arXiv subjects

J. H. Croston

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

At least 91 records · Page 5Linked to original sources

arXiv2009

Gas entropy in a representative sample of nearby X-ray galaxy clusters (REXCESS): relationship to gas mass fraction

(Abridged) We examine the radial entropy distribution and its scaling using 31 nearby galaxy clusters from the Representative XMM-Newton Cluster Structure Survey (REXCESS). The entropy profiles are robustly measured at least out to R_1000 in all systems and out to R_500 in 13 systems. Compared to theoretical expectations, the observed distributions show a radial and mass-dependent excess entropy that is greater and extends to larger radii in lower mass systems. At R_500, the mass dependence and entropy excess are both negligible within the uncertainties. Mirroring this behaviour, the scaling of gas entropy is shallower than self-similar in the inner regions, but steepens with radius, becoming consistent with self-similar at R_500. The dispersion in scaled entropy in the inner regions is linked to the presence of cool cores and dynamical activity; at larger radii the dispersion decreases by a factor of two and the dichotomy between subsamples disappears. Parameterising the profiles with a power law plus constant model, there are two peaks in central entropy K_0; however, we cannot distinguish between a bimodal or a left-skewed distribution. The outer slopes are correlated with system temperature; their distribution is unimodal with a median value of 0.98. Renormalising the dimensionless entropy profiles by the gas mass fraction profile f_gas(< R), leads to a remarkable reduction in the scatter, implying that gas mass fraction variations with radius and mass are the cause of the observed entropy properties. We discuss a tentative scenario to explain the behaviour of the entropy and gas mass fraction in the REXCESS sample, in which extra heating and merger mixing maintains an elevated central entropy level in the majority of the population, and a smaller fraction of systems develops a cool core.

astro-ph.CO↗

arXiv2009

Long Term Monitoring of the Dynamics and Particle Acceleration of Knots in the Jet of Centaurus A

We present new and archival multi-frequency radio and X-ray data for Centaurus A obtained over almost 20 years at the VLA and with Chandra, with which we measure the X-ray and radio spectral indices of jet knots, flux density variations in the jet knots, polarization variations, and proper motions. We compare the observed properties with current knot formation models and particle acceleration mechanisms. We rule out impulsive particle acceleration as a formation mechanism for all of the knots as we detect the same population of knots in all of the observations and we find no evidence of extreme variability in the X-ray knots. We find the most likely mechanism for all the stationary knots is a collision resulting in a local shock followed by a steady state of prolonged, stable particle acceleration and X-ray synchrotron emission. In this scenario, the X-ray-only knots have radio counterparts that are too faint to be detected, while the radio-only knots are due to weak shocks where no particles are accelerated to X-ray emitting energies. Although the base knots are prime candidates for reconfinement shocks, the presence of a moving knot in this vicinity and the fact that there are two base knots are hard to explain in this model. We detect apparent motion in three knots; however, their velocities and locations provide no conclusive evidence for or against a faster moving `spine' within the jet. The radio-only knots, both stationary and moving, may be due to compression of the fluid.

astro-ph.CO↗

arXiv2009

The universal galaxy cluster pressure profile from a representative sample of nearby systems (REXCESS) and the Y_SZ-M_500 relation

(abridged) We investigate the regularity of cluster pressure profiles with REXCESS, a representative sample of 33 local clusters observed with XMM-Newton. The sample spans a mass range of 10^14 M_sun <M_500<10^15 M_sun. We derive an average profile from observations scaled by mass and z according to the standard self-similar model, and find that the dispersion about the mean is remarkably low beyond 0.2R_500, but increases towards the centre. Deviations about the mean are related to both the mass and the thermo-dynamical state of the cluster. Unrelaxed systems have systematically shallower profiles while cooling core systems are more concentrated. The scaled profiles exhibit a residual mass dependence with a slope of about 0.12; however, the departure from standard scaling decreases with radius and is consistent with zero at R_500. The scatter in the core and departure from self-similar mass scaling is smaller compared to that of the entropy profiles, showing that the pressure is the quantity least affected by dynamical history and non-gravitational physics. Comparison with several state of the art numerical simulations shows good agreement outside the core. Combining the observational data below R_500 with simulation data above, we derive the universal pressure profile, that, in an analytical form, defines the physical pressure profile of clusters as a function of mass and z up to the cluster 'boundary'. Using this profile and the observed pressure profiles, we investigate the scaling properties of the integrated Compton parameter Y, considering both the spherically integrated quantity and the cylindrically integrated quantity, directly related to the Sunyaev-Zel'dovich (SZ) effect signal. We further derive the expected Y_SZ-M_500 and Y_SZ-L_X relations for any aperture.

astro-ph.CO↗

arXiv2009

The dynamics of the giant radio galaxy 3C 457

We present multi-frequency radio observations with the Giant Metrewave Radio Telescope and Very Large Array, and X-ray observations with the X-ray Multi-Mirror Mission ({\it XMM-Newton}) telescope of the giant radio source (GRS) 3C 457. We have detected the core, lobes and the environment of the GRS in X-ray. We examine the relationships between the radio and X-ray emission, determine the radio spectrum over a large frequency range and attribute the X-ray emission from the lobes to the inverse-Compton scattering of cosmic microwave background (CMB) photons. The magnetic field strength of the lobes is very close to the equipartition value. Both the lobes are in pressure balance near the hotspots and apparently under-pressured towards the core. The X-ray spectrum of the core of the GRS consists of an unabsorbed soft power-law component and a heavily absorbed hard power-law component. The soft unabsorbed component is likely to be related to the radio jets. There is no strong evidence of Fe K$α$ emission line in our data.

astro-ph.CO↗

arXiv2009

Luminosity functions of LMXBs in Centaurus A: globular clusters versus the field

We study the X-ray luminosity function (XLF) of low mass X-ray binaries (LMXB) in the nearby early-type galaxy Centaurus A, concentrating primarily on two aspects of binary populations: the XLF behavior at the low luminosity limit and comparison between globular cluster and field sources. The 800 ksec exposure of the deep Chandra VLP program allows us to reach a limiting luminosity of 8e35 erg/s, about 2-3 times deeper than previous investigations. We confirm the presence of the low luminosity break in the overall LMXB XLF at log(L_X)=37.2-37.6 below which the luminosity distribution follows a constant dN/d(ln L). Separating globular cluster and field sources, we find a statistically significant difference between the two luminosity distributions with a relative underabundance of faint sources in the globular cluster population. This demonstrates that the samples are drawn from distinct parent populations and may disprove the hypothesis that the entire LMXB population in early type galaxies is created dynamically in globular clusters. As a plausible explanation for this difference in the XLFs, we suggest that there is an enhanced fraction of helium accreting systems in globular clusters, which are created in collisions between red giants and neutron stars. Due to the 4 times higher ionization temperature of He, such systems are subject to accretion disk instabilities at approximately 20 times higher mass accretion rate, and therefore are not observed as persistent sources at low luminosities.

astro-ph.HE↗

arXiv2009

The active nuclei of z<1.0 3CRR radio sources

We combine Chandra and XMM-Newton X-ray data from our previous papers with new X-ray observations and with Spitzer mid-infrared data in order to study the nature of the nuclei of radio galaxies and radio-loud quasars with z<1.0 from the 3CRR sample. The significant increase in sample size over our previous work, the reduction of bias in the sample as a result of new observations, and the availability of more mid-infrared data allow us to show conclusively that almost all objects classed as low-excitation radio galaxies in optical spectroscopic studies lack a radiatively efficient active nucleus. We show that the distribution of absorbing columns in the narrow-line radio galaxies differs from the population of X-ray-selected radio-quiet type-2 quasars and from that in local Seyfert 2s. We comment on the current evidence for the nature of the soft X-ray component in radio-galaxy nuclear spectra, concluding that a jet origin for this component is very hard to evade. Finally, we discuss the recently discovered `fundamental plane' of black hole activity, showing that care must be taken when placing radio-loud AGN on such diagnostic diagrams.

astro-ph.GA↗

arXiv2009

The Jet-Heated X-ray Filament in the Centaurus A Northern Middle Radio Lobe

We present results from a 40 ks {\em XMM-Newton} observation of the X-ray filament coincident with the southeast edge of the Centaurus A Northern Middle Radio Lobe (NML). We find that the X-ray filament consists of five spatially resolved X-ray knots embedded in a continuous diffuse bridge. The spectrum of each knot is well fitted by a thermal model with temperatures ranging from 0.3-0.7 keV and subsolar elemental abundances. In four of the five knots, non-thermal models are a poor fit to the spectra, conclusively ruling out synchrotron or IC/CMB mechanisms for their emission. The internal pressures of the knots exceed that of the ambient ISM or the equipartition pressure of the NML by more than an order of magnitude, demonstrating that they must be short lived ($\sim3\times10^6$ yrs). Based on energetic arguments, it is implausible that these knots have been ionized by the beamed flux from the active galactic nucleus of Cen A or that they have been shock-heated by supersonic inflation of the NML. In our view, the most viable scenario for the origin of the X-ray knots is that they are the result of cold gas shock heated by a direct interaction with the jet. The lifetimes of the X-ray knots in the NML are roughly the same as the age of the strong shock around the SW inner lobe, suggesting they were created in the same AGN outburst. The most plausible model of the NML is that it is a bubble from a previous outburst that is being re-energized by the current outburst. We also report the discovery of a large scale (at least 35 kpc radius) gas halo around Cen A.

astro-ph.HE↗

arXiv2009

Galaxy cluster X-ray luminosity scaling relations from a representative local sample (REXCESS)

(Abridged) We examine the X-ray luminosity scaling relations of 31 nearby galaxy clusters from the Representative XMM-Newton Cluster Structure Survey (REXCESS). The objects are selected in X-ray luminosity only, optimally sampling the cluster luminosity function; temperatures range from 2 to 9 keV and there is no bias toward any particular morphological type. Pertinent values are extracted in an aperture corresponding to R_500, estimated using the tight correlation between Y_X and total mass. The data exhibit power law relations between bolometric X-ray luminosity and temperature, Y_X and total mass, all with slopes that are significantly steeper than self-similar expectations. We examine the causes for the steepening, finding that the primary driver appears to be a systematic variation of the gas content with mass. Scatter about the relations is dominated in all cases by the presence of cool cores. The natural logarithmic scatter about the raw X-ray luminosity-temperature relation is about 70%, and about the X-ray luminosity-Y_X relation it is 40%. Cool core and morphologically disturbed systems occupy distinct regions in the residual space with respect to the best fitting mean relation, the former lying systematically to the high luminosity side, the latter to the low luminosity side. Exclusion of the central regions serves to reduce the scatter by more than 50%. Using Y_X as a mass proxy, we derive a Malmquist bias corrected luminosity-mass relation and compare with previous determinations. Our results indicate that luminosity can be a reliable mass proxy with controllable scatter, which has important implications for upcoming all-sky cluster surveys, such as those to be undertaken with Planck and eROSITA, and ultimately for the use of clusters for cosmological purposes.

astro-ph↗

arXiv2009

High-energy particle acceleration at the radio-lobe shock of Centaurus A

We present new results on the shock around the southwest radio lobe of Centaurus A using data from the Chandra Very Large Programme observations. The X-ray spectrum of the emission around the outer southwestern edge of the lobe is well described by a single power-law model with Galactic absorption -- thermal models are strongly disfavoured, except in the region closest to the nucleus. We conclude that a significant fraction of the X-ray emission around the southwest part of the lobe is synchrotron, not thermal. We infer that in the region where the shock is strongest and the ambient gas density lowest, the inflation of the lobe is accelerating particles to X-ray synchrotron emitting energies, similar to supernova remnants such as SN1006. This interpretation resolves a problem of our earlier, purely thermal, interpretation for this emission, namely that the density compression across the shock was required to be much larger than the theoretically expected factor of 4. We estimate that the lobe is expanding to the southwest with a velocity of ~2600 km/s, roughly Mach 8 relative to the ambient medium. We discuss the spatial variation of spectral index across the shock region, concluding that our observations constrain gamma_max for the accelerated particles to be 10^8 at the strongest part of the shock, consistent with expectations from diffusive shock acceleration theory. Finally, we consider the implications of these results for the production of ultra-high energy cosmic rays (UHECRs) and TeV emission from Centaurus A, concluding that the shock front region is unlikely to be a significant source of UHECRs, but that TeV emission from this region is expected at levels comparable to current limits at TeV energies, for plausible assumed magnetic field strengths.

astro-ph.HE↗

arXiv2008

Chandra evidence for AGN feedback in the spiral galaxy NGC 6764

We report the Chandra detection of X-ray emission spatially coincident with the kpc-scale radio bubbles in the nearby (D_L ~ 31 Mpc) AGN-starburst galaxy NGC 6764. The X-ray emission originates in hot gas (kT ~ 0.75 keV), which may either be contained within the radio bubbles, or in a shell of hot gas surrounding them. We consider three models for the origin of the hot gas: (1) a starburst-driven galactic wind, (2) shocked gas associated with the expanding radio bubbles, and (3) gas heated and entrained into the bubbles by jet/ISM interactions in the inner AGN outflow. We rule out a galactic wind based on significant differences from known galactic wind systems. The tight correspondence between the brightest X-ray emission and the radio emission in the inner outflow from the Seyfert nucleus, as well as a correlation between X-ray and radio spectral features suggestive of shocks and particle acceleration, lead us to favour the third model; however, we cannot firmly rule out a model in which the bubbles are driving large-scale shocks into the galaxy ISM. In either AGN-driven heating scenario, the total energy stored in the hot gas is high, ~10^56 ergs, comparable to the energetic impact of low-power radio galaxies such as Centaurus A, and will have a dramatic impact on the galaxy and its surroundings.

astro-ph↗

arXiv2008

Galaxy-cluster gas-density distributions of the Representative XMM-Newton Cluster Structure Survey (REXCESS)

We present a study of the structural and scaling properties of the gas distributions in the intracluster medium (ICM) of 31 nearby (z < 0.2) clusters observed with XMM-Newton, which together comprise the Representative XMM-Newton Cluster Structure Survey (REXCESS). In contrast to previous studies, this sample is unbiased with respect to cluster dynamical state, and it fully samples the cluster X-ray luminosity function. The clusters cover a temperature range of 2.0 -- 8.5 keV and possess a variety of morphologies. The sampling strategy allows us to compare clusters with a wide range of central cooling times on an equal footing. We present non-parametric gas-density profiles out to distances ranging between 0.8 R_500 and 1.5 R_500. The central gas densities differ greatly from system to system, with no clear correlation with system temperature. At intermediate radii the scaled density profiles show much less scatter, with a clear dependence on system temperature, consistent with the presence of an entropy excess as suggested in previous literature. However, at large scaled radii this dependence becomes weaker: clusters with kT > 3 keV scale self-similarly, with no temperature dependence of gas-density normalisation. We find some evidence of a correlation between dynamical state and outer gas density slope, and between dynamical state and both central gas normalisation and cooling time. We find no evidence of a significant bimodality in the distributions of central density, density gradient, or cooling time. Finally, we present the gas mass-temperature relation for the REXCESS sample, which is consistent with the expectation of self-similar scaling modified by the presence of an entropy excess in the inner regions of the cluster, and has a logarithmic intrinsic scatter of ~10%.

astro-ph↗

arXiv2008

Evidence for Non-Hydrostatic Gas Motions in the Hot ISM of Centaurus A

We present preliminary results from a deep (600 ks) {\em Chandra} observation of the hot interstellar medium of the nearby early-type galaxy Centaurus A (Cen A). We find a surface brightness discontinuity in the gas $\sim$3.5 kpc from the nucleus spanning a 120$^\circ$ arc. The temperature of the gas is 0.60$\pm$0.05 and 0.68$\pm$0.10 keV, interior and exterior to the discontinuity, respectively. The elemental abundance is poorly constrained by the spectral fits, but if the abundance is constant across the discontinuity, there is a factor of 2.3$\pm$0.4 pressure jump across the discontinuity. This would imply that the gas is moving at 470$\pm$100 km s$^{-1}$, or Mach 1.0$\pm$0.2 (1.2$\pm$0.2) relative to the sound speed of the gas external (internal) to the discontinuity. Alternatively, pressure balance could be maintained if there is a large (factor of $\sim$7) discontinuity in the elemental abundance. We suggest that the observed discontinuity is the result of non-hydrostatic motion of the gas core (i.e. sloshing) due to the recent merger. In this situation, both gas motions and abundance gradients are important in the visibility of the discontinuity. Cen A is in the late stages of merging with a small late-type galaxy, and a large discontinuity in density and abundance across a short distance demonstrates that the gas of the two galaxies remains poorly mixed even several hundred million years after the merger. The pressure discontinuity may have had a profound influence on the temporal evolution of the kpc-scale jet. The jet could have decollimated crossing the discontinuity and thereby forming the northeast radio lobe.

astro-ph↗

arXiv2008

An XMM-Newton study of the environments, particle content and impact of low-power radio galaxies

We present a detailed study of the environments of a sample of nine low-power radio galaxies, based on new and archival XMM-Newton observations. We report new detections of group-scale environments around three radio galaxies, 3C 296, NGC 1044 and 3C 76.1. As with previous studies, we find that FR-I radio galaxies inhabit group environments ranging over nearly two orders of magnitude in bolometric X-ray luminosity, but find no evidence for a tight relationship between large-scale X-ray environment and radio-source properties such as size, radio luminosity, and axial ratio. We confirm earlier work showing that equipartition internal pressures are typically lower than the external pressures acting on the radio lobes, so that additional non-radiating particles must be present or the lobes must be magnetically dominated. We present the first direct observational evidence that entrainment may provide this missing pressure, in the form of a relationship between radio-source structure and apparent pressure imbalance. Finally, our study provides further support for the presence of an apparent temperature excess in radio-loud groups compared to the group population as a whole. For five of eight temperature excesses, the energy required to inflate the radio lobes is comparable to the energy required to produce this excess by heating of the group gas; however, in three cases the current radio source appears too weak to produce the temperature excess. It remains unclear whether the temperature excess can be directly associated with the current phase of AGN activity, or whether it is instead either a signature of previous AGN activity or simply an indicator of the particular set of group properties most conducive to the growth of an FR-I radio galaxy.

astro-ph↗

arXiv2008

Inverse-Compton emission from the lobes of 3C 353

X-ray emission due to inverse-Compton scattering of microwave background photons by electrons in the lobes of powerful radio galaxies has now been seen in a large number of objects. Combining an inverse-Compton model for the lobe X-ray emission with information obtained from radio synchrotron emission provides a method of constraining the electron population and magnetic field energy density, which cannot be accomplished using the radio data alone. Using six frequencies of new and archival radio data and new XMM-Newton observations of the Fanaroff & Riley class II radio galaxy 3C353, we show that inverse-Compton emission is detected in the radio lobes of this source at a level consistent with what is seen in other objects. We argue that variations in the X-ray/radio ratio in the brighter eastern lobe require positionally varying magnetic field strength. We also examine the X-ray nucleus and the cluster, Zw 1819.1-0108, spatially and spectrally.

astro-ph↗

arXiv2007

Where Centaurus A gets its X-ray knottiness

We report an X-ray spectral study of the transverse structure of the Centaurus A jet using new data from the Chandra Cen A Very Large Project. We find that the spectrum steepens with increasing distance from the jet axis, and that this steepening can be attributed to a change in the average spectrum of the knotty emission. Such a trend is unexpected if the knots are predominantly a surface feature residing in a shear layer between faster and slower flows. We suggest that the spectral steepening of the knot emission as a function of distance from the jet axis is due to knot migration, implying a component of transverse motion of knots within the flow.

astro-ph↗

arXiv2007

New results on particle acceleration in the Centaurus A jet and counterjet from a deep Chandra observation

We present new deep Chandra observations of the Centaurus A jet, with a combined on-source exposure time of 719 ks. These data allow detailed X-ray spectral measurements to be made along the jet out to its disappearance at 4.5 kpc from the nucleus. We distinguish several regimes of high-energy particle acceleration: while the inner part of the jet is dominated by knots and has properties consistent with local particle acceleration at shocks, the particle acceleration in the outer 3.4 kpc of the jet is likely to be dominated by an unknown distributed acceleration mechanism. In addition to several compact counterjet features we detect probable extended emission from a counterjet out to 2.0 kpc from the nucleus, and argue that this implies that the diffuse acceleration process operates in the counterjet as well. A preliminary search for X-ray variability finds no jet knots with dramatic flux density variations, unlike the situation seen in M87.

astro-ph↗

arXiv2007

X-Ray Nuclei in Radio Galaxies: Exploring the Roles of Hot and Cold Gas Accretion

We present results from Chandra and XMM-Newton spectroscopic observations of the nuclei of z<0.5 radio galaxies and quasars from the 3CRR catalog, and examine in detail the dichotomy in the properties of low- and high-excitation radio galaxies. The X-ray spectra of low-excitation sources (those with weak or absent optical emission lines) are dominated by unabsorbed emission from a parsec-scale jet, with no contribution from accretion-related emission. These sources show no evidence for an obscuring torus, and are likely to accrete in a radiatively inefficient manner. High-excitation sources (those with prominent optical emission lines), on the other hand, show a significant contribution from a radiatively efficient accretion disk, which is heavily absorbed in the X-ray when they are oriented close to edge-on with respect to the observer. However, the low-excitation/high-excitation division does not correspond to the FRI/FRII division: thus the Fanaroff-Riley dichotomy remains a consequence of the interaction between the jet and the hot-gas environment through which it propagates. Finally, we suggest that accretion of the hot phase of the IGM is sufficient to power all low-excitation radio sources, while high-excitation sources require an additional contribution from cold gas that in turn forms the cold disk and torus. This model explains a number of properties of the radio-loud active galaxy population, and has important implications for AGN feedback mechanisms.

astro-ph↗

arXiv2007

A Chandra study of particle acceleration in the multiple hotspots of nearby radio galaxies

We present Chandra observations of a small sample of nearby classical double radio galaxies which have more than one radio hotspot in at least one of their lobes. The X-ray emission from the hotspots of these comparatively low-power objects is expected to be synchrotron in origin, and therefore to provide information about the locations of high-energy particle acceleration. In some models of the relationship between the jet and hotspot the hotspots that are not the current jet termination point should be detached from the energy supply from the active nucleus and therefore not capable of accelerating particles to high energies. We find that in fact some secondary hotspots are X-ray sources, and thus probably locations for high-energy particle acceleration after the initial jet termination shock. In detail, though, we show that the spatial structures seen in X-ray are not consistent with naive expectations from a simple shock model: the current locations of the acceleration of the highest-energy observable particles in powerful radio galaxies need not be coincident with the peaks of radio or even optical emission.

astro-ph↗