arXiv subjects

D. M. Worrall

Publications and source records attributed to D. M. Worrall.

At least 109 records · Page 6Linked to original sources

arXiv2003

The temperature and distribution of gas in CL 0016+16 measured with XMM-Newton

We present results of a 37 ks observation of CL 0016+16 with the XMM-Newton EPIC instrument. Within 1.5 arcmin of the cluster centre we measure a gas temperature of kT = 9.13^{+0.24}_{-0.22} keV and an abundance of 0.22^{+0.04}_{-0.03} times the solar value (1 sigma uncertainties). This significant improvement over previous measurements has allowed us to revise the estimate of the Hubble constant based on CL 0016+16 to 68 +/- 8 km s^{-1} Mpc^{-1} (random error only), close to the value from the Hubble Space Telescope distance-scale project. The total gravitating mass within a radius of 248 kpc of the cluster centre is in good agreement with that found from gravitational lensing over the same region, supporting the assumption of isothermal gas in hydrostatic equilibrium. The gas mass fraction of 0.13 +/- 0.02 is in remarkable agreement with that given by cosmological parameters for the Universe as a whole, suggesting that CL 0016+16 is a fair sample of the matter content of the Universe. While there is no spectral or spatial evidence to suggest a cooling flow in CL 0016+16, we find an asymmetrical central X-ray structure which may have a harder spectrum than the cluster as a whole, and be evidence for some merger activity, in addition to a previously reported asymmetry to the west of the cluster. The nearest companion cluster to CL 0016+16 is measured to have a gas temperature of kT = 3.8^{+0.3}_{-0.3} keV and an abundance of 0.6^{+0.3}_{-0.2} times the solar value (1 sigma uncertainties). We also present spectral data for the companion quasar whose spectrum was confused with CL 0016+16 in previous ASCA data.

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arXiv2002

Results from a Complete Chandra Survey of Radio Jets

We report preliminary results from the first targets observed as part of a program to image the X-ray jets in a complete sample of radio selected jets. We have acquired Australian Telescope Compact Array and Very Large Array data with resolution 1", matching the Chandra resolution. PKS 1202-262 and PKS 0208-512 show dramatic coincidence with the initial radio jet, with the X-ray emission dropping off at the large angle radio bends, similar to the structure of PKS 0637-752.

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arXiv2002

Unifying B2 radio galaxies with BL Lac objects

In an earlier paper we presented nuclear X-ray flux densities, measured with ROSAT, for the B2 bright sample of nearby low-luminosity radio galaxies. In this paper we construct a nuclear X-ray luminosity function for the B2 radio galaxies, and discuss the consequences of our results for models in which such radio galaxies are the parent population of BL Lac objects. Based on our observations of the B2 sample, we use Monte Carlo techniques to simulate samples of beamed radio galaxies, and use the selection criteria of existing samples of BL Lac objects to compare our simulated results to what is observed. We find that previous analytical results are not applicable since the BL Lac samples are selected on beamed flux density. A simple model in which BL Lacs are the moderately beamed (gamma ~ 3) counterparts of radio galaxies, with some random dispersion (~ 0.4 decades) in the intrinsic radio-X-ray relationship, can reproduce many of the features of the radio-selected and X-ray-selected BL Lac samples, including their radio and X-ray luminosity functions and the distributions of their radio-to-X-ray spectral indices. In contrast, models in which the X-ray and radio emission have systematically different beaming parameters cannot reproduce important features of the radio-galaxy and BL Lac populations. The redshift distribution and number counts of the X-ray-selected EMSS sample are well reproduced by our best models, supporting a picture in which these objects are beamed FRI radio galaxies with intrinsic luminosities similar to those of the B2 sample. However, we cannot match the redshift distribution of the radio-selected 1-Jy sample, and it is likely that a population of FRII radio galaxies is responsible for the high-redshift objects in this sample. [abridged]

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arXiv2002

Magnetic field strengths in the hotspots and lobes of three powerful FRII radio sources

We have made deep Chandra observations of three powerful FRII radio sources: two quasars (3C263 and 3C351) and one radio galaxy (3C330). X-ray emission from hotspots and lobes, as well as from the active nucleus, is detected in each source. We model the hotspots' synchrotron spectra using VLA, BIMA and HST data. In 3C263 and 3C330, the hotspots' X-ray emission is at a level consistent with being synchrotron self-Compton (SSC) emission, with a hotspot magnetic field close to the equipartition value. In the two hotspots of 3C351, however, an SSC origin for the X-rays would require the magnetic field strength to be an order of magnitude below the equipartition value in our models: in addition, there are offsets between the radio, optical and X-ray emission from the secondary hotspot which are hard to explain in a simple SSC model. We discuss the emission mechanisms that may be responsible for these observations. On our preferred model, the X-ray emission from the radio lobes of the three sources is due to inverse-Compton scattering of the microwave background radiation. If this is the case, the magnetic field strengths in the lobes are typically about a factor 2 below the equipartition values, assuming uniform lobe electron and magnetic field distributions. We detect extended X-ray emission, which we attribute to a cluster/group environment, around 3C263 and 3C330. This detection allows us to show that the lobes are close to pressure balance with their surroundings, as long as no non-radiating particles contribute to the internal pressure of the lobes.

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arXiv2002

High Resolution Soft X-ray Spectroscopy of M87 with the Reflection Grating Spectrometers on XMM-Newton

We present high-resolution X-ray spectroscopic observations of M87 with the Reflection Grating Spectrometers on XMM-Newton. We detect strong K-shell line emission from N, O, Ne, Mg, some emission from He-like Si, a fully resolved set of Fe L-shell emission spectra, and some emission from C. The angular intensity distributions of the strong emission lines are detectably resolved on scales (15-160) arcsec. The gas in the inner arcmin of M87 has a multi-phase structure, as indicated by the similarity of the emission line profiles of Fe L shell ions with widely separated ionization potentials. The global Fe L spectrum is approximately consistent with an isothermal plasma at kT_e ~ 1.8 keV, in addition to a component with a temperature distribution appropriate to an isobaric cooling flow, but with a minimum temperature cutoff of kT_min ~ 600 eV. The behaviour of this cooling-flow component is qualitatively similar to what is seen in other cooling flow clusters. Finally, we do not find any strong evidence for a spatial variation in abundances due to resonance scattering redistribution in the inner arcminute of the core.

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arXiv2002

The X-ray jet and halo of PKS 0521-365

Chandra ACIS observations of PKS 0521-365 find that the X-ray emission of this BL Lac object consists of emission from an unresolved core, a diffuse halo, and a 2 arcsec jet feature coincident with the inner radio/optical jet. A comparison with a new ATCA 8.6-GHz map also finds X-ray emission from the bright hotspot south-east of the nucleus. The jet spectrum, from radio to X-ray, is probably synchrotron emission from an electron population with a broken power-law energy distribution, and resembles the spectra seen from the jets of low-power (FR I) radio galaxies. The hotspot X-ray flux is consistent with the expectations of synchrotron self-Compton emission from a plasma close to equipartition, as seen in studies of high-power (FR II) radio galaxies. While the angular structure of the halo is similar to that found by an analysis of the ROSAT HRI image, its brightness is seen to be lower with Chandra, and the halo is best interpreted as thermal emission from an atmosphere of similar luminosity to the halos around FR I radio galaxies. The X-ray properties of PKS 0521-365 are consistent with it being a foreshortened, beamed, radio galaxy.

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arXiv2002

A Chandra observation of the X-ray environment and jet of 3C31

We have used a deep Chandra observation of the central regions of the twin-jet FRI radio galaxy 3C31 to resolve the thermal X-ray emission in the central few kiloparsecs of the host galaxy, NGC 383, where the jets are thought to be decelerating rapidly. This allows us to make high-precision measurements of the density, temperature and pressure distributions in this region, and to show that the X-ray emitting gas in the centre of the galaxy has a cooling time of only 5 times 10^7 years. In a companion paper these measurements are used to place constraints on models of the jet dynamics. A previously unknown one-sided X-ray jet in 3C31, extending up to 8 arcsec from the nucleus, is detected and resolved. Its structure and steep X-ray spectrum are similar to those of X-ray jets known in other FRI sources, and we attribute the radiation to synchrotron emission from a high-energy population of electrons. In situ particle acceleration is required in the region of the jet where bulk deceleration is taking place. We also present X-ray spectra and luminosities of the galaxies in the Arp 331 chain of which NGC 383 is a member. The spectrum and spatial properties of the nearby bright X-ray source 1E 0104+3153 are used to argue that the soft X-ray emission is mostly due to a foreground group of galaxies rather than to the background broad absorption-line quasar.

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arXiv2002

The XMM-Newton Bright Serendipitous Source Sample (XMM-Newton BSS)

The XMM-Newton Survey Science Center is constructing a large (~1000 sources) and complete sample of bright serendipitous XMM-Newton sources at high galactic latitude, so as to allow both the discovery of sources of high individual interest as well as statistical population studies. The sample, known as ``The XMM-Newton Bright Serendipitous Source Sample" (XMM-Newton BSS), has a flux limit of ~10^(-13) cgs in the 0.5--4.5 keV energy band and will be fundamental in complementing other medium and deep survey programs (having fluxes 10 to 100 times fainter) in order to: a) characterize the X-ray sky and its constituents, b) understand the evolution of the population selected and c) define statistical identification procedures to select rare and interesting populations of X-ray sources (e.g. high z clusters of galaxies, BL Lacs, type 2 QSOs, etc..). We discuss here the scientific rationale and the details of the project and we present some preliminary results.

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arXiv2001

Chandra Observations of the X-ray Jet in Centaurus A

We present high angular resolution X-ray images and spectra from two {\em Chandra} ACIS-I observations of the X-ray jet in the nearby radio galaxy Centaurus A. We find that the X-ray emission from the jet is composed of a low surface brightness diffuse component extending continuously from within at least 60 pc of the active nucleus into the NE radio lobe 4 kpc from the nucleus, along with 31 discrete compact knots, most of which are extended at the resolution of our observation. We find that there are small but significant differences between the X-ray and radio morphologies of the inner jet at the arcsecond level, making it unlikely that a single, spatially coincident population of ultrarelativistic electrons is responsible for the emission in both energy regimes. We suggest that the X-ray knots of the inner jet are indeed the sites of particle acceleration and shocks, and the X-ray and radio knot offsets are caused by a combination of particle diffusion and energy loss. These offsets may be a common feature of all jets in radio galaxies, or at least jets in FR I type galaxies, and may be fundamental to the physics of such jets. The X-ray spectra of several regions of the jet are well fit by absorbed power-law models with photon indices $\sim$2.2-2.5, although the spectrum of one bright knot located $\sim$1 kpc from the nucleus (knot B) is harder (photon index = 2.0).

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arXiv2001

The XMM-Newton Serendipitous Survey. II. First results from the AXIS high galactic latitude medium sensitivity survey

We present the first results on the identifications of a medium sensitivity survey (X-ray flux limit 2E-14 erg/cm2/s in the 0.5-4.5 keV band) at high galactic latitude (|b|>20 deg) carried out with the XMM-Newton X-ray observatory within the AXIS observing programme. This study is being conducted as part of the XMM-Newton Survey Science Centre activities towards the identification of the sources in the X-ray serendipitous sky survey. The sample contains 29 X-ray sources in a solid angle of 0.26deg2 (source density 113 +/- 21 sources/deg2), out of which 27 (93%) have been identified. The majority of the sources are broad-line AGN (19), followed by narrow emission line X-ray emitting galaxies (6, all of which turn out to be AGN), 1 nearby non-emission line galaxy (NGC 4291) and 1 active coronal star. Among the identified sources we find 2 broad-absorption line QSOs (z~1.8 and z~1.9), which constitute ~10% of the AGN population at this flux level, similar to optically selected samples. Identifications of a further 10 X-ray sources fainter than our survey limit are also presented.

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arXiv2001

Chandra finds X-ray jets are common in low-power radio galaxies

We present results for the first three low-power radio galaxies from the B2 bright sample to have been observed with Chandra. Two have kpc-scale radio jets, and in both Chandra resolves jet X-ray emission, and detects soft X-ray core emission and an X-ray emitting galaxy-scale atmosphere of luminosity a few 10^41 ergs s^-1. These are the first detections of X-ray jets in low-power radio galaxies more distant than Cen A and M 87. The cooling time of the galaxy-scale gas implies mass infall rates of order one solar mass per year. The gas pressure near the jets is comparable to the minimum pressure in the jets, implying that the X-ray emitting gas may play an important role in jet dynamics. The third B2 radio galaxy has no kpc-scale radio jet, and here only soft X-ray emission from the core is detected. The ratio of X-ray to radio flux is similar for the jets and cores, and the results favour a synchrotron origin for the emission. kpc-scale radio jets are detected in the X-ray in ~7 ks exposures with Chandra more readily than in the optical via HST snapshot surveys.

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arXiv2001

A Chandra detection of the radio hotspot of 3C123

Chandra X-ray Observatory observations of the powerful, peculiar radio galaxy 3C123 have resulted in an X-ray detection of the bright eastern hotspot, with a 1-keV flux density of ~ 5 nJy. The X-ray flux and spectrum of the hotspot are consistent with the X-rays being inverse-Compton scattering of radio synchrotron photons by the population of electrons responsible for the radio emission (`synchrotron self-Compton emission') if the magnetic fields in the hotspot are close to their equipartition values. 3C123 is thus the third radio galaxy to show X-ray emission from a hotspot which is consistent with being in equipartition. Chandra also detects emission from a moderately rich cluster surrounding 3C123, with Lx (2-10 keV) = 2 x 10^44 ergs/s and kT ~ 5 keV, and absorbed emission from the active nucleus, with an inferred intrinsic column density of 1.7 x 10^22 cm^-2 and an intrinsic 2-10 keV luminosity of 10^44 ergs/s.

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arXiv2001

Chandra observations of the X-ray jet in 3C66B

Our Chandra observation of the FRI radio galaxy 3C66B has resulted in the first detection of an X-ray counterpart to the previously known radio, infra-red and optical jet. The X-ray jet is detected up to 7 arcsec from the core and has a steep X-ray spectrum, alpha ~ 1.3 +/- 0.1. The overall X-ray flux density and spectrum of the jet are consistent with a synchrotron origin for the X-ray emission. However, the inner knot in the jet has a higher ratio of X-ray to radio emission than the others. This suggests that either two distinct emission processes are present or that differences in the acceleration mechanism are required; there may be a contribution to the inner knot's emission from an inverse-Compton process or it may be the site of an early strong shock in the jet. The peak of the brightest radio and X-ray knot is significantly closer to the nucleus in the X-ray than in the radio, which may suggest that the knots are privileged sites for high-energy particle acceleration. 3C66B's jet is similar both in overall spectral shape and in structural detail to those in more nearby sources such as M87 and Centaurus A.

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arXiv2001

Chandra measurements of 3C 220.1's X-ray core and cluster

We report results of an 18 ks exposure with the ACIS instrument on Chandra of the powerful z = 0.62 radio galaxy 3C 220.1. The X-ray emission separates into cluster gas of emission-weighted kT ~ 5 keV and 0.7-12 keV luminosity (to a radius of 45 arcsec) 5.6 x 10^44 ergs s^-1, and unresolved emission (coincident with the radio core). While the extended X-ray emission is clearly thermal in nature, a straightforward cooling-flow model, even in conjunction with a point-source component, is a poor fit to the radial profile of the X-ray emission. This is despite the fact that the measured properties of the gas suggest a massive cooling flow of ~ 130 M_\odot yr^-1, and the data show weak evidence for a temperature gradient. The central unresolved X-ray emission has a power-law spectral energy index α~ 0.7 and 0.7-12 keV luminosity 10^45 ergs s^-1, and any intrinsic absorption is relatively small. The two-point spectrum of the core emission between radio and X-ray energies has α_rx = 0.75. Since this is a flatter spectrum than seen in other sources where the X-ray emission is presumed to be radio-related, regions close to the AGN in this source may dominate the central X-ray output, as is believed to be the case for lobe-dominated quasars. Simple unification models would be challenged if this were found to be the case for a large fraction of high-power radio galaxies.

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arXiv2000

The X-ray emission of 3C 346 and its environment

We present a detailed spectral and spatial analysis of the X-ray properties of the compact and unusual radio galaxy 3C 346, combining information from ROSAT and ASCA. The dominant component of X-ray emission (~ 10^44 ergs/s in each of the 0.5-3 keV and 2-10 keV bands) is unresolved and not heavily absorbed (intrinsic N_H ~< 2 10^21 cm^-2), with evidence for variability of 32 +/- 13% over 18 months. We relate the X-ray emission to radio structures on both milliarcsecond scales and the arcsecond scales which Chandra can resolve. The absence of X-ray absorption, and the radio/optical/X-ray colors, when combined with previous radio evidence that the source is a foreshortened FRII, suggest that the radio jets are seen at an angle to the line of sight of about 30 degrees, intermediate between the radio-galaxy and quasar classes. Roughly a third of the soft X-ray emission is from a cluster atmosphere, for which we measure a temperature of 1.9 (+1.3, -0.7) keV, making this the second low-redshift (z < 0.2) powerful radio galaxy, after Cygnus A, with a measured cluster temperature. At a jet angle of ~ 30 degrees, all the radio structures lie within the core radius of the cluster, for which the cooling time is sufficiently long that there is no reason to expect the presence of a cooling flow. The radio lobes of 3C 346 are roughly in pressure balance with the external medium under the assumptions that the energy densities in the magnetic field and radiating particles balance and that a source of excess pressure in the radio lobes, commonly invoked in other radio galaxies, is absent here.

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arXiv2000

XMM-Newton observations of M87 and Its X-Ray Halo

We report performance verification observations of the giant elliptical galaxy M87 in the Virgo Cluster with the MOS, pn, and optical monitor instruments on board of XMM-Newton. With the energy sensitive imaging instruments MOS and pn we obtain the first spatially constrained X-ray spectra of the nucleus and the jet of the galaxy. The good photon statistics of the pn and MOS allow a detailed analysis of the radial temperature and abundance distribution of 6 elements. The data provide no indication of a multi-temperature structure for radii less than 2 arcmin. An apparent sharp metal abundance drop deduced for the regions inside this radius is probably due to resonant line scattering.

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arXiv2000

An XMM-Newton study of the sub-structure in M87's halo

The high signal to noise and good point spread function of XMM have allowed the first detailed study of the interaction between the thermal and radio emitting plasma in the central regions of M87. We show that the X-ray emitting structure, previously seen by ROSAT, is thermal in nature and that the east and southwest extensions in M87's X-ray halo have a significantly lower temperature (kT= 1.5 keV) than the surrounding ambient medium (kT= 2.3 keV). There is little or no evidence for non-thermal emission with an upper limit on the contribution of a power law component of spectral index flatter than 3 being less than 1% of the flux in the region of the radio lobes.

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arXiv2000

NGC 6251 at multiple scales and wavelengths

We have studied the FR I radio galaxy NGC 6251 and its environment at several wavelengths and scale lengths. On the large scale, we have probed the gravity field by measuring the velocity dispersion of the cluster members associated with NGC 6251 and relating this to the cluster's X-ray emission. On the small scale, the gravitational information is provided by cold HI near the nucleus and the distribution of stars and gas near the centre of the galaxy. The cold HI gas which we have measured explains the absorption of the central X-ray emission and is consistent with the extinction through the recently discovered HST gas disc of NGC 6251.

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