Showing papers by "A. C. Fabian published in 1995"

Warm absorbers in active galactic nuclei

Abstract: Recent {\it ASCA} observations confirm the presence of X-ray absorption due to partially ionized gas in many Seyfert 1 galaxies; the so-called warm absorber. Constraints on the location of the warm material are presented with the conclusion that this material lies at radii coincident with, or just outside, the broad-line region. The stability of this warm material to isobaric perturbations under the assumptions of thermal and photoionization equilibrium is also studied. It is shown that there is a remarkably small range of ionization parameter, $\xi$, for which the warm absorber state is stable. The robustness of this result to changes in the shape of the primary continuum, the assumed density and optical depth is investigated. Given the constraints on the location and the stability properties of the material, several models for the environments of Seyfert nuclei are discussed. These attempt to explain the presence of significant amounts of partially ionized material. In particular, various models of the broad-line region are discussed. The simple two-phase model of the broad-line region proves to be unsatisfactory. A model of the broad-line region is presented in which a turbulent, hot intercloud medium is mechanically heated. Turbulent mixing layers could then give rise to warm absorption features. Finally, a model is discussed in which the warm absorber is due to a steady state, radiatively driven outflow.

The variable OVIII Warm Absorber in MCG-6-30-15

Abstract: We present the results of a 4 day ASCA observation of the Seyfert galaxy MCG-6-30-15, focussing on the nature of the X-ray absorption by the warm absorber, characterizd by the K-edges of the intermediately ionized oxygen, OVII and OVIII. We confirm that the column density of OVIII changes on a timescale of $\sim 10^4$~s when the X-ray continuum flux decreases. The significant anti-correlation of column density with continuum flux gives direct evidence that the warm absorber is photoionized by the X-ray continuum. From the timescale of the variation of the OVIII column density, we estimate that it originates from gas within a radius of about $10^{17}\cm$ of the central engine. In contrast, the depth of the OVII edge shows no response to the continuum flux, which indicates that it originates in gas at larger radii. Our results strongly suggest that there are two warm absorbing regions; one located near or within the Broad Line Region, the other associated with the outer molecular torus, scattering medium or Narrow Line Region.

ASCA and ROSAT observations of the QSF3 field: the X-ray background in the 0.1--7 keV band

Abstract: The X-ray background (XRB) from 0.1 to 7 keV has been studied using high spectral and spatial resolution data from the ASCA SIS and ROSAT PSPC. Analysing both the diffuse background radiation and resolved sources, we have carried out a series of joint spectral fits of the ASCA and ROSAT data. The spectrum of the XRB can be fit well by a single power-law from 1 to 7 keV; overall, the photon index of the power-law model ranges from 1.4 to 1.5, and no obvious excess is found between 1 and 3 keV. Below 1 keV, the models become more complicated and involve a mixture of extragalactic and Galactic sources. As some of the extragalactic contributions should be from point sources, we have examined the ASCA and ROSAT spectra of resolved sources individually: a stellar source having a well-fit thermal spectrum and two AGNs having a much steeper power-law spectrum (with photon index of about 3); the accumulated spectrum of other non-stellar sources resolved by ROSAT also has a steeper-than-average spectrum. Fitting the XRB spectrum observed by ASCA and the accumulated point source spectrum by ROSAT together by varying the contribution from steep spectrum sources, such as quasars, to the background, we find that they contribute less than 30 per cent in the 0.5--2 keV band and drop to below 10 per cent over 2--10 keV. This fraction is provided by sources brighter than a few times $10^{-15}$ \ergpercmpers (in the 0.5--2 keV band). Constrained by our spectral fitting results, the major contributor of the XRB must be a single population with similar flat spectra.

ASCA PV observations of the Seyfert 1 galaxy MCG – 6 – 30 – 15: rapid variability of the warm absorber

Abstract: We present a detailed re-analysis of the two {\it ASCA} Performance Verification observations of the nearby Seyfert 1 galaxy MCG-6-30-15. Confirming the results of Fabian et al. (1994), we find definite evidence for the {\sc O\,vii} and {\sc O\,viii} K-shell absorption edges of the warm absorber and a doubling of the warm absorber column density within the 3 weeks separating the two observations. No intra-day {\it flux-correlated} variability of the warm absorber is found. However, we report the discovery of an `event' in which the warm absorber parameters temporarily change for \sim10\,000\thinspace s before returning to their original values. Possible interpretations are discussed but a contradiction remains: the constancy of the ionization state of the warm absorber argues that it lies at large distances from the central source whereas the short term change in column density argues for small distances. Fluorescent iron emission is examined. As found by Fabian et al. (1994), the iron line is broad and strong (equivalent width \sim300\thinspace eV). The line profile is also suggestive of it being skewed. Such a line would be expected from a relativistic accretion disk. We also find very rapid primary X-ray variability. Assuming relativistic beaming to be unimportant, the derived efficiency is comparable to the maximum obtainable from accretion onto a Schwarzschild black hole. Correlated variability outside of the energy range of {\it ASCA} might exceed this maximum, thus requiring efficient accretion onto a Kerr hole.

Joint ROSAT-Compton GRO observations of the X-ray bright Seyfert galaxy IC 4329A

Abstract: We report a simultaneous ROSAT and Gamma Ray Observatory (GRO) observation of the X-ray-bright Seyfert galaxy IC 4329A. For the GRO Oriented Scintillation Spectrometer Experiment (OSSE) detector, we also present the sum of the data for this and earlier observations. The overall spectrum is very well described as a power law with an energy spectral index of approximately 1 absorbed at low energies plus a strong Compton reflection component, typical for Seyfert 1 galaxies. The low energy absorption can be well described by a sum of a neutral column density of approximately 3 x 10(exp 21)sq cm, most of which is associated with the edge-on galactic disk of IC 4329A, plus an edgelike feature at approximately 700 eV; this feature implies either complex absorption (due to additional ionized material, or due to a partial covering), or a soft excess. The data only weakly constrain the presence of a high-energy cutoff in the underlying power law; they are compatible with an exponential cutoff at any energy E(sub c) approximately greater than 100 keV. The relative steepness of the OSSE data, with the power-law energy index of 1.6 +/- 0.2, can be accounted for entirely by the contribution of the high-energy tail of the reflection component when E(sub c) approaches infinity. (We find that the definite cutoff at an energy E(sub c)approximately 130 keV suggested in the recently published analysis of the OSSE data for this subject is due to a data reduction error.) Including nonsimultaneous Ginga observations with 2 keV fluxes matching well that of ROSAT gives us likely broad-band X-ray/gamma-ray spectra of the object from approximately 0.1 keV up to several hundred keV. We also report the ROSAT spectrum of the companion object to the Seyfert galaxy, the elliptical galaxy IC 4329.

Cooling flows, central galaxy-cluster alignments, X-ray absorption and dust

Abstract: We present the analysis of pointed ROSAT PSPC observations of five of t he most luminous, intermediate redshift (0 1 0 15) clusters of galaxies detected in the ROSAT All-Sky Survey. The PSPC data are combined with optical CCD images and spectra to examine the relationship between clusters and their central cluster galaxies (CCGs). Abell 1068, Abell 1361 and Abell 1664 contain three of the most optically line-luminous CCGs known. The PSPC X-ray data show that these galaxies lie at the centres of massive (200 ‐ 400 M yr 1 ) cooling flows. The alignment between CCGs and their host clusters has been investigated. For those clusters with cooling flows, the position angles of the X-ray emission from t he clusters and the optical emission from the CCGs agrees within 5 degrees. For the one probable non-cooling flow cluster in the sample, Abell 2208, the alignment is significantly poorer. We examine the evidence for intrinsic X-ray absorption in the clusters. The X-ray spectra for Ab ell 1068 and Abell 1664 show that the cooling flows in these clusters are intrinsically absorbed by equivalent hydrogen column densities 10 21 atom cm 2 . The optical spectra of the CCGs in these clusters exhibit substantial intrinsic reddening, at levels consistent with the X-ray absorption results if standard dust to gas ratios are assumed.

ASCA observations of the Seyfert 1 galaxies MRK 1040 and MS 0225.5+3121

Abstract: We present {\it ASCA} observations of the Seyfert 1 galaxies Mrk~1040 and MS~02255+3121 Mrk~1040 was found to have decreased in flux by almost a factor of 4 since an {\it EXOSAT} observation 10\,years ago The energy spectrum of Mrk~1040 displays complexity both at soft energies (below 08\thinspace keV) and at hard energies (6--7\thinspace keV) The latter is readily interpreted as fluorescent K$\alpha$ emission from cold iron expected when the primary X-ray source illuminates cold optically-thick material This line is both broad (with FWHM 16\,000--70\,000 km\,s$^{-1}$) and strong (equivalent width $\sim 550\pm 250$\, eV) suggesting that it originates from material close to the compact object with non-solar abundances Abundance effects on the equivalent width of such a line are investigated via Monte Carlo simulations We find that strong lines can be produced with physically plausible abundances The effect of abundances on the associated reflection continuum is also discussed The soft spectral complexity implies either a strong soft excess together with intrinsic absorption, or a complex absorber Various models for the nature of such a complex absorber are discussed MS~02255+3121 shows no evidence for any variability and has a spectrum that is well described by a power law with Galactic absorption

ROSAT HRI observations of IRAS P09104+4109: a massive cooling flow

Abstract: A ROSAT HRI image of the hyperluminous infrared galaxy IRAS P09104+4109 shows extended X-ray emission peaked around the central galaxy in a profile characteristic of a cooling flow. The inferred soft X-ray luminosity is comparable to that detected with ASCA. The ASCA spectrum is well fit by a cooling flow spectrum, indicating a mass cooling rate of about 1000 Msunpyr. The cooling flow dominates the total luminosity of the surrounding cluster. Any contribution from a hidden AGN in the host galaxy to either the total ASCA spectrum or the HRI image is small.

On broad iron Kalpha lines in Seyfert 1 galaxies

Abstract: The X-ray spectrum obtained by Tanaka et al from a long observation of the active galaxy MCG$-6-30-15$ shows a broad iron K$\alpha$ line skewed to low energies. The simplest interpretation of the shape of the line is that it is due to doppler and gravitational redshifts from the inner parts of a disk about a massive black hole. Similarly broad lines are evident in shorter observations of several other active galaxies. In this paper we investigate other line broadening and skewing mechanisms such as Comptonization in cold gas and doppler shifts from outflows. We have also fitted complex spectral models to the data of MCG$-6-30-15$ to see whether the broad skewed line can be mimicked well by other absorption or emission features. No satisfactory mechanism or spectral model is found, thus strengthening the relativistic disk line model.

The Accretion luminosity of a massive black hole in an elliptical galaxy

Abstract: Most large elliptical galaxies should now host a massive black hole leftover from an earlier quasar phase. Such galaxies also have an extensive hot gaseous halo. Here we consider why the nuclei of elliptical galaxies are not luminous sources due to the accretion of the hot gas by the central black hole, yet have weak radio sources. In particular we suggest that accretion from the hot medium has low radiative efficiency and forms a hot ion torus surrounding the black hole. Synchrotron emission from the torus can account for at least part of the weak radio sources commonly seen in otherwise normal elliptical galaxies. The inner funnel of the ion torus can also begin the collimation of a narrow jet. We speculate on the difference between jets from the nuclei of spiral and elliptical galaxies and suggest that the extensive hot atmosphere seen in ellipticals is essential for maintaining the collimation and provoking the observed radio structures of radio galaxies. We also note that low radiative efficiency, and not necessarity a large change in accretion rate, may be responsible for the demise of quasars and may contribute to the hard X-ray Background. Hot ion tori may be common in galactic nuclei.

The role of cooling flows in galaxy formation

Abstract: The present structure of galaxies is governed by the radiative dissipation of the gravitational and supernova energy injected during formation. A crucial aspect of this process is whether the gas cools as fast as it falls into the gravitational potential well. If it does then rapid normal star formation is assumed to ensue. If not, and the gas can still cool by the present time, then the situation resembles that of a cooling flow, such as commonly found in clusters of galaxies. The cooled matter is assumed to accumulate as very cold clouds and/or low mass stars, i.e. as baryonic dark matter. In this paper we investigate the likelihood of a cooling flow phase during the hierarchical formation of galaxies. We concentrate on the behaviour of the gas, using a highly simplified treatment of the evolution of the dark matter potential within which the gas evolves. We assume that normal star formation is limited by how much gas the associated supernovae can unbind and allow the gas profile to flatten as a consequence of supernova energy injection. We find that cooling flows are an important phase in the formation of most galaxies with total (dark plus luminous) masses approxgt 10^12 Msun , creating about 20 per cent of the total dark halo in a galaxy such as our own and a smaller but comparable fraction of an elliptical galaxy of similar mass. The onset of a cooling flow determines the upper mass limit for the formation of a visible spheroid from gas, setting a characteristic mass scale for normal galaxies. We argue that disk formation requires a cooling flow phase and that dissipation in the cooling flow phase is the most important factor in the survival of normal galaxies during subsequent hierarchical mergers.

Nearby galaxies and the ginga x-ray background

Abstract: XB thanks the DGICYT for financial support, under project PB92-00501. FJC, XB and ACF acknowledge partial financial support from a NATO collaborative research grant. XB and ACF were partly supported by the 'Human Capital and Mobility' programme of the EU under contract CHRXCT92-0033.

Recent x‐ray spectral results from seyfert 1 galaxies

Abstract: Recent results from GINGA, ROSAT and, in particular, ASCA show that the X-ray spectra of Seyfert 1 galaxies are complex. The primary power-law continuum is commonly modified by oxygen absorption edges indicative of highly ionized gas along the line of sight (the warm absorber) and fluorescent iron emission features, with an associated hard continuum, indicative of reflection from relatively cold material. The excellent sensitivity and spectral resolution of ASCA now enables us to resolve the spectral features and so probe and map out the innermost structures of AGN.