse emission from the radio lobe to the W. The X-ray emission from the eastern knots cannot be explained by an inverse Compton model unless they are far from equipartition. Using archival HST data, optical emission is detected from two knots, and the radio/optical/X-ray spectra are well fitted by simple synchrotron models. This is one of the strongest examples to date of X-ray synchrotron emission from multiple knots in the jet of an FR II radio galaxy. X-ray emission is also detected from the radio wings at a flux consistent with inverse Compton scattering of CMB photons from relativistic electrons if the wings are near equipartition. The nuclear spectrum is well described by a multi-component model that includes a heavily absorbed power law and a bright, broadened Fe line. A second, less absorbed, power-law component, likely to represent unresolved emission from a pc-scale jet, is also required.

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arXiv2004

A Chandra observation of the X-ray environment and jet of 3C296

We have observed the twin-jet radio galaxy 3C296 with Chandra. X-ray emission is detected from the nucleus, from the inner parts of the radio jet, and from a small-scale thermal environment around the jet deceleration region. As we have found in previous observations of other twin-jet radio galaxies, the X-ray jet and a steep pressure gradient in the external thermal environment are associated with the region where strong bulk deceleration of the jet material is suggested by radio observations. Our observations provide additional evidence that the inner jets of twin-jet objects are always associated with a relatively cool, dense central X-ray emitting component with a short cooling time.

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arXiv2004

Evidence for radio-source heating of groups

We report evidence that the gas properties of X-ray groups containing radio galaxies differ from those of radio-quiet groups. For a well-studied sample of ROSAT-observed groups, we found that more than half of the elliptical-dominated groups can be considered ``radio-loud'', and that radio-loud groups are likely to be hotter at a given X-ray luminosity than radio-quiet groups. We tested three different models for the origin of the effect and conclude that radio-source heating is the most likely explanation. We found several examples of groups where there is strong evidence from Chandra or XMM-Newton images for interactions between the radio source and the group gas. A variety of radio-source heating processes are important, including shock-heating by young sources and gentler heating by larger sources. The heating effects can be longer-lasting than the radio emission. We show that the sample of X-ray groups used in our study is not significantly biased in the fraction of radio-loud groups that it contains. This allows us to conclude that the energy per particle that low-power radio galaxies can inject over the group lifetime is comparable to the requirements of structure formation models.

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arXiv2004

Cen A and its interaction with the x-ray emitting interstellar medium

As the closest radio galaxy, Centaurus A is a powerful laboratory for the X-ray study of radio-emitting structures and their interactions with the hot interstellar medium (ISM). This paper details our interpretation of the remarkable X-ray enhancement which caps the inner southwest radio lobe, at a radius of about 6 kpc from the galaxy center. The shell of X-ray-emitting gas is hotter than the ambient ISM, and overpressured by a factor of 100. We argue that it is heated compressed material behind the supersonically-advancing bow shock of the radio lobe, the first example of the phenomenon to be clearly detected. The results demonstrate that Cen A is actively re-heating nearby X-ray-emitting gas. The shell's kinetic energy is $\sim 5$ times its thermal energy, and exceeds the thermal energy of the ISM within 15 kpc of the center of the galaxy. As the shell dissipates it will have a major effect on Cen A's ISM, providing distributed heating.

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arXiv2004

Chandra and XMM-Newton Observations of the Nucleus of Centaurus A

We present X-ray spectra of the nucleus of the nearby radio galaxy Centaurus A from observations with the XMM-Newton EPIC CCD cameras (two exposures separated by 12 months) and the Chandra HETGS. For the first time in an FRI type galaxy, we resolve fluorescent K$α$ emission from cold, neutral, or near-neutral iron at 6.4 keV, with an rms line width of $\sim$20 eV. The Fe line parameters observed are consistent with fluorescent emission from material at a large distance from the AGN, either in the form of an absorber that nearly completely surrounds the central engine or a torus that lies predominantly out of the line of sight. Unresolved emission lines from neutral Si K$α$ at 1.74 keV and neutral S K$α$ at 2.30 keV are also detected. We find no evidence in the data for a previously reported 6.8 keV broadened Fe line. The continuum spectrum is well fitted with a combination of a heavily absorbed power-law component that we relate, using Bondi theory, to accretion phenomena in the form of a standard, geometrically thin, optically thick disk, and a second, less absorbed, power-law component that we associate with emission from the subparsec VLBI radio jet.

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arXiv2004

An all-sky optical catalogue of radio / X-ray sources

We present an all-sky catalogue that aligns and overlays the ROSAT HRI, RASS, PSPC and WGA X-ray catalogues and the NVSS, FIRST and SUMSS radio catalogues onto the optical APM and USNO-A catalogues. Objects presented are those APM/USNO-A optical objects which are calculated with >= 40% confidence to be associated with radio/X-ray detections, or which are identified as known QSOs, AGN or BL Lacs, totalling 501,761 objects in all, including 48,285 QSOs and 21,498 double radio lobe detections. For each radio/X-ray associated optical object we display the calculated percentage probabilities of its being a QSO, galaxy, star, or erroneous radio/X-ray association, plus any identification from the literature. The catalogue includes 86,009 objects which were not previously identified and which we list as being 40% to >99% likely to be a QSO. As a byproduct of the construction of this catalogue, we are able to list comprehensive ROSAT field shifts as determined by our whole-sky likelihood algorithm, and also plate-by-plate photometric recalibration of the complete APM and USNO-A2.0 optical catalogues, significantly improving accuracy for objects of >15 mag. The catalogue is available wholly and in subsets at http://quasars.org/qorg-data.htm

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arXiv2004

The origins of X-ray emission from the hotspots of FRII radio sources

We use new and archival Chandra data to investigate the X-ray emission from a large sample of compact hotspots of FRII radio galaxies and quasars from the 3C catalogue. We find that only the most luminous hotspots tend to be in good agreement with the predictions of a synchrotron self-Compton model with equipartition magnetic fields. At low hotspot luminosities inverse-Compton predictions are routinely exceeded by several orders of magnitude, but this is never seen in more luminous hotspots. We argue that an additional synchrotron component of the X-ray emission is present in low-luminosity hotspots, and that the hotspot luminosity controls the ability of a given hotspot to produce synchrotron X-rays, probably by determining the high-energy cutoff of the electron energy spectrum. It remains plausible that all hotspots are close to the equipartition condition.

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arXiv2004

X-ray emission from the nuclei, lobes and hot-gas environments of two FR-II radio galaxies

We report the detection of multiple components of X-ray emission from the two FR-II radio galaxies 3C 223 and 3C 284, based on new XMM-Newton observations. We attribute the detected X-ray emission from the lobes of both sources to inverse-Compton scattering of cosmic microwave background photons. With this model, we find that the magnetic field strength in the lobes is at the equipartition value for 3C 284, and within a factor of two of the equipartition value for 3C 223. We also detect group-scale hot atmospheres around both sources, and determine temperatures and pressures in the gas. The lobes of both sources are in pressure balance with the hot-gas environments, if the lobes contain only the synchrotron-emitting particles and the measured magnetic field strength. The core spectra of both sources contain an unabsorbed soft component, likely to be related to the radio jet, and an additional heavily absorbed power-law component. 3C 223 also displays a bright (EW ~ 500 eV) Fe K-alpha emission line.

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arXiv2004

XMM-Newton observations of three high-redshift radio galaxies

We present the results of XMM-Newton observations of three high-redshift powerful radio galaxies 3C 184, 3C 292 and 3C 322. Although none of the sources lies in as rich an X-ray-emitting environment as is seen for some powerful radio galaxies at low redshift, the environments provide sufficient pressure to confine the radio lobes. The weak gas emission is particularly interesting for 3C 184, where a gravitational arc is seen, suggesting the presence of a massive cluster. Here Chandra data complement the XMM-Newton measurements by spatially separating X-rays from the extended atmosphere, the nucleus and the small-scale radio source. For 3C 292 the X-ray-emitting gas has a temperature of ~2 keV and luminosity of 6.5E43 erg/s, characteristic of a poor cluster. In all three cases, structures where the magnetic-field strength can be estimated through combining measurements of radio-synchrotron and inverse-Compton-X-ray emission, are consistent with being in a state of minimum total energy. 3C 184 and 3C 292 (and possibly 3C 322) have a heavily absorbed component of nuclear emission of N_H ~ $ few 10^{23} cm^{-2}.

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arXiv2004

XMM-Newton observations of three high redshift radio galaxies

We present results on the physical states of three high-redshift powerful radio galaxies (3C 292 at z=0.7, 3C 184 at z=1, and 3C322 at z=1.7). They were obtained by combining radio measurements with X-ray measurements from XMM-Newton that separate spectrally and/or spatially radio-related and hot-gas X-ray emission. Originally observed as part of a programme to trace clusters of galaxies at high redshift, none of the sources is found to lie in a rich X-ray-emitting environment similar to those of some powerful radio galaxies at low redshift, although the estimated gas pressures are sufficient to confine the radio lobes. The weak gas emission is a particularly interesting result for 3C 184, where a gravitational arc is seen, suggesting the presence of a very massive cluster. Here Chandra data complement the XMM-Newton measurements in spatially separating X-ray extended emission from that associated with the nucleus and rather small radio source. 3C 292 is the source for which the X-ray-emitting gas is measured with the greatest accuracy, and its temperature of 2 keV and luminosity of 6.5E43 erg/s are both characteristic of a poor cluster. This source allows the most accurate measurement of inverse-Compton X-ray emission associated with the radio lobes. In all structures where the magnetic-field strength can be estimated through combining measurements of radio-synchrotron and inverse-Compton-X-ray emission, the field strengths are consistent with sources being in a state of minimum total energy.

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arXiv2004

Jet/environment interactions of FR-I and FR-II radio galaxies

We present the results of new XMM observations of FR-I and FR-II radio galaxies that show the importance of jet/environment interactions for both radio-source structure and the properties of the surrounding gas. The FR-I observations reveal that the distribution of hot gas determines the radio-lobe morphology, and provide evidence that the subsonic expansion of the lobes heats the surrounding gas. The FR-II observations show that the sources are at equipartition and in pressure balance with their surroundings. Finally, we present an analysis of a sample of elliptical-dominated groups, showing that radio-source heating is common.

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arXiv2003

Jet termination in wide-angle tailed radio sources

Wide-angle tail radio galaxies (WATs) are an uncommon class of radio sources with luminosities near the FRI/FRII break, and are usually associated with central cluster galaxies. Their defining characteristic when imaged sensitively at high resolution is their twin, well-collimated jets, which can persist with low opening angle for tens of kpc before flaring into long, often bent plumes. Although several models for the jet termination have been proposed, the majority of them are unsatisfactory when confronted with observations. Here we present the results of a programme of radio observations made with the aims of showing that objects classified as WATs do all have well-collimated jets and seeing in detail how the jets disrupt as they enter the plumes. We show that compact, `hotspot-like' features at the ends of the jets are common but by no means universal, and discuss the constraints that this places on models of the jet-plume transition. We discuss the properties of the observed well-collimated jets and, using relativistic beaming models, estimate their speed to be ~ 0.3c. Finally, we show that the distance from the galactic centre at which the base of the plume is found is related to the temperature of the host cluster.

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arXiv2003

B_gg revisited: The environments of low-excitation radio galaxies and unified models

Recent measurements of the galaxy clustering environments around intermediate-redshift radio sources have suggested a systematic environmental difference between radio galaxies and radio-loud quasars, in contradiction to the predictions of simple unified models for the two classes of object. I show that the apparent difference arises mainly as a result of the properties of low-excitation radio galaxies included in the radio-galaxy sample, which tend to lie in significantly richer environments. The environmental properties of high-excitation radio galaxies and quasars are statistically consistent in the redshift range 0.15 < z <0.4, as unified models would predict.

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arXiv2003

The relationship between the X-ray and radio components in the Compact Steep Spectrum Quasar 3C 48

We combine results from ROSAT, Chandra, and multifrequency VLBA observations of the Compact Steep Spectrum quasar 3C 48. We infer that the X-ray spectrum of the nucleus is made up of a soft, variable, steep-spectrum component, and harder, power-law emission of slope consistent with the 1 GHz radio spectrum. The similarity of 3C 48's X-ray to radio ratio to that seen in core-dominated radio-loud quasars leads us to examine the possibility that the harder X-ray emission is inverse Compton radiation from the radio source, which is more than 99% resolved in our VLBA data. The weak (3 sigma) evidence that we find for a proper motion of 0.5\pm0.2c in a compact radio component about 0.05 arcsec from the core implies that if this component has a highly relativistic bulk motion, it is at a very small angle to the line of sight. However, stringent requirements on the jet opening angle make it unlikely that all the X-ray emission is from a fast jet which sees boosted Cosmic Microwave Background emission and emits beamed X-rays in the observer's frame. If the unusual radio structures are intrinsically one-sided and unbeamed, the inverse Compton mechanism can provide an appreciable fraction of the X-ray emission if the magnetic field strength is a factor of six to ten below that which gives equal energy in radiating relativistic particles and magnetic fields and roughly minimizes the total energy in the source. It remains possible that the unresolved X-ray emission arises from close to the central engine, either as an embedded blazar or associated with the accretion processes. (Slightly abridged).

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arXiv2003

XMM-Newton observations of the hot-gas atmospheres of 3C 66B and 3C 449

We present new XMM-Newton observations of the hot-gas environments of two low-power twin-jet radio galaxies, 3C 66B and 3C 449, showing direct evidence for the interactions between X-ray-emitting gas and radio plasma that are thought to determine the large-scale radio structure of these sources. The temperatures that we measure for the two environments are significantly higher than those predicted by standard luminosity-temperature relations for clusters and groups. We show that luminosity-temperature relations for radio-quiet and radio-loud X-ray groups differ, in the sense that radio-source heating may operate in most groups containing radio galaxies. If the radio lobes are expanding subsonically, we find minimum ages of 3e8 years for 3C 66B, and 5e8 years for 3C 449, older than the values obtained from spectral ageing, which would give the radio source sufficient time to heat the groups to the observed temperatures for plausible values of the jet power. The external pressures in the atmospheres of both radio galaxies are an order of magnitude higher than equipartition estimates of their radio-lobe pressures, confirming that the radio lobes are either out of equipartition or require a pressure contribution from non-radiating particles. Constraints from the level of X-ray emission we measure from the radio lobes allow us to conclude that a departure from equipartition must be in the direction of magnetic domination, and that the most plausible candidates for a particle contribution to lobe pressure are relativistic protons, an additional population of low-energy electrons, or entrained and heated thermal material.

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