arXiv subjects

A. C. Fabian

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

At least 775 records · Page 43Linked to original sources

arXiv1999

Low radiative efficiency accretion in the nuclei of elliptical galaxies

The discovery of hard, power-law X-ray emission from a sample of six nearby elliptical galaxies, including the dominant galaxies of the Virgo,Fornax and Centaurus clusters (M87, NGC 1399 and NGC 4696, respectively), and NGC 4472, 4636 and 4649 in the Virgo cluster, has important implications for the study of quiescent supermassive black holes. We describe how the broad band spectral energy distributions of these galaxies, which accrete from their hot gaseous halos at rates comparable to their Bondi rates, can be explained by low-radiative efficiency accretion flows in which a significant fraction of the mass, angular momentum and energy is removed from the flows by winds. The observed suppression of the synchrotron component in the radio band and the systematically hard X-ray spectra, which are interpreted as thermal bremsstrahlung emission, support the conjecture that significant mass outflow is a natural consequence of systems accreting at low-radiative efficiencies. We briefly discuss an alternative model for the X-ray emission, namely that it is due to nonthermal synchrotron-self Compton processes in the accretion flow, or wind. This appears to require implausibly weak magnetic fields. Emission from a collimated jet viewed off axis should be distinguishable from the bremsstrahlung model by variability and thermal line emission studies. We argue that the difference in radiative efficiency between the nuclei of spiral and elliptical galaxies arises from the different manner in which interstellar gas is fed into the nuclei. In ellipticals, matter fed from the hot (slowly cooling) ISM is likely to be highly magnetized and with low specific angular momentum, both of which favor low-radiative efficiency accretion solutions and possibly the formation of the observed jets.

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arXiv1999

Detection of X-ray emission from the host clusters of 3CR quasars

We report the detection of extended X-ray emission around several powerful 3CR quasars with redshifts out to 0.73. The ROSAT HRI images of the quasars have been corrected for spacecraft wobble and compared with an empirical point-spread function. All the quasars examined show excess emission at radii of 15 arcsec and more; the evidence being strong for the more distant objects and weak only for the two nearest ones, which are known from other wavelengths not to lie in strongly clustered environments. The spatial profiles of the extended component is consistent with thermal emission from the intracluster medium of moderately rich host clusters to the quasars. The total luminosities of the clusters are in the range 4x10^44 - 3x10^45 erg/s, assuming a temperature of 4keV. The inner regions of the intracluster medium are, in all cases, dense enough to be part of a cooling flow.

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arXiv1999

ROSAT PSPC observations of 36 high-luminosity clusters of galaxies: constraints on the gas fraction

We present a detailed and homogeneous analysis of the ROSAT PSPC surface brightness profiles of 36 clusters of galaxies with high X-ray luminosity (L_X > 10^{45} erg s^{-1}) and redshifts between 0.05 and 0.44. Using recent ASCA estimates of the temperature of the gas for most of the clusters in the sample, we apply both the deprojection technique and model fitting to the surface brightness profiles to constrain the gas and dark matter distributions under the assumption that the gas is both isothermal and hydrostatic. Applying robust estimators, we find that the gas fraction within r_{500} of the clusters in our sample has a distribution centred on f_gas(r_{500}) = 0.168 h_{50}^{-1.5}. The gas fraction ranges from 0.101 to 0.245 at the 95 per cent confidence level. The values of f_gas show highly significant variations between individual clusters, which may be explained if the dark matter has a significant baryonic component. Within a cluster, the average radial dependence of the gas mass fraction increases outward as r^s, with s~0.20. Combining these results with those of primordial nucleosynthesis calculations and the current estimate of H_0, the above central location implies Ω_{0, m} < 0.56 at the 95 per cent confidence level. This upper limit decreases to 0.34 if we take the highest significant estimates for f_gas. A significant decrease in cluster gas fraction with redshift from the local value, f_{gas, 0}, of 0.21, found assuming Ω_{0, m} =1, is also reduced if Ω_{0, m} is low.

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arXiv1999

Variation of the broad X-ray iron line in MCG-6-30-15 during a flare

We report results on the broad iron emission line of the Seyfert galaxy MCG-6-30-15, obtained from the second long ASCA observation in 1997. The time-averaged profile of the broad line is very similar to that seen with ASCA in 1994, so confirming the detailed model fit then obtained. A bright flare is seen in the light curve, during which the continuum was soft. At that time the emission line peaks around 5 keV and most of its emission is shifted below 6 keV with no component detected at 6.4 keV (EW<60 eV). This can be interpreted as the result of an extraordinarily large gravitational redshift due to a dominant flare occurring very close to the black hole at a radius of <5m.

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arXiv1999

ASCA and ROSAT observations of NGC5548: discrepant spectral indices

We report on simultaneous ASCA and ROSAT observations of the Seyfert galaxy NGC5548 made during the ASCA Performance Verification phase. Spectral features due to a warm absorber and reflection are clearly seen in the X-ray spectra. We find that the continuum spectral shape differs between the ASCA and ROSAT datasets. The photon-index obtained from the ROSAT PSPC exceeds that from the ASCA SIS about 0.4. The discrepancy is clear even in the 0.5-2 keV energy band over which both detectors are sensitive. The spectra cannot be made consistent by choosing a more complex model. The problem likely lies in the response curve (estimated effective area) of one, or both, detectors. There may be important consequences for a wide range of published results.

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arXiv1999

An RXTE study of M87 and the core of the Virgo cluster

We present hard X-ray observations of the nearby radio galaxy M87 and the core of the Virgo cluster using the Rossi X-ray Timing Explorer. These are the first hard X-ray observations of M87 not affected by contamination from the nearby Seyfert 2 galaxy NGC4388. Thermal emission from Virgo's intracluster medium is clearly detected and has a spectrum indicative of kT=2.5keV plasma with approximately 25% cosmic abundances. No non-thermal (power-law) emission from M87 is detected in the hard X-ray band, with fluctuations in the Cosmic X-ray Background being the limiting factor. Combining with ROSAT data, we infer that the X-ray spectrum of the M87 core and jet must be steep (Gamma_core>1.90$ and Gamma_jet>1.75), and we discuss the implications of this result. In particular, these results are consistent with M87 being a mis-aligned BL-Lac object.

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arXiv1999

The optical variability of the narrow line Seyfert 1 galaxy IRAS 13224-3809

We report on a short optical monitoring programme of the narrow-line Seyfert 1 Galaxy IRAS 13224-3809. Previous X-ray observations of this object have shown persistent giant variability. The degree of variability at other wavelengths may then be used to constrain the conditions and emission processes within the nucleus. Optical variability is expected if the electron population responsible for the soft X-ray emission is changing rapidly and Compton-upscattering infrared photons in the nucleus, or if the mechanism responsible for X-ray emission causes all the emission processes to vary together. We find that there is no significant optical variability with a firm upper limit of 2 per cent and conclude that the primary soft X-ray emission region produces little of the observed optical emission. The X-ray and optical emission regions must be physically distinct and any reprocessing of X-rays into the optical waveband occurs some distance from the nucleus. The lack of optical variability indicates that the energy density of infrared radiation in the nucleus is at most equal to that of the ultraviolet radiation since little is upscattered into the optical waveband. The extremely large X-ray variability of IRAS 13224-3809 may be explained by relativistic boosting of more modest variations. Although such boosting enhances X-ray variability over optical variability, this only partially explains the lack of optical variability.

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arXiv1999

The ROSAT Brightest Cluster Sample (BCS): -- III. Optical spectra of the central cluster galaxies

We present new spectra of dominant galaxies in X-ray selected clusters, which combine with our previously published spectra to form a sample of 256 dominant galaxies in 215 clusters; 177 of these are members of the ROSAT Brightest Cluster Sample. 27% of the central dominant galaxies have emission-line spectra, all but 5 with line intensity ratios typical of cooling flow nebulae. A further 6% show only [NII] with H-alpha in absorption. We find no evidence for an increase in the frequency of line emission with X-ray luminosity. Purely X-ray selected clusters at low redshift have a higher probability of containing line emission. The more H-alpha luminous galaxies have larger emission-line regions and show a higher ratio of Balmer to forbidden line emission, although there is a continuous trend of ionization behaviour across four decades in H-alpha luminosity. Galaxies with the more luminous line emission show a significantly bluer continuum, whereas lower-luminosity and [NII]-only line emitters have continua that differ little from those of non-line emitting dominant galaxies. Values of the Balmer decrement in the more luminous systems commonly imply intrinsic reddening of E(B-V)~0.3, and when this is corrected for, the excess blue light can be characterized by a population of massive young stars, which can also provide sufficient photoionization to produce the observed H-alpha luminosity. The lower H-alpha luminosity systems show a higher ionization state and few massive stars, requiring instead the introduction of a harder source of photoionization.

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arXiv1999

X-ray reflection spectra from ionized slabs

X-ray reflection spectra are an important component in the X-ray spectra of many active galactic nuclei and Galactic black hole candidates. It is likely that reflection takes place from highly ionized surfaces of the accretion disc in some cases. This can lead to strong Comptonization of the emergent iron, and other, absorption and emission features. We present such reflection spectra here, computed in a self-consistent manner with the method described by Ross and Fabian. In particular we emphasise the range where the ionization parameter (the flux to density ratio) ξis around and above 10^4. Such spectra may be relevant to the observed spectral features found in black hole candidates such as Cygnus X-1 in the low/hard state.

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arXiv1999

X-ray and lensing results on the cluster around the powerful radio galaxy 4C+55.16

We present results from ASCA and ROSAT HRI observations of the powerful radio galaxy 4C+55.16 at redshift 0.24. Extended soft X-ray emission is imaged by the ROSAT HRI. The X-ray brightness profile is sharply peaked on the radio galaxy, characteristic of a strong cooling flow. The X-ray spectrum obtained from ASCA is consistent with multi-phase intracluster gas. There is evidence for an absorbed cool component as well as ambient cluster medium in the ASCA spectrum. A spectral fit, taking a cooling-flow component into account, gives a temperature of kT=5.4(+1.4/-0.9) keV, metal abundance 0.5(+/-0.1)Zsun, excess absorption on the cool component, nH=4.9(+3.4/-1.3)E+21 cm-2, and an absorption-corrected bolometric luminosity of 2.2E+45 erg/s (H0=50km/s/Mpc, q0=0.5). The mass deposition rate is estimated to be about 1100 Msun/yr from the spectral analysis, in good agreement with that derived from imaging analysis of the ROSAT data when corrected for absorption. We tentatively identify a blue feature, seen about 15 arcsec from the centre of the radio galaxy in a published optical image, as a gravitationally-lensed arc. The inferred lensing mass is consistent with the gravitational mass derived from the X-ray data. The best-estimate of the redshift of the lensed object is 1.5(>0.7). All the observed properties suggest that the environment of 4C+55.16 is similar to known massive cooling flow clusters. This is the first massive cooling flow to be found around a powerful, radio source with a compact, GHz-peaked spectrum core.

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arXiv1999

BeppoSAX observations of Mkn 3: Piercing through the torus of a Seyfert 2 galaxy

A new BeppoSAX broad-band (0.6-150 keV) spectrum of the Seyfert 2 galaxy Mkn 3 is presented. The spectrum provides a direct measurement of a large, neutral column of gas with Nh~10**24 cm**-2 in the source direction. The source, as bright as 3C 273 above 10 keV, has a steep (Gamma~1.8) spectrum without any evidence of a high-energy cutoff up to at least 150 keV. At lower energies, the data are best modeled with the addition of an unabsorbed reflection component. Combining these data with previous Ginga and ASCA observations, the Fe K$_α$ and reflection continuum indicate that the reprocessed emission is responding slower than the intrinsic continuum variations suggesting a size of the reprocessor >= 2 pc. Identifying such a reprocessor with a (close to edge-on) obscuring torus, the overall result fits well into unified models since, presumably, one can interpret the strong absorption as due to transmission through the rim of the torus and the unabsorbed (directly viewed) reflection component as due to reprocessing from the torus inner surface.

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arXiv1999

ROSAT HRI Monitoring of Extreme X-ray Variability in the Narrow-Line Quasar PHL 1092

We report results from an 18-day ROSAT HRI monitoring campaign on the ultrasoft Narrow-Line Seyfert 1 (NLS1) class quasar PHL 1092. This luminous, radio-quiet quasar showed strong X-ray variability in a short ROSAT PSPC observation, and ROSAT HRI monitoring of the similar object IRAS 13224-3809 revealed extreme variability on intermediate timescales. We wanted to determine whether remarkable X-ray variability persistently occurs in PHL 1092, and we also wanted to search for outstanding variability events that constrain emission processes. Given the large luminosity of PHL 1092 (about 5x10^{45} erg/s in the HRI band), we detect extremely rapid and large-amplitude X-ray variability throughout our monitoring. The maximum observed variability amplitude is a factor of about 14, and in the most rapid variability event the HRI count rate increases by a factor of about 3.8 in a rest-frame time interval of < 3580 s. The most rapid event has a rate change of luminosity of > 1.3x10^{42} erg/s^2, making it the most extreme such event we are aware of from a radio-quiet quasar. Standard `radiative efficiency limit' arguments imply a radiative efficiency larger than can be achieved by accretion onto a Kerr black hole rotating at the maximum plausible rate, although we point out that such arguments depend upon the geometry of initial radiation release. Relativistic motions of the X-ray source are probably causing the radiative efficiency limit to break down; such relativistic motions have also been inferred in the similar NLS1-class quasar PKS 0558--504.

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arXiv1999

BeppoSAX uncovers the hidden Seyfert 1 nucleus in the Seyfert 2 galaxy NGC 2110

The Seyfert 2 galaxy NGC 2110 has been observed with BeppoSAX between 0.5 and 150 keV. The high energy instrument onboard, PDS, has succeeded in measuring for the first time the spectrum of this source in the 13-150 keV range. The PDS spectrum, having a photon index Gamma~1.86 is fully compatible with that expected from a Seyfert 1 nucleus. In the framework of unified models, the harder (Gamma~1.67) 2-10 keV spectrum is well explained assuming the presence of a complex partial + total absorber (Nh~30x10^22 cm^-2 x25% + Nh~4x10^22 cm^-2 x100%). The high column density of this complex absorber is consistent both with the FeK_alpha line strength and with the detection of an absorption edge at E~7.1 keV in the power-law spectrum.

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arXiv1999

The mass density in black holes inferred from the X-ray background

The X-ray Background (XRB) probably originates from the integrated X-ray emission of active galactic nuclei (AGN). Modelling of its flat spectrum implies considerable absorption in most AGN. Compton down-scattering means that sources in which the absorption is Compton thick are unlikely to be major contributors to the background intensity so the observed spectral intensity at about 30 keV is little affected by photoelectric absorption. Assuming that the intrinsic photon index of AGN is 2, we then use the 30 keV intensity of the XRB to infer the absorption-corrected energy density of the background. Soltan's argument then enables us to convert this to a mean local density in black holes, assuming an accretion efficiency of 0.1 and a mean AGN redshift of 2. The result is within a factor of two of that estimated by Haehnelt et al from the optically-determined black hole masses of Magorrian et al. We conclude that there is no strong need for any radiatively inefficient mode of accretion for building the masses of black holes. Furthermore we show that the absorption model for the XRB implies that about 85 per cent of accretion power in the Universe is absorbed. This power probably emerges in the infrared bands where it can be several tens per cent of the recently inferred backgrounds there. The total power emitted by accretion is then about one fifth that of stars.

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arXiv1998

The ROSAT Brightest Cluster Sample (BCS) - I. The compilation of the sample and the cluster log N-log S distribution

We present a 90 per cent flux-complete sample of the 201 X-ray brightest clusters of galaxies in the northern hemisphere (dec > 0 deg), at high Galactic latitudes (|b| > 20 deg), with measured redshifts z < 0.3 and fluxes higher than 4.4 x 10^(-12) erg cm^(-2) s^{-1) in the 0.1-2.4 keV band. The sample, called the ROSAT Brightest Cluster Sample (BCS), is selected from ROSAT All-Sky Survey data and is the largest X-ray selected cluster sample compiled to date. In addition to Abell clusters, which form the bulk of the sample, the BCS also contains the X-ray brightest Zwicky clusters and other clusters selected from their X-ray properties alone. Effort has been made to ensure the highest possible completeness of the sample and the smallest possible contamination by non-cluster X-ray sources. X-ray fluxes are computed using an algorithm tailored for the detection and characterization of X-ray emission from galaxy clusters. These fluxes are accurate to better than 15 per cent (mean 1 sigma error). We find the cumulative log N-log S distribution of clusters to follow a power law k S^(-alpha) with alpha=1.31 (+0.06)(-0.03) (errors are the 10th and 90th percentiles) down to fluxes of 2 x 10^(-12) erg cm^(-2) s^(-1), i.e. considerably below the BCS flux limit. Although our best-fitting slope disagrees formally with the canonical value of -1.5 for a Euclidean distribution, the BCS log N-log S distribution is consistent with a non-evolving cluster population if cosmological effects are taken into account.

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arXiv1998

A Simultaneous ASCA and RXTE Long Look at the Seyfert 1 Galaxy MCG--6-30-15

We report on a joint RXTE and ASCA observation spanning $\sim$ 400~ks of the Seyfert 1 galaxy MCG--6-30-15. The low energy coverage of ASCA coupled with the high energy coverage of RXTE have allowed us not only to confirm features of Compton reflection but also to set bounds on the abundances as a function of reflective fraction.

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arXiv1998

On the Origin of the Hard X-Ray Background

Several models for the hard X-ray Background (XRB) suggest that it is due to the emission from heavily obscured AGN. Recent studies have revealed the presence of a new population of hard X-ray sources which must contribute significantly to the XRB. However, whether the majority of these sources are obscured AGN or some other type of object at present remains unclear. Here, we further examine the possibility that a significant fraction of the XRB comes from a population of galaxies undergoing advection-dominated accretion in the high-\mdot regime and thus produce intrinsically hard spectra. When the accretion rate is close to its critical value, above which an advection-dominated accretion flow (ADAF) no longer exists, the major contribution to X-ray emission is due to inverse Compton scattering of the soft seed photons produced by cyclo-synchrotron radiation. In this regime, the resulting ADAF spectra are relatively hard with a fairly constant X-ray spectral index α\sim 0.2-0.4 and a spectral cut-off at \sim 200 keV. We show that the integrated emission from such sources can provide a good fit to the hard (> 2 keV) X-ray background, provided that the spectrum is dominated by the contribution from objects located at redshifts z \sim 2-3. The model requires most of the contribution to the XRB to be due to objects accreting at the critical rate.

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arXiv1998

One more surprise from the Circinus Galaxy: BeppoSAX discovery of a transmission component in hard X-rays

The Circinus Galaxy has been observed, for the first time above 10 keV, by BeppoSAX. An excess emission above the extrapolation of the best fit 0.1--10 keV spectrum is apparent in the PDS data. The most likely explanation is that we are observing the nucleus through a column density of about 4x10**24 cm-2, i.e. large enough to completely block the radiation below 10 keV but small enough to permit partial transmission above this energy.

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