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A. C. Fabian

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

At least 613 records · Page 34Linked to original sources
arXiv2004

Relativistic Iron Lines in Galactic Black Holes: Recent Results and Lines in the ASCA Archive

Recent observations with Chandra and XMM-Newton, aided by broad-band spectral coverage from RXTE, have revealed skewed relativistic iron emission lines in stellar-mass Galactic black hole systems. Such systems are excellent laboratories for testing General Relativity, and relativistic iron lines provide an important tool for making such tests. In this contribution to the Proceedings of the 10th Annual Marcel Grossmann Meeting on General Relativity, we briefly review recent developments and present initial results from fits to archival ASCA observations of Galactic black holes. It stands to reason that relativistic effects, if real, should be revealed in many systems (rather than just one or two); the results of our archival work have borne-out this expectation. The ASCA spectra reveal skewed, relativistic lines in XTE J1550-564, GRO J1655-40, GRS 1915+105, and Cygnus X-1.

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arXiv2004

High redshift quasars and the supermassive black hole mass budget: constraints on quasar formation models

We investigate the constraints on models of supermassive black hole (SMBH) and quasar formation obtainable from two recent observational developments: the discovery of luminous quasars at z~6, and estimates of the local mass density of SMBHs. If ~90 per cent of this mass was accreted at redshifts z<3, as suggested by the observed quasar luminosity functions, these joint constraints pose a challenge for models, which must account for the observed luminous quasar population at z~6 within a very limited `mass budget'. We investigate a class of models based within the hierarchical structure formation scenario, in which major mergers lead to black hole formation and fuelling, and the resulting quasars shine at their Eddington-limited rate until their fuel is exhausted. We show that the simplest such model, in which a constant fraction of the gas within the halo is accreted in each major merger, cannot satisfy both constraints simultaneously. When this model is normalized to reproduce the number density of luminous quasars at z~6, the mass budget is grossly exceeded due to an overabundance of lower mass SMBHs. We explore a range of modifications to the simple model designed to overcome this problem. We show that both constraints can be satisfied if the gas accretion fraction scales as a function of the halo virial velocity. Similar scalings have been proposed in order to reproduce the local M-sigma relation. Successful models can also be constructed by restricting the formation of seed black holes to redshifts above z~11.5 or to haloes above a velocity threshold ~55 km/s, or assuming that only a fraction of major mergers result in formation of a seed SMBH. (abridged)

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arXiv2004

A light bending model for the X-ray temporal and spectral properties of accreting black holes

Uncorrelated variability between direct X-ray continuum and reflection components (including the Fe line, if present) has been reported in many accreting black hole systems, challenging theoretical models of disc reflection. We explore a model based on gravitational light bending in the near vicinity of the central black hole in which the X-ray variability is controlled by the height of the primary source of X-rays above the accretion disc. The relationship between the direct power law component (PLC) and the reflection-dominated component (RDC) is not trivial and allows to identify three different regimes in which the RDC (and the Fe line) is correlated, anti-correlated or almost independent with respect to the PLC. X-ray observations of the active galaxy MCG-6-30-15 and the black hole candidate XTE J1650-500 (and possibly other sources, e.g. NGC 4051) reveal that a series of predictions are actually observed: the behaviour of the Fe line flux, profile, and EW and of the reflection fraction with respect to the continuum supports our model which may explain what are otherwise puzzling properties of these sources.

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arXiv2004

Mapping small-scale temperature and abundance structures in the core of the Perseus cluster

We report further results from a 191 ks Chandra observation of the core of the Perseus cluster, Abell 426. The emission-weighted temperature and abundance structure is mapped detail. There are temperature variations down to ~1 kpc in the brightest regions. Globally, the strongest X-ray surface brightness features appear to be caused by temperature changes. Density and temperature changes conspire to give approximate azimuthal balance in pressure showing that the gas is in hydrostatic equilibrium. Si, S, Ar, Ca, Fe and Ni abundance profiles rise inward from about 100 kpc, peaking at about 30-40 kpc. Most of these abundances drop inwards of the peak, but Ne shows a central peak, all of which may be explained by resonance scattering. There is no evidence for a widespread additional cooler temperature component in the cluster with a temperature greater than a factor of two from the local temperature. There is however evidence for a widespread hard component which may be nonthermal. The temperature and abundance of gas in the cluster is observed to be correlated in a manner similar to that found between clusters.

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arXiv2003

IZw1 observed with XMM-Newton: Low-energy spectral complexity, iron lines, and hard X-ray flares

We present a 20 ks XMM-Newton observation of the prototypical Narrow-Line Seyfert 1 galaxy IZw1. The best-fit model to the data is a double blackbody plus a dominant power-law, on which complex soft absorption (possibly a blended edge or absorption lines) and/or OVII emission are superimposed, as well as strong Fe Kalpha emission. The iron feature in the high-energy spectra appears broad; however, on close examination of the EPIC pn data, there exists the possibility that the broad emission feature can be attributed to a neutral Fe Kalpha line in addition to a blend of He- and H-like Fe Kalpha lines. The light curve shows a strong, hard X-ray flare concentrated in the 3-12 keV band. The flare appears to induce spectral variability, showing spectral hardening to be occuring as the flare intensifies. A detailed examination suggests that the spectral variability is most likely due to an increase in the 3-12 keV flux relative to the soft flux during the flare. A difference spectrum and complete modelling of the flare and non-flare spectra show intrinsic changes only in the normalisation of the continuum components and not in their shape parameters. The timing results are consistent with the flare originating in the accretion disc corona. The iron emission line(s) do not appear to respond to changes in the continuum flux during the flare; the iron lines are stronger in equivalent width during the low-flux (non-flare) states, and weaker during the flare.

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arXiv2003

A long, hard look at MCG-6-30-15 with XMM-Newton II: detailed EPIC analysis and modelling

The bright Seyfert 1 galaxy MCG-6-30-15 has provided some of the best evidence to date for the existence of supermassive black holes in active galactic nuclei. Observations with ASCA revealed an X-ray iron line profile shaped by strong Doppler and gravitational effects. In this paper the shape of the iron line, its variability characteristics and the robustness of this spectral interpretation are examined using the long XMM-Newton observation taken in 2001. A variety of spectral models, both including and excluding the effects of strong gravity, are compared to the data in a uniform fashion. The results strongly favour models in which the spectrum is shaped by emission from a relativistic accretion disc. It is far more difficult to explain the 3-10 keV spectrum using models dominated by absorption (either by warm or partially covering cold matter), emission line blends, curved continua or additional continuum components. These provide a substantially worse fit to the data and fail to explain other observations (such as the simultaneous BeppoSAX spectrum). This reaffirms the veracity of the relativistic `disc line' interpretation. The short term variability in the shape of the energy spectrum is investigated and explained in terms of a two-component emission model. Using a combination of spectral variability analyses the spectrum is successfully decomposed into a variable power-law component (PLC) and a reflection dominated component (RDC). The former is highly variable while the latter is approximately constant throughout the observation, leading to the well-known spectral variability patterns. (Abridged)

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arXiv2003

An X-ray absorption analysis of the high-velocity system in NGC 1275

We present an X-ray absorption analysis of the high-velocity system (HVS) in NGC 1275 using results from a deep 200 ks Chandra observation. We are able to describe the morphology of the HVS in more detail than ever before. We present an HST image for comparison, and note close correspondence between the deepest X-ray absorption and the optical absorption. A column density map of the HVS shows an average column density NH of 1x10^21 cm^-2 with a range from ~5x10^20 to 5x10^21 cm^-2. From the NH map we calculate a total mass for the absorbing gas in the HVS of (1.32+-0.05)x10^9 solar masses at solar abundance. 75 per cent of the absorbing mass is contained in the four regions of deepest absorption. We examine temperature maps produced by spectral fitting and find no direct evidence for shocked gas in the HVS. Using deprojection methods and the depth of the observed absorption, we are able to put a lower limit on the distance of the HVS from the nucleus of 57 kpc, showing that the HVS is quite separate from the body of NGC 1275.

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arXiv2003

Powerful obscured AGN among X-ray hard, optically-dim serendipitous Chandra sources

We present a small sample of Chandra X-ray sources selected from the fields of ACIS observations which probe fluxes around the break in the hard band source counts. The targets of these fields include 9 nearby galaxy clusters, 1 distant cluster and 2 powerful radio galaxy fields. The follow-up of this serendipitous sample was biased towards X-ray hard and optically-dim sources mostly not seen on the Digitized Sky Survey; for these, we present X-ray fluxes, optical and near-infrared photometry leading to 51 photometric redshifts in all and 18 independently measured spectroscopic redshifts. Few sources are associated with the target fields themselves. Fifty-six of 58 sources imaged in the K-band are detected at K<~20 with K_median=18, and of these, 38 have hard X-ray count ratios and 24 of these are significantly hard with most of the counts emerging about 2 keV. We find that almost all are AGN hosted in massive early-type host galaxies with a photometric redshift distribution peaking at z~1. Two type 2 quasars with intrinsic X-ray luminosity L>~10^{45} erg/s, Fe K_alpha emission lines and absorbing column density nH>10^{23} cm^{-2} -- and nH>~10^{24} cm^{-2} in one case -- are discussed in detail; the sample contains at least 12 potential type 2 quasars in all. We discuss various detection strategies for type 2 quasars and calculate their inferred space density. This combines and extends a number of results from subsamples already published by us.

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arXiv2003

Arakelian 564: An XMM-Newton view

We report on two XMM-Newton observations of the bright narrow-line Seyfert 1 galaxy Ark 564 taken one year apart (2000 June and 2001 June). The 0.6-10 keV continuum is well described by a soft blackbody component (kT~140-150 eV) plus a steep power law (Gamma~2.50-2.55). No significant spectral changes are observed between the two observations, although the X-ray flux in the second observation is ~40-50 per cent lower. In both observations we detect a significant absorption edge at a rest-frame energy of ~0.73 keV, corresponding to OVII. The presence of the absorption feature is confirmed by a simultaneous Chandra grating observation in 2000 June, although the best-fitting edge threshold is at a slightly lower energy in the Chandra data, possibly because of a different parameterisation of the underlying X-ray continuum. We find tentative evidence for a broad iron emission line in the 2000 June observation. The results from an analysis of the power spectral density (PSD) function are also presented. The present XMM-Newton data support the idea that the PSD shows two breaks, although the location of the high-frequency break requires further constraints.

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arXiv2003

Chandra/HETGS Spectroscopy of the Galactic Black Hole GX 339-4: A Relativistic Iron Line and Evidence for a Seyfert-like Warm Absorber

We observed the Galactic black hole GX 339-4 with the Chandra High Energy Transmission Grating Spectrometer (HETGS) for 75 ksec during the decline of its 2002-2003 outburst. The sensitivity of this observation provides an unprecedented glimpse of a Galactic black hole at about a tenth of the luminosity of the outburst peak. The continuum spectrum is well described by a model consisting of multicolor disk blackbody (kT = 0.6 keV) and power-law (Gamma = 2.5) components. X-ray reflection models yield improved fits. A strong, relativistic Fe K-alpha emission line is revealed, indicating that the inner disk extends to the innermost stable circular orbit. The breadth of the line is sufficient to suggest that GX 339-4 may harbor a black hole with significant angular momentum. Absorption lines from H-like and He-like O, and He-like Ne and Mg are detected, as well as lines which are likely due to Ne II and Ne III. The measured line properties make it difficult to associate the absorption with the coronal phase of the interstellar medium. A scenario wherein the absorption lines are due to an intrinsic AGN-like warm-absorber geometry -- perhaps produced by a disk wind in an extended disk-dominated state -- may be more viable. We compare our results to Chandra observations of the Galactic black hole candidate XTE J1650-500, and discuss our findings in terms of prominent models for Galactic black hole accretion flows and connections to supermassive black holes.

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arXiv2003

Extended X-ray emission in the high redshift quasar GB 1508+5714 at z=4.3

We report the discovery of extended X-ray emission around the powerful high-redshift quasar GB 1508+5714 at z=4.3, revealed in a long Chandra ACIS observation. The emission feature is 3-4 arcsec away from the quasar core, which corresponds to a projected distance of about 25 kpc. The X-ray spectrum is best fitted with a power law of photon index 1.92+-0.35 (90 per cent c.l.). The X-ray flux and luminosity reach 9.2x10^-15 ergs (0.5-8 keV) and 1.6x10^45 erg s^-1 (2.7-42.4 keV rest frame, Omega_Lambda=0.73, Omega_m=0.27, H_0=71 km s^-1 Mpc^-1), which is about 2 percent of the total X-ray emission of the quasar. We interpret the X-ray emission as inverse Compton scattering of Cosmic Microwave Background photons. The scattering relativistic electron population could either be a quasi-static diffuse cloud fed by the jet, or an outer extension of the jet with a high bulk Lorentz factor. We argue that the lack of an obvious detection of radio emission from the extended component could be a consequence of Compton losses on the electron population, or of a low magnetic field. Extended X-ray emission produced by inverse Compton scattering may be common around high redshift radio galaxies and quasars, demonstrating that significant power is injected into their surrounding by powerful jets.

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arXiv2003

The hard X-ray spectrum of the Seyfert galaxy IRAS 18325-5926: cool corona, reflection from an ionized disk and variable iron K emission

We report our analysis of X-ray spectra of the Seyfert galaxy IRAS 18325-5926 (=Fairall 49) obtained from various X-ray observatories prior to XMM-Newton, including new results from two RXTE and one BeppoSAX observations. A relatively steep continuum slope (photon-index of ~2.2) in the 2-15 keV band is confirmed. The continuum spectrum observed with the BeppoSAX PDS shows a possible roll-over at energies above 30 keV, indicating a Comptonizing corona cooler than in other Seyfert nuclei. The X-ray spectrum above 2 keV is best explained with a model including reflection from a highly ionized disk with significant relativistic blurring. The iron K-alpha emission feature is then mainly due to FeXXV. The recent seven observations shows that the iron K emission flux appears to follow the continuum between the observations separated by a few months to years, although some exceptions suggest that the line strength may be determined in a more complex way.

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arXiv2003

The X-ray variability of the Narrow-Line Seyfert 1 galaxy IRAS 13224-3809 from an XMM-Newton observation

We report on the XMM-Newton timing properties of the most X-ray variable, radio-quiet, NLS1 IRAS 13224-3809. IRAS 13224-3809 continues to display the extremely variable behavior that was previously observed with ROSAT and ASCA; however, no giant, rapid flaring events are observed. We detect variations by a factor as high as ~8 during the 64 ks observation, and the variability is persistent throughout the light curve. Dividing the light curve into 9 minute segments we found almost all of the segments to be variable at > 3 sigma. When the time-averaged cross-correlation function is calculated for the 0.3-0.8 keV band with the 3-10 keV band, the cross-correlation profile is skewed indicating a possible smearing of the signal to longer times (soft band leading the hard). A correlation between count rate and hardness ratio is detected in four energy bands. In three cases the correlation is consistent with spectral hardening at lower count rates which can be explained in terms of a partial-covering model. The other band displays the reverse effect, showing spectral hardening at higher count rates. We can explain this trend as a more variable power-law component compared to the soft component. We also detect a delay between the 0.3-1.5 keV count rate and the 0.8-1.5 keV to 0.3-0.8 keV hardness ratio, implying flux induced spectral variability. Such delays and asymmetries in the cross correlation functions could be suggesting reprocessing of soft and hard photons. In general, much of the timing behavior can be attributed to erratic eclipsing behavior associated with the partial covering phenomenon, in addition to intrinsic variability in the source. The variability behavior of IRAS 13224-3809 suggests a complicated combination of effects which we have started to disentangle with this present analysis.

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arXiv2003

Thermal conduction and reduced cooling flows in galaxy clusters

Conduction may play an important role in reducing cooling flows in galaxy clusters. We analyse a sample of sixteen objects using Chandra data and find that a balance between conduction and cooling can exist in the hotter clusters (T > 5 keV), provided the plasma conductivity is close to the unhindered Spitzer value. In the absence of any additional heat sources, a reduced mass inflow must develop in the cooler objects in the sample. We fit cooling flow models to deprojected data and compare the spectral mass deposition rates found to the values required to account for the excess luminosity, assuming Spitzer-rate heat transfer over the observed temperature gradients. The mass inflow rates found are lower than is necessary to maintain energy balance in at least five clusters. However, emission from cooling gas may be partially absorbed. We also compute the flux supplied by turbulent heat transport (Cho et al. 2003) and find conductivity profiles which follow a strikingly similar temperature dependence to the conductivity values required to prevent cooling. Finally, we show that the cluster radio luminosities vary by over five orders of magnitude in objects with X-ray luminosities differing by no more than a factor of a few. This suggests that there is unlikely to be a straightforward correlation between the mechanical power provided by the radio lobes and the rate of energy loss in cooling flow clusters.

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arXiv2003

A softer look at MCG--6-30-15 with XMM-Newton

We present analysis and results from the Reflection Grating Spectrometer during the 320 ks XMM observation of the Seyfert 1 galaxy MCG-6-30-15. The spectrum is marked by a sharp drop in flux at 0.7 keV which has been interpreted by Branduardi-Raymont et al. as the blue wing of a relativistic OVIII emission line and by Lee at al. as a dusty warm absorber. We find that the drop is well explained by the FeI L2,3 absorption edges and obtain reasonable fits over the 0.32-1.7 keV band using a multizone, dusty warm absorber model. Some residuals remain which could be due to emission from a relativistic disc, but at a much weaker level than from any model relying on relativistic emission lines alone. A model based on such emission lines can be made to fit if sufficient (warm) absorption is added, although the line strengths exceed those expected. The EPIC pn difference spectrum between the highest and lowest flux states of the source indicates that this is a power-law in the 3-10 keV band which, if extrapolated to lower energies, reveals the absorption function acting on the intrinsic spectrum, provided that any emission lines do not scale exactly with the continuum. We find that this function matches our dusty warm absorber model well. The soft X-ray spectrum is therefore dominated by absorption structures, with the equivalent width of any individual emission lines in the residuals being below about 30 eV. (abridged)

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arXiv2003

The lack of variability of the iron line in MCG-6-30-15: general relativistic effects

The spectrum and variability of the Seyfert galaxy MCG-6-30-15 can be decomposed into two apparently disconnected components: a highly variable power law and an almost constant component which contains a broad and strong iron line. We explore a possible explanation of the puzzling lack of variability of the iron line, by assuming that the variations of the power law component are due to changes in the height of the primary source in the near vicinity of a rotating black hole. Due to the bending of light in the strong field of the central black hole, the apparent brightness of the power-law component can vary by about a factor 4 according to its position, while the total iron line flux variability is less than 20 percent. This behaviour is obtained if the primary source is located within 3-4 gravitational radii ($r_{\rm g}$) from the rotation axis with a variable height of between $\sim$ 3 and 8 $r_{\rm g}$. These results revive the possibility that future X-ray observations of MCG-6-30-15 can map out the strong gravity regime of accreting black holes.

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arXiv2003

The relationship between the optical Halpha filaments and the X-ray emission in the core of the Perseus cluster

NGC 1275 in the centre of the Perseus cluster of galaxies, Abell 426, is surrounded by a spectacular filamentary Halpha nebula. Deep Chandra X-ray imaging has revealed that the brighter outer filaments are also detected in soft X-rays. This can be due to conduction and mixing of the cold gas in the filaments with the hot, dense intracluster medium. We show the correspondence of the filaments in both wavebands and draw attention to the relationship of two prominent curved NW filaments to an outer, buoyant radio bubble seen as a hole in the X-ray image. There is a strong resemblance in the shape of the hole and the disposition of the filaments to the behaviour of a large air bubble rising in water. If this is a correct analogy, then the flow is laminar and the intracluster gas around this radio source is not turbulent. We obtain a limit on the viscosity of this gas.

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