arXiv subjects

J. H. Croston

Publications and source records attributed to J. H. Croston.

At least 109 records · Page 6Linked to original sources

arXiv2007

Inverse-Compton emission from the lobes of 3C353

Analysis of the radio synchrotron and X-ray inverse-Compton emission from radio-loud active galaxies allows us to determine their particle acceleration processes and electron energy spectra. Previous studies have provided new constraints on the total energy budget and particle content of powerful radio galaxies and quasars; however, in most cases the sources are too faint in the X-ray to obtain spatial information. We present archival and new multi-frequency radio observations from the VLA and GMRT, and XMM-Newton observations of the bright FRII radio galaxy 3C353 which lies on the edge of the X-ray-luminous cluster Zw 1718.1-0108. The X-ray observations detect both the inverse-Compton emission from the radio galaxy lobes and thermal emission from the hot phase of the intracluster medium. We discuss the properties of the particle energy spectrum as a function of position in the lobe, as well as the properties of the merging cluster.

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arXiv2007

Jet/environment interactions in low-power radio galaxies

The interactions between low-power radio galaxies and their environments are thought to play a crucial role in supplying energy to offset cooling in the centres of groups and clusters. Such interactions are also important in determining large-scale radio structures and radio-source dynamics. I will discuss new XMM-Newton observations of the hot-gas environments of a representative sample of nine FRI radio galaxies, which show strong evidence for the importance of such interactions (including evidence for heating) and provide important new constraints on source dynamics and particle content. In particular I will show that the widely discussed apparent imbalance between the internal lobe pressure available from relativistic electrons and magnetic field and the external pressure of hot gas correlates with radio structure, so that naked jets require a large contribution from non-radiating particles, whereas lobed sources do not. This may provide the first direct observational evidence that entrainment of the ICM supplies the missing pressure.

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arXiv2007

The Representative XMM-Newton Cluster Structure Survey (REXCESS) of an X-ray Luminosity Selected Galaxy Cluster Sample

The largest uncertainty for cosmological studies using clusters of galaxies is introduced by our limited knowledge of the statistics of galaxy cluster structure, and of the scaling relations between observables and cluster mass. To improve on this situation we have started an XMM-Newton Large Programme for the in-depth study of a representative sample of 33 galaxy clusters, selected in the redshift range z=0.055 to 0.183 from the REFLEX Cluster Survey, having X-ray luminosities above 0.4 X 10^44 h_70^-2 erg s^-1 in the 0.1 - 2.4 keV band. This paper introduces the sample, compiles properties of the clusters, and provides detailed information on the sample selection function. We describe the selection of a nearby galaxy cluster sample that makes optimal use of the XMM-Newton field-of-view, and provides nearly homogeneous X-ray luminosity coverage for the full range from poor clusters to the most massive objects in the Universe. For the clusters in the sample, X-ray fluxes are derived and compared to the previously obtained fluxes from the ROSAT All-Sky Survey. We find that the fluxes and the flux errors have been reliably determined in the ROSAT All-Sky Survey analysis used for the REFLEX Survey. We use the sample selection function documented in detail in this paper to determine the X-ray luminosity function, and compare it with the luminosity function of the entire REFLEX sample. We also discuss morphological peculiarities of some of the sample members. The sample and some of the background data given in this introductory paper will be important for the application of these data in the detailed studies of cluster structure, to appear in forthcoming publications.

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arXiv2007

The interaction between radio lobes and hot gas in the nearby radio galaxies 3C285 and 3C442A

We present Chandra observations of two nearby radio galaxies in group environments, 3C285 and 3C442A. The host galaxies of both sources are involved in mergers with nearby massive galaxies, and the hot gas in the systems is extended along lines joining the interacting galaxies. Both sources show strong evidence for interactions between the radio lobes and the asymmetrical hot gas. We argue that the structure in the hot gas is independent of the existence of the radio lobes in these systems, and argue that hot gas shaped by an ongoing massive galaxy merger may play an important role in the dynamics of radio lobes in other objects. For 3C442A, our observations show that gas is being driven out of both members of the host interacting galaxy pair, and the implied constraints on galaxy velocities are consistent with mildly supersonic motions with respect to the group-scale hot gas. The previously known filamentary radio structure in the center of 3C442A may be a result of the interaction between hot gas expelled from these galaxies and pre-existing radio-emitting plasma. In 3C285, where there is no ongoing galaxy merger, the powerful radio source is probably having a significant effect on the energetics of the host group.

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arXiv2007

Shock heating in the nearby radio galaxy NGC 3801

We report the Chandra detection of shock-heated shells of hot gas surrounding the radio lobes of the nearby (D_L ~ 53 Mpc) low-power radio galaxy NGC 3801. The shells have temperatures of 1 keV and 0.7 keV, compared to an ISM temperature of 0.23 keV. The estimated expansion speed of the shells is 850 km/s, corresponding to a Mach number of ~ 4. This is the second X-ray detection of strong shocks produced by a low-power radio galaxy, and allows us to measure directly the contribution of shock heating to the radio galaxy's total energetic input to the ISM. We show that the gas properties of the shells and surrounding ISM are consistent with the Rankine-Hugoniot shock jump conditions. We estimate the energy stored in the hot gas shells (thermal + kinetic energy) to be 1.7x10^56 ergs, which is equivalent to the thermal energy of the ISM within ~11 kpc of the galaxy centre, and a factor of ~25 larger than the inferred PdV work required to inflate the lobe cavities, indicating that energy transfer from the AGN to its environment is dominated by shock heating during this stage of radio-source evolution. Our results provide direct evidence that shock heating in the early supersonic phase of FRI radio-source expansion can have important long-term effects on the properties of the host galaxy ISM. Finally, we discuss the merger history of NGC 3801, the fuelling of its AGN and the role of this type of system in feedback models.

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arXiv2007

Hot and cold gas accretion and feedback in radio-loud active galaxies

We have recently shown that X-ray observations of the population of `low-excitation' radio galaxies, which includes most low-power, Fanaroff-Riley class I sources as well as some more powerful Fanaroff-Riley class II objects, are consistent with a model in which the active nuclei of these objects are not radiatively efficient at any waveband. In another recent paper Allen et al. have shown that Bondi accretion of the hot, X-ray emitting phase of the intergalactic medium (IGM) is sufficient to power the jets of several nearby, low-power radio galaxies at the centres of clusters. In this paper we combine these ideas and suggest that accretion of the hot phase of the IGM is sufficient to power all low-excitation radio sources, while high-excitation sources are powered by accretion of cold gas that is in general unrelated to the hot IGM. This model explains a number of properties of the radio-loud active galaxy population, and has important implications for the energy input of radio-loud active galactic nuclei into the hot phase of the IGM: the energy supply of powerful high-excitation sources does not have a direct connection to the hot phase.

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arXiv2007

A Radio through X-ray Study of the Hot Spots, Active Nucleus, and Environment of the Nearby FR II Radio Galaxy 3C 33

We present results from {\em Chandra}/ACIS-S, {\em Spitzer}, {\em XMM-Newton}, {\em HST}, and VLA observations of the radio hot spots, extended environment, and nucleus of the nearby ($z$=0.0597) FR II radio galaxy 3C 33. This is a relatively low-power FR II radio galaxy, and so we expect, {\it a priori}, to detect a significant X-ray synchrotron component to the emission from the hot spots. We detect X-ray emission coincident with the two knots of polarized optical emission from the southern hot spot (SHS), as well as along the northwest arm of this hot spot. We also detect X-ray emission from two compact regions of the northern hot spot (NHS), as well as diffuse emission behind the radio peak. The X-ray flux density of the region at the tip of the southern hot spot, the most compact radio feature of the southern lobe, is consistent with the synchrotron self-Compton (SSC) process. The X-ray flux densities of the other three regions of the SHS and the two compact regions of the NHS are an order of magnitude or more above the predictions from either the SSC and inverse-Compton scattering of the CMB (IC/CMB) mechanisms, thus strongly disfavoring these scenarios unless they are far from equipartition ($B$ $\sim$4-14 times smaller than the equipartition values). The X-ray flux from the diffuse region behind the NHS is consistent with the IC/CMB prediction assuming a small departure from equipartition. We conclude that the X-ray emission is synchrotron emission from multiple populations of ultrarelativistic electrons unless these regions are far from equipartition. There must therefore be unresolved substructure within each knot, similar to that which is found in Chandra observations of nearby FR I jets such as Centaurus A and 3C 66B.

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arXiv2006

Shock heating by nearby AGN

Shock heating by radio jets is potentially an important process in a range of environments, as it will increase the entropy of the heated gas. Although this process is expected to occur in the most powerful radio-loud AGN, strong shocks have so far only been detected in nearby low-power radio galaxies. Here we discuss X-ray detections of strong shocks in nearby galaxies, including a new detection of shocked gas around both lobes of the nearby radio galaxy NGC 3801 with inferred Mach numbers of 3 - 6 and a total injected energy comparable to the thermal energy of the ISM within 11 kpc. We discuss possible links between shock heating, AGN fuelling and galaxy mergers and the role of this type of system in feedback models.

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arXiv2006

Temperature profiles of a representative sample of nearby X-ray galaxy clusters

A study of the structural and scaling properties of the temperature distribution of the hot, X-ray emitting intra-cluster medium of galaxy clusters, and its dependence on dynamical state, can give insights into the physical processes governing the formation and evolution of structure. We analyse the X-ray temperature profiles from XMM-Newton observations of 15 nearby (z < 0.2) clusters, drawn from a statistically representative sample. The clusters cover a temperature range from 2.5 keV to 8.5 keV, and present a variety of X-ray morphologies. We derive accurate projected temperature profiles to ~ 0.5 R_200, and compare structural properties (outer slope, presence of cooling core) with a quantitative measure of the X-ray morphology as expressed by power ratios. We also compare the results to recent cosmological numerical simulations. Once the temperature profiles are scaled by an average cluster temperature (excluding the central region) and the estimated virial radius, the profiles generally decline in the region 0.1 R_200 < R < 0.5 R_200. The central regions show the largest scatter, attributable mostly to the presence of cool core clusters. There is good agreement with numerical simulations outside the core regions. We find no obvious correlations between power ratio and outer profile slope. There may however be a weak trend with the existence of a cool core, in the sense that clusters with a central temperature decrement appear to be slightly more regular. The present results lend further evidence to indicate that clusters are a regular population, at least outside the core region.

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arXiv2006

An improved deprojection and PSF-deconvolution technique for galaxy-cluster X-ray surface-brightness profiles

We have developed a regularisation procedure for the direct deprojection and PSF-deconvolution of X-ray surface brightness profiles of clusters of galaxies. This procedure allows us to obtain accurate density profiles in a straightforward manner from X-ray observations (in particular data from XMM-Newton, where the PSF correction is important), while retaining information about substructure in the gas distribution, in contrast to analytic modelling of the profiles. In addition to describing our procedure, we present here a detailed investigation of the accuracy of the method and its error calculations over a wide range of input profile characteristics and data quality using Monte Carlo simulations. We also make comparisons with gas density profiles obtained from Chandra observations, where the PSF correction is small, and with profiles obtained using analytic modelling, which demonstrate that our procedure is a useful improvement over standard techniques. This type of method will be especially valuable in the ongoing analysis of unbiased and complete samples of X-ray clusters, both local and distant, helping to improve the quality of their results.

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arXiv2006

The X-ray nuclei of intermediate-redshift radio sources

We present a Chandra and XMM-Newton spectral analysis of the nuclei of the radio galaxies and radio-loud quasars from the 3CRR sample in the redshift range 0.1 < z < 0.5. In the range of radio luminosity sampled by these objects, mostly FRIIs, it has been clear for some time that a population of radio galaxies (`low-excitation radio galaxies') cannot easily participate in models that unify narrow-line radio galaxies and broad-line objects. We show that low-excitation and narrow-line radio galaxies have systematically different nuclear X-ray properties: while narrow-line radio galaxies universally show a heavily absorbed nuclear X-ray component, such a heavily absorbed component is rarely found in sources classed as low-excitation objects. Combining our data with the results of our earlier work on the z<0.1 3CRR sources, we discuss the implications of this result for unified models, for the origins of mid-infrared emission from radio sources, and for the nature of the apparent FRI/FRII dichotomy in the X-ray. The lack of direct evidence for accretion-related X-ray emission in FRII LERGs leads us to argue that there is a strong possibility that some, or most, FRII LERGs accrete in a radiatively inefficient mode. However, our results are also consistent with a model in which the accretion mode is the same for low- and high-excitation FRIIs, with the lower accretion luminosities in FRII LERGs attributed instead to more efficient radio luminosity production in those objects.

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arXiv2005

FRII sources at z>0.5: X-ray properties of the core and extended emission

Active galaxies are the most powerful engines in the Universe for converting gravitational energy into radiation, and their study at all epochs of evolution is therefore important. Powerful radio-loud quasars and radio galaxies have the added advantage that, since their radio jets need X-ray-emitting gas as a medium in which to propagate, the sources can be used as cosmological probes to trace significant atmospheres at high redshift. The radio emission can be used as a measure of source orientation, and sensitive X-ray measurements, especially when used in combination with multi-wavelength data, can be used to derive important results on the physical structures on a range of sizes from the cores to the large-scale components. In this paper we present new results on a significant sample of powerful radio galaxies and quasars at z > 0.5, drawn from the 3CRR catalogue and selected to sample a full range of source orientation. Using high-quality observations from XMM-Newton and Chandra, we discuss the X-ray properties of the cores, jets, lobes and cluster gas, and, through the incorporation of multi-wavelength data, draw conclusions about the nature of the emission from the different components.

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arXiv2005

The Chandra view of extended X-ray emission from Pictor A

We discuss the extended X-ray emission seen in three archival Chandra observations, and one archival XMM-Newton observation, of the FRII radio galaxy Pictor A. The overall properties of the X-ray lobes are consistent with the conclusions of earlier workers that the extended X-ray emission is largely due to the inverse-Compton process, and the implied departure from equipartition is in the range seen by us in other sources. In detail, we show that the X-ray/radio flux ratio varies quite strongly as a function of position throughout the source, and we discuss possible implications of this observation for the spatial variation of electron energy spectra and magnetic field strength through the lobe. We show that the radio and X-ray properties of the lobe are not consistent with a simple model in which variations in the magnetic field strength alone are responsible for the observed differences between emission at different frequencies. We also discuss the origins of the extended emission seen around the eastern hotspot, arguing that it may be diffuse synchrotron radiation tracing a region of distributed particle acceleration, and the implications of a possible weak X-ray counterjet detection which, taken together with the other properties of the bright X-ray jet, leads us to suggest that the X-ray jet and possible counterjet are also produced by synchrotron emission.

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arXiv2005

An X-ray study of magnetic field strengths and particle content in FRII radio sources

We present a Chandra and XMM-Newton study of X-ray emission from the lobes of 33 classical double radio galaxies and quasars. We report new detections of lobe-related X-ray emission in 11 sources. Together with previous detections we find that X-ray emission is detected from at least one radio lobe in ~75 percent of the sample. For all of the lobe detections, we find that the measured X-ray flux can be attributed to inverse-Compton scattering of the cosmic microwave background radiation, with magnetic field strengths in the lobes between (0.3 - 1.3) B_eq, where the value B_eq corresponds to equipartition between the electrons and magnetic field assuming a filling factor of unity. There is a strong peak in the magnetic field strength distribution at B ~ 0.7 B_eq. We find that > 70 percent of the radio lobes are either at equipartition or electron dominated by a small factor. The distribution of measured magnetic field strengths differs for narrow-line and broad-line objects, in the sense that broad-line radio galaxies and quasars appear to be further from equipartition; however, this is likely to be due to a combination of projection effects and worse systematic uncertainty in the X-ray analysis for those objects. Our results suggest that the lobes of classical double radio sources do not contain an energetically dominant proton population, because this would require the magnetic field energy density to be similar to the electron energy density rather than the overall energy density in relativistic particles.

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arXiv2005

Chandra and XMM-Newton Observations of NGC 6251

We present new X-ray observations of the nucleus, jet and extended emission of the nearby radio galaxy NGC 6251 using the Chandra/ACIS-S camera, together with a reanalysis of archival Chandra/ACIS-I and XMM-Newton/EPIC data. We find that the nuclear X-ray spectrum is well-fitted with an absorbed power-law, and that there is tentative, but not highly significant, evidence for Fe K$α$ emission. We argue that the observed nuclear X-ray emission is likely to originate in a relativistic jet, based on the double-peaked nature, and our synchrotron self-Compton modelling, of the radio-to-X-ray spectral energy distribution. However, we cannot rule out a contribution from an accretion flow. We resolve X-ray jet emission in three distinct regions, and argue in favour of a synchrotron origin for all three; inverse-Compton emission models are possible but require extreme parameters. We detect thermal emission on both galaxy and group scales, and demonstrate that hot gas can confine the jet, particularly if relativistic beaming is important. We show evidence that the radio lobe has evacuated a cavity in the X-ray-emitting gas, and suggest that the lobe is close to the plane of the sky, with the jet entering the lobe close to the surface nearest to the observer.

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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

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

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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