arXiv subjects

A. C. Fabian

Publications and source records attributed to A. C. Fabian.

At least 559 records · Page 31Linked to original sources

arXiv2005

The low-power nucleus of PKS 1246-410 in the Centaurus Cluster

We present Chandra, Very Large Array (VLA), and Very Long Baseline Array (VLBA) observations of the nucleus of NGC 4696, a giant elliptical in the Centaurus cluster of galaxies. Like M87 in the Virgo cluster, PKS 1246-410 in the Centaurus cluster is a nearby example of a radio galaxy in a dense cluster environment. In analyzing the new X-ray data we have found a compact X-ray feature coincident with the optical and radio core. While nuclear emission from the X-ray source is expected, its luminosity is low, <10^{40} erg/s. We estimate the Bondi accretion radius to be 30 pc and the accretion rate to be 0.01 M_sun/yr which under the canonical radiative efficiency of 10% would overproduce by 3.5 orders of magnitude the radiative luminosity. Much of this energy can be directed into the kinetic energy of the jet, which over time inflates the observed cavities seen in the thermal gas. The VLBA observations reveal a weak nucleus and a broad, one-sided jet extending over 25 parsecs in position angle -150 degrees. This jet is deflected on the kpc-scale to a more east-west orientation (position angle of -80 degrees).

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arXiv2005

Radio Bubbles in Clusters

We extend our earlier work on cluster cores with distinct radio bubbles, adding more active bubbles, i.e. those with Ghz radio emission, to our sample, and also investigating ``ghost bubbles,'' i.e. those without GHz radio emission. We have determined k, which is the ratio of the total particle energy to that of the electrons radiating between 10 MHz and 10 GHz. Constraints on the ages of the active bubbles confirm that the ratio of the energy factor, k, to the volume filling factor, f lies within the range 1 < k/f < 1000. In the assumption that there is pressure equilibrium between the radio-emitting plasma and the surrounding thermal X-ray gas, none of the radio lobes has equipartition between the relativistic particles and the magnetic field. A Monte-Carlo simulation of the data led to the conclusion that there are not enough bubbles present in the current sample to be able to determine the shape of the population. An analysis of the ghost bubbles in our sample showed that on the whole they have higher upper limits on k/f than the active bubbles, especially when compared to those in the same cluster. A study of the Brightest 55 cluster sample shows that 17, possibly 20, clusters required some form of heating as they have a short central cooling time, t_cool < 3 Gyr, and a large central temperature drop, T_centre/T_outer< 1/2. Of these between 12 (70 per cent) and 15 (75 per cent), contain bubbles. This indicates that the duty cycle of bubbles is large in such clusters and that they can play a major role in the heating process.

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arXiv2005

INTEGRAL observations of AGN in the Galactic Plane

We present results on approximately one year of INTEGRAL observations of six AGN detected during the regular scans of the Galactic Plane. The sample is composed by five Seyfert 2 objects (MCG -05-23-16, NGC 4945, the Circinus galaxy, NGC 6300, ESO 103-G35) and the radio galaxy Centaurus A. The continuum emission of each of these sources is well represented by a highly absorbed (NH > 1e22 1/cm^2) power law, with average spectral index Gamma = 1.9 +/- 0.3. A high energy exponential cut-off at Ec ~ 50 keV is required to fit the spectrum of the Circinus galaxy, whereas a lower limit of 130 keV has been found for NGC 4945 and no cut-off has been detected for NGC 6300 in the energy range covered by these INTEGRAL data. The flux of Centaurus A was found to vary by a factor of ~ 2 in 10 months, showing a spectral change between the high and low state, which can be modelled equally well by a change in the absorption (NH from 17e22 to 33e22 1/cm^2) or by the presence of a cut-off at >~ 120 keV in the low state spectrum. A comparison with recently reprocessed BeppoSAX/PDS data shows a general agreement with INTEGRAL results. The high energy cut-off in the hard X-ray spectra appears to be a common but not universal characteristic of Seyfert 2 and to span a wide range of energies.

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arXiv2005

A possible line-like emission feature at 8 keV in the Seyfert 1.2 UGC 3973

Two short X-ray exposures (< 3600 s each) of the radio-quiet Seyfert 1.2 galaxy UGC 3973 (Mrk 79) were conducted with XMM-Newton as part of an AGN snap-shot survey. In this paper we concentrate on the significance and possible origin of a narrow 8 keV (rest-frame) line-like emission feature detected with the pn instrument during the second observation. Simulations show that the feature is significant at 96.0-98.4% confidence, depending on what a priori assumptions are made. The feature cannot be attributed to background contamination and appears to be variable (or transient) since it was not detected in the first observation of UGC 3973 six months earlier. However, a constant feature cannot be completely dismissed, based on the 90% upper-limit on the flux from the first observation. There is some indication that the feature is variable over the duration of the second observation as well. We discuss various models (e.g. Ni emission, recombination edges, outflows, disc lines) which could potentially produce an emission feature at such energies.

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arXiv2005

On viscosity, conduction and sound waves in the intracluster medium

Recent X-ray and optical observations of the Perseus cluster indicate that the viscous and conductive dissipation of sound waves is the mechanism responsible for heating the intracluster medium and thus balancing radiative cooling of cluster cores. We discuss this mechanism more generally and show how the specific heating and cooling rates vary with temperature and radius. It appears that the heating mechanism is most effective above 10^7K, which allows for radiative cooling to proceed within normal galaxy formation but will stifle the growth of very massive galaxies. The scaling of the wavelength of sound waves with cluster temperature and feedback in the system are investigated.

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arXiv2005

The extended H-alpha emitting filaments surrounding NGC4696, the central galaxy of the Centaurus cluster

We present images of NGC4696, the central galaxy in the Centaurus cluster, showing the large extent of cool filaments which are bright in H-alpha line emission. These filaments share the detailed structure of both the central dust lane and the inner regions of the arc-like plumes seen in soft X-ray emission. The X-ray gas is at its coolest, and most absorbed in this same region. The smoothness of the features implies that the local environment is not strongly turbulent. We suggest that these filaments are either shaped by confinement due to a strong magnetic field, or by bulk flows within the intracluster medium. We propose that like similar filamentary systems in the core of clusters, these cooler components have been drawn out of the central galaxy behind buoyant gas bubbles from previous episodes of radio activity. We find a spur of low-frequency radio emission leading to a region of low X-ray pressure within the intracluster medium supporting this interpretation.

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arXiv2005

XMM-Newton observations of the Lockman Hole IV: spectra of the brightest AGN

This paper presents the results of a detailed X-ray spectral analysis of a sample of 123 X-ray sources (46 being type-1 AGN and 28 type-2 AGN) detected with XMM-Newton in the Lockman Hole field. Using a single power law model our sources' average spectral slope hardens at faint 0.5-2 keV fluxes but not at faint 2-10 keV fluxes. We explain the effect in terms of an increase in X-ray absorption at faint fluxes. The average spectral slope of our sources is ~1.9 with an intrinsic dispersion of ~0.28. We detected X-ray absorption in 37% of the sources, ~10% in type-1 AGN and ~77% in type-2 AGN. Using X-ray fluxes corrected for absorption, the fraction of absorbed objects and the absorbing column density distribution did not vary with X-ray flux. Our type-1 and type-2 AGN do not appear to have different continuum shapes, but the distribution of intrinsic absorption is different among both classes. A significant fraction of our type-2 AGN (5 out of 28) were found to display no substantial absorption. We discuss possible interpretations to this in terms of Compton-thick AGN and intrinsic Broad Line Region properties. An emission line compatible with Fe Kalpha was detected in 8 sources with rest frame equivalent widths 120-1000 eV. The AGN continuum or intrinsic absorption did not depend on X-ray luminosity and/or redshift. Soft excess emission was detected in 18 objects, but only in 9 could we fit this spectral component with a black body model. The 0.5-2 keV luminosities of the fitted black body were not significantly different in type-1 and type-2 AGN, although the temperatures were slightly higher in type-2 AGN than in type-1 AGN. For 9 sources (including 1 type-1 AGN and 3 type-2 AGN) a scattering model provided a better fit of the soft excess emission.

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arXiv2005

X-rays from the Type II quasar in the hyperluminous infrared galaxy IRAS F15307+3252

We report the detection of X-ray emission from the hyperluminous infrared galaxy IRAS F15307+3252 at z=0.93 and its properties obtained from XMM-Newton observations. Although the X-ray emission is very faint and the data are noisy, a prominent line-like feature in the observed 3-4 keV range is inferred from both photometric and spectroscopic techniques. It indicates an X-ray spectrum dominated by 6.4 keV Fe K emission and the presence of a Compton-thick AGN. Our estimate of the luminosity of the illuminating source (L(2-10keV)>=1e45 erg/s), required to produce the observed Fe K emission in reflection from cold matter, means that the hidden quasar nucleus accounts for a significant fraction of the large bolometric luminosity. The soft X-ray emission below 2 keV is found to be spatially extended and probably of a separate origin. The temperature and bolometric luminosity (kT=2 keV and L(bol,CL)=1e44 erg/s) obtained from a thermal spectrum place this X-ray source on the L-Tx relation of galaxy clusters. The possible association with a galaxy cluster can be added to the list of remarkable similarities between IRAS F15307+3252 and another hyperluminous infrared galaxy IRAS 09104+4109 (z=0.44), both of which have bolometric luminosities dominated by hidden quasar nuclei. Our result on IRAS F15307+3252 illustrates how difficult it is to detect Compton thick Type II quasars at z>=1, particularly if their bolometric outputs do not rival the hyperluminous population.

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arXiv2005

Investigating ionized disc models of the variable narrow-line Seyfert 1 PG 1404+226

We investigate the use of relativistically blurred photoionized disc models on an XMM-Newton observation of the Narrow Line Seyfert 1 galaxy PG 1404+226. The model is designed to reproduce the radiation from the inner accretion disc around a Kerr black hole, and is more successful at fitting the spectrum than models based on a thermal soft excess. The source varies strongly over the course of the observation, and the disc model works over all observed flux states. We conclude that it is a useful tool in the study of certain quasars.

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arXiv2005

A Chandra HETGS Spectral Study of the Iron K Bandpass in MCG-6-30-15: A Narrow View of the Broad Iron Line

We present a high resolution X-ray spectrum of the iron K bandpass in MCG-6-30-15 based on a 522 ksec observation with Chandra's High Energy Transmission Grating Spectrometer. The Chandra spectrum is consistent with the presence of a relativistically broadened, highly redshifted iron K alpha emission line with a similar profile to previous observations. A number of narrow features are detected above 2 keV, including a narrow Fe K alpha emission line and narrow absorption lines from H- and He-like Fe, H-like S and H-like Si. This absorption is well described by a photoionized plasma with a column density log N_H = 23.2 and an ionization parameter log xi = 3.6, assuming the iron abundance has the Solar value and a velocity dispersion parameter b = 100 km/s. Applying this absorption model to a high fidelity XMM-Newton EPIC-pn spectrum we find that a broad iron line is still required with emission extending to within 1.9 gravitational radii of the black hole. If the iron line comes from an accretion disk truncated at the innermost stable circular orbit, this indicates that the black hole must be spinning rapidly with a>0.95. Ionized absorption models attempting to explain the 3-6 keV spectral curvature without strong gravity predict absorption lines in the 6.4-6.6 keV range that are inconsistent with the Chandra spectrum. The H- and He-like iron absorption lines in the Chandra spectrum are blueshifted by 2.0 (+0.7/-0.9) x 10^3 km/s compared to the source frame, and may originate in a high velocity, high ionization component of the warm absorber outflow. This high ionization component may dominate the energy budget of the outflow, and account for a significant fraction of the outflowing mass. (Abstract truncated).

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arXiv2005

The giant H-alpha/X-ray filament in the cluster of galaxies A1795

The cluster of galaxies A1795 hosts a 46 kpc-long filament at its core, which is clearly visible in the light of H-alpha and X-ray emission. We present optical slit spectroscopy and deeper Chandra X-ray data of the filament. The optical spectra reveal that the the bulk of the filament is quiescent (with sigma < 130 km/s), although considerable velocity structure is apparent around the powerful radio source in the central cluster galaxy, where a direct interaction is occurring between the radio plasma and the surrounding intracluster medium. The filament contains a clump of UV/blue continuum halfway along its length, which we resolve into a chain of at least 5 distinct knots using archival HST images; the optical spectrum of this clump confirm it to be mostly comprised of O stars. It is well-removed from the central galaxy and radio source, and is most likely an example of a group of young star clusters condensing directly from the cooling gas in the filament. The observed spatial offset between these knots of star formation and the peak in the optical line emission confirms that the massive star formation is most unlikely to be responsible for the bulk of the observed emission-line luminosity in the filament. Some other (as yet undetermined) source of energy is required to power and maintain the optical line-emission, yet it must not completely impede the cooling of the X-ray gas within the filament to form the star clusters.

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arXiv2005

X-ray reflection in the Seyfert galaxy 1H0419-577 revealing strong relativistic effects in the vicinity of a Kerr black hole

We report results obtained from six XMM-Newton observations of the Seyfert galaxy 1H 0419-577. Here we show that the X-ray spectrum and variability are well described by a two-component model comprising a power law with constant spectral shape and variable normalisation and a much more constant ionised reflection component from the inner accretion. One of the observations was performed when the source was in a particularly low flux state in which the X-ray spectrum is rather peculiar and exhibits a very flat hard spectrum with broad residuals below 6.6 keV and a steep soft excess below about 1 keV. We interpret the spectrum as being reflection-dominated by X-ray reprocessed emission from the inner accretion disc. The primary continuum is almost completely unobserved possibly because of strong light bending towards the central black hole. The ionised reflection model simultaneously accounts for the broad residuals and hard flat spectrum and for the soft excess. The same model provides an excellent description of the data at all the other flux levels, the most important difference being a variation in the power law normalisation. Our results imply that most of the X-ray emission in this source originates from within few gravitational radii from the central black hole and requires that the compact object is an almost maximally spinning Kerr black hole. (abridged)

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arXiv2005

Non-thermal X-rays, a high abundance ridge and fossil bubbles in the core of the Perseus cluster of galaxies

Using a deep Chandra observation of the Perseus cluster of galaxies, we find a high-abundance shell 250 arcsec (93 kpc) from the central nucleus. This ridge lies at the edge of the Perseus radio mini-halo. In addition we identify two Halpha filaments pointing towards this shell. We hypothesise that this ridge is the edge of a fossil radio bubble, formed by entrained enriched material lifted from the core of the cluster. There is a temperature jump outside the shell, but the pressure is continuous indicating a cold front. A non-thermal component is mapped over the core of the cluster with a morphology similar to the mini-halo. Its total luminosity is 4.8x10^43 erg/s, extending in radius to ~75 kpc. Assuming the non-thermal emission is the result of inverse Compton scattering of the CMB and infrared emission from NGC 1275, we map the magnetic field over the core of the cluster.

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arXiv2005

The Prevalence of Cooling Cores in Clusters of Galaxies at z~0.15-0.4

We present a Chandra study of 38 X-ray luminous clusters of galaxies in the ROSAT Brightest Cluster Sample (BCS) that lie at z~0.15-0.4. We find that the majority of clusters at moderate redshift generally have smooth, relaxed morphologies with some evidence for mild substructure perhaps indicative of recent minor merger activity. Using spatially-resolved spectral analyses, cool cores appear to still be common at these redshifts. At a radius of 50 kpc, we find that at least 55 per cent of the clusters in our sample exhibit signs of mild cooling [t(cool)<10 Gyr], while in the central bin at least 34 per cent demonstrate signs of strong cooling [t(cool)<2 Gyr]. These percentages are nearly identical to those found for luminous, low-redshift clusters of galaxies, suggesting little evolution in cluster cores since z~0.4 and that heating and cooling mechanisms may already have stabilised by this epoch. Comparing the central cooling times to central Halpha emission in BCS clusters, we find a strong correspondence between the detection of Halpha and central cooling time. (Abridged)

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arXiv2005

A deep Chandra observation of the Centaurus cluster: bubbles, filaments and edges

X-ray images and gas temperatures taken from a deep ~200 ks Chandra observation of the Centaurus cluster are presented. Multiple inner bubbles and outer semicircular edges are revealed, together with wispy filaments of soft X-ray emitting gas. The frothy central structure and eastern edge are likely due to the central radio source blowing bubbles in the intracluster gas. The semicircular edges to the surface brightness maps 32kpc to the east and 17.5kpc to the west are marked by sharp temperature increases and abundance drops. The edges could be due to sloshing motions of the central potential, or are possibly enhanced by earlier radio activity. The high abundance of the innermost gas (about 2.5 times Solar) limits the amount of diffusion and mixing taking place.

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arXiv2005

Evidence for an Untruncated Accretion Disk in the Broad-Line Radio Galaxy 4C+74.26

We present evidence for a broad, ionized Fe K line in the XMM-Newton spectrum of the broad-line radio galaxy (BLRG) 4C+74.26. This is the first indication that the innermost regions of the accretion flow in BLRGs contain thin, radiatively efficient disks. Analysis of the 35 ks XMM-Newton observation finds a broad line with an inner radius close to the innermost stable circular orbit for a maximally spinning black hole. The outer radius of the relativistic line is also found to be within 10 gravitational radii. The Fe K line profile gives an inclination angle of ~40 degrees, consistent with the radio limit. There are two narrow components to the Fe K complex: one at 6.4 keV from neutral Fe, and one at 6.2 keV. These may form the blue and red horns of a diskline from farther out on the disk, but a longer observation is required to confirm this hypothesis. We discuss the implications of this observation for models of jet production, and suggest that BLRGs and radio-loud quasars will have larger than average black hole masses, thus resulting in thicker accretion flows close to the black hole.

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arXiv2005

An XMM-Newton Observation of Abell 2597

We report on a 120 ks XMM-Newton observation of the galaxy cluster Abell 2597. Results from both the European Photon Imaging Camera (EPIC) and the Reflection Grating Spectrometer (RGS) are presented. From EPIC we obtain radial profiles of temperature, density and abundance, and use these to derive cooling time and entropy. We illustrate corrections to these profiles for projection and point spread function (PSF) effects. At the spatial resolution available to XMM-Newton, the temperature declines by around a factor of two in the central 150 kpc or so in radius, and the abundance increases from about one-fifth to over one-half solar. The cooling time is less than 10 Gyr inside a radius of 130 kpc. EPIC fits to the central region are consistent with a cooling flow of around 100 solar masses per year. Broad-band fits to the RGS spectra extracted from the central 2 arcmin are also consistent with a cooling flow of the same magnitude; with a preferred low-temperature cut-off of essentially zero. The data appear to suggest (albeit at low significance levels below formal detection limits) the presence of the important thermometer lines from Fe XVII at 15, 17 Angstrom rest wavelength, characteristic of gas at temperatures ~ 0.3 keV. The measured flux in each line is converted to a mass deposition estimate by comparison with a classical cooling flow model, and once again values at the level of 100 solar masses per year are obtained. These mass deposition rates, whilst lower than those of previous generations of X-ray observatories, are consistent with those obtained from UV data for this object. This raises the possibility of a classical cooling flow, at the level of around 100 solar masses per year, cooling from 4 keV by more than two orders of magnitude in temperature.

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arXiv2005

XMM observations of the high-redshift quasar RXJ1028.6-0844 at z=4.276: soft X-ray spectral flattening

We present results from a new XMM-Newton observation of the high-redshift quasar RXJ1028.6-0844 at a redshift of 4.276. The soft X-ray spectral flattening, as reported by a study with ASCA previously (Yuan et al. 2000, ApJ 545, 625), is confirmed to be present, however, with reduced column density when modelled by absorption. The inferred column density for absorption intrinsic to the quasar is 2.1(+0.4-0.3)x10^22 (cm^-2) for cold matter, and higher for ionised gas. The spectral flattening shows remarkable similarity with those of two similar objects, GB1428+4217 (Worsley et al. 2004, MNRAS 350, L67) and PMNJ0525-3343 (Worsley et al. 2004, MNRAS 350, 207). The results improve upon those obtained from a previous short-exposure observation for RXJ1028.6-0844 with XMM-Newton (Grupe et al. 2004, AJ 127, 1). A comparative study of the two XMM-Newton observations reveals a change in the power-law photon index from Gamma ~1.3 to 1.5 on timescales of about one year. A tentative excess emission feature in the rest-frame 5-10keV band is suggested, which is similar to that marginally suggested for GB1428+4217.

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