arXiv subjects

A. C. Fabian

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

At least 631 records · Page 35Linked to original sources

arXiv2003

A deep Chandra observation of the Perseus cluster: shocks and ripples

We present preliminary results from a deep observation lasting almost 200 ks, of the centre of the Perseus cluster of galaxies around NGC 1275. The X-ray surface brightness of the intracluster gas beyond the inner 20 kpc, which contains the inner radio bubbles, is very smooth apart from some low amplitude quasi-periodic ripples. A clear density jump at a radius of 24 kpc to the NE, about 10 kpc out from the bubble rim, appears to be due to a weak shock driven by the northern radio bubble. A similar front may exist round both inner bubbles but is masked elsewhere by rim emission from bright cooler gas. The continuous blowing of bubbles by the central radio source, leading to the propagation of weak shocks and viscously-dissipating sound waves seen as the observed fronts and ripples, gives a rate of working which balances the radiative cooling within the inner 50 kpc of the cluster core.

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arXiv2003

The X-ray nebula around the Seyfert 2 galaxy NGC4388

We report on X-ray emission from the Seyfert 2 galaxy NGC4388 observed with the Chandra X-ray Observatory. A hard X-ray peak is found at the position of the active nucleus suggested by optical and radio observations. Extended soft X-ray emission correlates well with the ionization cone found in optical line emission. A large soft X-ray extension is found up to 16 kpc to the north of the galaxy. Photoionized gas with low ionization parameters (xi<3) appears to be the likely explanation of this emission. The same ionized gas clouds could be responsible for the optical [OIII] emission. Fe K line emission from cold material is found to be extended by a few kpc.

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arXiv2003

A BeppoSAX observation of MKN6

We have used the BeppoSAX satellite to study the broad band (0.5-100 keV) X-ray spectrum of the Seyfert 1.5 galaxy MKN6. The source is characterized by a power law of Gamma=1.7 [+0.08, -0.07] and there is no strong evidence for either a reflection bump or a high energy cut-off. We have detected a narrow iron line at 6.4 keV (rest frame) with an equivalent width of 98 [+33, -35] eV. MKN6 also exhibits strong and complex absorption. At least two components (NH_1 =1.34 [+0.4,-0.4] x 10^(22) cm^(-2) and NH_2 = 4.18 [+2.2, -1.3] x 10^(22) cm^(-2)) are present and they both partially cover the source with covering fractions of ~90% and ~50% respectively. Comparison with a previous ASCA observation indicates that in both absorbing columns the NH is variable over a 2 year timescale, while the covering fractions are constant over the same amount of time. The state of each absorber is cold or mildly photoionized. The Broad Line Region (BLR) is suggested as the possible location for this complex absorption.

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arXiv2003

The Contribution of Particle Impact to the Production of Fe K Emission from Accreting Black Holes

The iron K line is perhaps the most important spectral diagnostic available in the study of accreting black holes. The line is thought to result from the reprocessing of external X-rays by the surface of the accretion disk. However, as is observed in the solar corona, illumination by energetic particles may also produce line emission. In principle, such a process may be uncorrelated with the observed X-rays and could explain some of the unexpected variability behavior of the Fe line. This paper compares predictions of iron K flux generated by impacting electrons and protons to that from photoionization. Non-thermal power-laws of electrons are considered as well as thermal distributions of electrons and virialized protons. The electrons are thought to originate in a magnetically dominated accretion disk corona, while the protons are considered in the context of a two phase (hot/cold) accretion scenario. In each case, the Fe K flux from particle impact is found to be < 1% of that produced by photoionization by a hard X-ray power-law (normalized to the same energy flux as the particles). Thus, the electrons or protons must strike the disk with 100--10,000 times more energy flux than radiation for particle impact to be a significant producer of Fe K flux. This situation is difficult to reconcile with the observations of hard X-ray spectra, or the proposed particle acceleration mechanisms in the accretion disk corona. Truncated accretion flows must be externally illuminated by hard X-rays in order to produce the Fe line, as proton impact is very inefficient in generating line emission. In contrast to the Sun, our conclusion is that, with the possible exception for localized regions around magnetic footpoints, particle impact will not be an important contributor to the X-ray emission in accreting black holes.

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arXiv2003

Obscured Active Galactic Nuclei and Obscured Accretion

Most of the local active galactic nucleus (AGN) population is obscured and much of the X-ray background originates in obscured AGNs. The contribution of obscured accretion to the growth of massive black holes is discussed here. The recent identification of significant samples of the X-ray sources that dominate the X-ray background intensity has shown a redshift peak at 0.7-0.8, rather than the redshift of 2 found for bright optical quasars. Obscured accretion has a faster evolution than unobscured accretion. The lower redshift and luminosity of most obscured AGNs mean that although they dominate the absorption-corrected intensity of the X-ray background by a factor of about 3 over unobscured objects, they make only an equal contribution to the local mass density in black holes. Obscured and unobscured AGNs together contribute about 4 x 10^5 Msun/Mpc^3. Type 2 quasars and Compton-thick objects may give another 10^5 Msun/Mpc^3, but no more unless direct determinations from the M-sigma relation seriously underestimate the local black hole mass density, or unless most massive black holes are rapidly spinning (so having a higher radiative efficiency than the 10% assumed above). Obscured accretion probably dominates the growth of black holes with masses below a few times 10^8 Msun, whereas optically bright quasars dominate at higher masses. The luminosity absorbed by the dusty gas in obscured AGNs is reradiated in the mid-infrared and far-infrared bands. The contribution of AGNs drops from about 20% of the mid-infrared background to just a few percent of the far-infrared background.

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arXiv2003

X-rays from Clusters of Galaxies

The X-ray emission from clusters of galaxies enables them to be used as good cosmological probes and as an example for massive galaxy formation. The gas mass fraction in clusters should be a universal standard which by means of Chandra observations enables Omega_m to be determined to better than 15 per cent accuracy. Future observations of its apparent variation with redshift will enable Omega_Lambda to be measured. The interplay of radiative cooling and heating in cluster cores may reveal the dominant processes acting during the formation of the baryonic part of massive galaxies.

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arXiv2003

Iron K alpha line profiles and the inner boundary condition of accretion flows

Recent X-ray observations have shown evidence for exceptionally broad and skewed iron K alpha emission lines from several accreting black hole systems. The lines are assumed to be due to fluorescence of the accretion disk illuminated by a surrounding corona and require a steep emissivity profile increasing in to the innermost radius. This appears to question both standard accretion disc theory and the zero torque assumption for the inner boundary condition, both of which predict a much less extreme profile. Instead it argues that a torque may be present due to magnetic coupling with matter in the plunging region or even to the spinning black hole itself. Discussion so far has centered on the torque acting on the disc. However the crucial determinant of the iron line profile is the radial variation of the power radiated in the corona. Here we study the effects of different inner boundary conditions on the coronal emissivity and on the profiles of the observable Fe K alpha lines. We argue that in the extreme case where a prominent highly redshifted component of the iron line is detected, requiring a steep emissivity profile in the innermost part and a flatter one outside, energy from the gas plunging into the black hole is being fed directly to the corona.

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arXiv2003

Cosmological constraints from the local X-ray luminosity function of the most X-ray luminous galaxy clusters

We present precise constraints on the normalization of the power spectrum of mass fluctuations in the nearby universe, sigma_8, as a function of the mean local matter density, Omega_m. Using the observed local X-ray luminosity function of galaxy clusters from the extended BCS and REFLEX studies, a mass-luminosity relation determined from Chandra and ROSAT X-ray data and weak gravitational lensing observations, and the mass function predicted by the Hubble Volume simulations of Evrard et al., we obtain sigma_8 = (0.508\pm0.019) Omega_m^-(0.253\pm0.024), with Omega_m < 0.34 at 68 per cent confidence. The degeneracy between sigma_8 and Omega_m can be broken using Chandra measurements of the X-ray gas mass fractions in dynamically relaxed clusters. Using this information and including Gaussian priors on the mean baryon density of the universe and the Hubble constant, we obtain sigma_8=0.695\pm0.042 and Omega_m=0.287\pm0.036, for an assumed flat LambdaCDM cosmology (marginalized 68 per cent confidence limits). Our results are in good agreement with some recent studies based on the local X-ray temperature function of clusters, the redshift evolution of the X-ray luminosity and temperature functions of clusters, early results from the Sloan Digitized Sky Survey, the most recent results from studies of cosmic shear, and combined analyses of the 2dF galaxy redshift survey and cosmic microwave background anisotropies.

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arXiv2003

Chandra and XMM-Newton observations of Tololo 0109-383

We present and discuss Chandra and XMM-Newton observations of the Seyfert 2 galaxy and Compton-thick absorbed source, Tololo~0109-383. The hard X-ray emission (i.e. above $\sim$2 keV), is dominated by a spatially unresolved reflection component, as already discovered by previous ASCA and BeppoSAX observ ations. The soft X-ray emission is partly ($\sim$15%) extended over about 1 kpc. Below 2 keV, the spectrum is very steep and two emission lines, probably due to recombination to He-like ions of oxygen and neon, are clearly present. Combining X-rays and optical information taken from the literature, we propose an overall scenario for the nuclear regions of this source.

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arXiv2003

A two-component ionized reflection model of MCG--6-30-15

Ionized reflection has often been considered as the explanation for the unusual Fe K variability observed in MCG--6-30-15. In this paper, we test this model using a 325 ks observation of MCG--6-30-15 by XMM-Newton and BeppoSAX. The data are fit between 2.5 and 80 keV with the constant density models of Ross & Fabian. The best fit ionized reflection model requires the Fe K line to be split into two reprocessing events: one from the inner disc to build up the red wing, and the other from the outer accretion disc to fit the blue horn. The implied geometry is a disc which becomes strongly warped or flared at large radii. A good fit was obtained with a solar abundance of iron and a reflection fraction (R) of unity for the inner reflector. The combination of the two reflection spectra can appear to have R>2 as required by the BeppoSAX data. The inner reflector has an ionization parameter \log ξ=3.8, but the outer one is neutral with an inner radius ~70 gravitational radii (r_g), corresponding to a light crossing time of about an hour for a 10^7 Msun black hole. Applying this model to time-resolved spectra shows that the inner reflector becomes more ionized as the source brightens. This reduces the strength of the red wing at high flux states. The X-ray source is constrained to arise from a narrow annulus at ~5 r_g, with only 6 per cent of the 2-10 keV flux due to the outer reprocessor. This amount of localized energy generation is extremely difficult to produce without resorting to other energy sources such as the black hole spin. In fact, all the Fe K models fit to XMM-Newton spectra of MCG--6-30-15 require a large increase in energy production at the inner edge of the accretion disc.

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arXiv2003

Evidence for an intermediate mass black hole and a multi-zone warm absorber in NGC 4395

We report on the results of an analysis in the X-ray band of a recent long ASCA observation of NGC 4395, the most variable low-luminosity AGN known. A relativistically-broadened iron line at ~6.4 keV is clearly resolved in the time-averaged spectrum, with an equivalent width of 310^{+70}_{-90} eV. Time-resolved spectral analysis of the heavily absorbed soft X-ray band confirms the existence of a variable, multi-zone warm absorber in this source, as proposed in a previous analysis of a shorter ASCA observation. The light curve of the source is wildly variable on timescales of hours or less, and a factor of nearly 10 change in count-rate was recorded in a period of less than 2000 s. The long observation and variability of the source allowed the power density spectrum (PDS) to be constructed to an unprecedented level of detail. There is evidence for a break in the PDS from a slope of α~1 to α~1.8 at a frequency of around 3 \times 10^{-4} Hz. The central black hole mass of NGC 4395 is estimated to be approximately 10^4-10^5 solar masses using the break in the PDS, a result consistent with previous analyses using optical and kinematical techniques.

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arXiv2003

The iron line in MCG-6-30-15 from XMM-Newton: evidence for gravitational light-bending?

The lack of variability of the broad iron line seen in the Seyfert 1 galaxy MCG-6-30-15 is studied using the EPIC pn data obtained from a 2001 observation of 325 ks, during which the count rate from the source varied by a factor of 5. The spectrum of 80 ks of data from when the source was bright, using the the lowest 10 ks as background, is well fitted over the 3-10 keV band with a simple power-law of photon index Gamma=2.11. The source spectrum can therefore be decomposed into an approximately constant component, containing a strong iron emission line, and a variable power law component. Assuming that the power-law continuum extends down to 0.5 keV enables us to deduce the absorption acting on the nucleus. A simple model involving Galactic absorption and 2 photoelectric edges (at 0.72 and 0.86 keV) is adequate for the EPIC spectrum above 1 keV. It still gives a fair representation of the spectrum at lower energies where absorption lines, UTAs etc are expected. Applying the absorption model to the low flux spectrum enables us to characterize it as reflection-dominated with iron about 3 times Solar. Fitting this model, with an additional power-law continuum, to 32 x 10 ks spectra of the whole observation reveals that Gamma lies mostly between 2 and 2.3 and the normalization of the reflection-dominated component varies by up to 25 per cent. The breadth of the iron line suggests that most of the illumination originates from about 2 gravitational radii. Gravitational light bending will be very strong there, causing the observed continuum to be a strong function of the source height and much of the continuum radiation to return to the disk. Together these effects can explain the otherwise puzzling disconnectedness of the continuum and reflection components of MCG-6-30-15.

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arXiv2003

A deep Chandra observation of the cluster environment of the z=1.786 radio galaxy 3C294

We report the results from a 200 ks Chandra observation of the z=1.786 radio galaxy 3C294 and its cluster environment, increasing by tenfold our earlier observation. The diffuse emission, extending about 100 kpc around the nucleus, has a roughly hourglass shape in the N-S direction with surprisingly sharp edges to the N and S. The spectrum of the diffuse emission is well fitted by either a thermal model of temperature 3.5 keV and abundance <0.9 solar (2-sigma), or a power-law with photon index 2.3. If the emission is due to hot gas then the sharp edges mean that it is probably not in hydrostatic equilibrium. Much of the emission is plausibly due to inverse Compton scattering of the Cosmic Microwave Background (CMB) by nonthermal electrons produced earlier by the radio source. The required relativistic electrons would be of much lower energy and older than those responsible for the present radio lobes. This could account for the lack of detailed spatial correspondence between the X-rays and the radio emission, the axis of which is at a position angle of about 45 deg. Hot gas would still be required to confine the relativistic plasma; the situation could parallel that of the radio bubbles seen as holes in nearby clusters, except that in 3C294 the bubbles are bright in X-rays owing to the extreme power in the source and the sixty fold increase in the energy density of the CMB. The X-ray spectrum of the radio nucleus is hard, showing a reflection spectrum and iron line. The source is therefore an obscured radio-loud quasar.

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arXiv2003

X-ray Spectroscopic Evidence for Intermediate Mass Black Holes: Cool Accretion Disks in Two Ultra--Luminous X-ray Sources

We have analyzed an XMM-Newton observation of the nearby spiral galaxy NGC 1313, which contains two "ultra-luminous" X-ray (ULX) sources. We measure isotropic luminosities of L_X = 2.0 * 10^(40) erg/s and L_X = 6.6 * 10^(39) erg/s for NGC 1313 X-1 and X-2 (0.2-10.0 keV, assuming a distance of 3.7 Mpc). The spectra statistically require soft and hard spectral components to describe the continuum emission; some prior studies of ULXs have claimed cool soft components with lower statistics. The improvement over several single-component models exceeds the 8 sigma level of confidence for X-1; the improvement for X-2 is significant at the 3 sigma level. The soft components in these ULX spectra are well-fit by multi-color disk blackbody models with color temperatures of kT = 150 eV. This temperature differs markedly from those commonly measured in the spectra of stellar-mass (10 M_sun) black holes in their brightest states (kT ~ 1 keV). It is expected that the temperature of an accretion disk orbiting a black hole should decrease with increasing black hole mass. If the soft components we measure are due to emission from the inner region of an accretion disk, and disks extend close to the innermost stable circular orbit at the accretion rates being probed, the low color temperatures may be interpreted as spectroscopic evidence of black holes with intermediate masses: M_BH ~ 10^(3) M_sun. Simple Eddington scaling arguments suggest a mimum mass of M_BH ~ 10^(2) M_sun. NGC 1313 X-1 and X-2 are found in optical nebulae (Pakull and Mirioni 2002), which may indicate that anisotropic emission geometries are unlikely to account for the fluxes observed.

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arXiv2003

The high frequency power spectrum of Markarian 766

An analysis is presented of the power spectrum of X-ray variability of the bright Seyfert 1 galaxy Mrk 766 as observed by XMM-Newton. Over the 0.2-10 keV energy range the power spectral density (PSD) is well-represented by a power-law with a slope of alpha_low ~ 1 at low frequencies, breaking to a slope of alpha_hi = 2.8 (-0.4/+0.2) at a frequency f_br ~ 5 x 10^-4 Hz. As has been noted before this broken power-law PSD shape is similar to that observed in the Galactic black hole candidate Cygnus X-1. If it is assumed that Mrk 766 shows a power spectrum similar in form to that of Cyg X-1, and that the break timescale scales linearly with black hole mass, then the mass of the black hole in Mrk 766 is inferred to be < 5 x 10^5 M_sun. This rather low mass would mean Mrk 766 radiates above the Eddington limit. The coherence between different energy bands is significantly below unity implying that variations in the different energy bands are rather poorly correlated. The low coherence can be explained in the framework of standard Comptonisation models if the properties of the Comptonising medium are rapidly variable or if there are several distinct emission sites.

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arXiv2003

Chandra observations of the galaxy cluster A478: the interaction of hot gas and radio plasma in the core, and an improved determination of the Compton y-parameter

We present the results from a 42 ks \chandra ACIS observation of the galaxy cluster A478. This cluster is generally considered to be highly relaxed. The \chandra image reveals, for the first time, X-ray cavities in the hot gas within the central 15 kpc radius of A478. Two weak and small ($\sim$ 4 kpc) radio lobes that extend from the central nucleus, are detected in a 1.4 GHz VLA observation. The radio lobes are roughly along the direction of the X-ray cavities, but are much smaller than the X-ray cavities. We propose a ``donut'' configuration for the hot gas within the central 15 kpc, created by the interaction of the gas with the radio plasma that originated from the nucleus. The current radio activity of the central radio source is weak ($\sim$ 0.2% of Hydra A) and the total radio power is at least 10 times smaller than the minimum power needed to create the cavities. We compare A478 with other galaxy clusters where similar X-ray cavities were found. A478 and A4059 host much weaker central radio sources than do others with similar size X-ray cavities. On larger scales, deprojected temperature and density profiles are obtained for A478. We used these to derive the Compton y-parameter, for the first time, through direct integration. The result has a much smaller statistical uncertainty than previous ones. This serves as an example of how high quality X-ray data can help constrain H$_{0}$. The corresponding H$_{0}$ also was derived combining the available Sunyaev-Zeldovich effect (SZE) measurement.

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arXiv2002

Ionized Disc Models of MCG-6-30-15

Ionized reflection is one of the possible mechanisms proposed to explain the small short-timescale variability of the MCG-6-30-15 Fe K line. In this contribution, we show fits of the Ross & Fabian ionized reflection models to the 325 ks XMM-Newton observation of MCG--6-30-15. It is found that the spectrum above 2.5 keV can be fitted with a combination of an ionized reflector from 5 r_g and a neutral reprocessor beyond about 70 r_g. The ionization parameter of the inner reflector increases as the source brightens.

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arXiv2002

X-ray continuum variability of MCG-6-30-15

This paper presents a comprehensive examination of the X-ray continuum variability of the bright Seyfert 1 galaxy MCG-6-30-15. The source clearly shows the strong, linear correlation between rms variability amplitude and flux first seen in Galactic X-ray binaries. The high frequency power spectral density (PSD) of MCG-6-30-15 is examined in detail using a Monte Carlo fitting procedure and is found to be well represented by a steep power-law at high frequencies (with a power-law index alpha ~ 2.5), breaking to a flatter slope (alpha ~ 1) below f_br ~ 0.6 - 2.0 x 10^-4 Hz, consistent with the previous results of Uttley, McHardy & Papadakis. The slope of the power spectrum above the break is energy dependent, with the higher energies showing a flatter PSD. At low frequencies the variations between different energy bands are highly coherent while at high frequencies the coherence is significantly reduced. Time lags are detected between energy bands, with the soft variations leading the hard. The magnitude of the lag is small (<200 s for the frequencies observed) and is most likely frequency dependent. These properties are remarkably similar to the temporal properties of the Galactic black hole candidate Cygnus X-1. The characteristic timescales in these two types of source differ by ~10^5; assuming that these timescales scale linearly with black hole mass then suggests a black hole mass ~10^6 M_sun for MCG-6-30-15. We speculate that the timing properties of MCG-6-30-15 may be analogous to those of Cyg X-1 in its high/soft state and discuss a simple phenomenological model, originally developed to explain the timing properties of Cyg X-1, that can explain many of the observed properties of MCG-6-30-15.

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