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Using two complementary X-ray galaxy cluster studies we present new
cosmological constraints on dark energy. Using Chandra measurements
of the X-ray gas mass fraction, fgas, we obtain a detection
of the effects of dark energy comparable in significance to recent
type Ia supernovae (SNIa) studies. Using X-ray luminosity function
(XLF) data from galaxy cluster surveys, we obtain the first
interesting determination of the dark energy equation of state from
measurements of the growth of cosmic structure in clusters. Both of
our experiments provide strong, independent support to the
G. Matt, Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, England,
A.C. Fabian, Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, England,
R.R. Ross, Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, England, Physics Department, College of the Holy Cross, Worcester, MA 01610, USA
The properties of the iron Kα line emitted by a photoionized accretion disc have been calculated for different source geometries.
The properties of the iron Kα fluorescence line emitted by an α-viscosity accretion disc illuminated by an external X-ray source have been calculated for different values of the disc accretion rate ṁ. The vertical ionization structure of the matter has been computed by using the numerical code described in. Two different source geometries have been studied: a point source located at 20 rg (=GM/c2) above the disc on its symmetry axis, and an extended source above the innermost part (r = 6 – 50rg) of the disc. Assuming α=0.1, a Schwarzschild black hole and a hard luminosity equal to the disc luminosity, we find that for large values of ṁ (≲ 0.2, in units of the critical value) the matter can be significantly ionized, and the iron line equivalent width can reach values as high as 250 eV for the point source, and up to about 400 eV for the extended source (while for neutral matter it is ∼150eV for a face-on disc). The line centroid energy, in the emitting rest frame, is significantly higher than 6.4 keV, the value for neutral iron. A further increase of ṁ (≲ 0.5) leads to a strong decrease of the line intensity, because the iron becomes fully stripped in the inner region of the disc.
A.C. Fabian, Institute of Astronomy, Madingley Road, Cambridge CB3 0HA,
R.R. Ross, Institute of Astronomy, Madingley Road, Cambridge CB3 0HA and Physics Department, College of the Holy Cross, Worcester, MA, USA
Luminous accretion discs around black holes are expected to be optically thick and radiate much of their emission in the EUV and soft X-ray bands. Quasiblackbody emission consistent with such discs is observed in many Seyfert 1 galaxies and from Galactic black hole candidates such as Cygnus X-1. The harder, rapidly variable, X-rays from such objects must originate above the disc, probably from non-thermal processes involving magnetic fields. The disc is therefore irradiated by a hard X-ray continuum. Backscattering and fluorescence from the disc produce a reflection spectrum, which is now observed in X-rays. Features in the reflection spectrum act as a diagnostic of the geometry and conditions of the inner disc, offering the strong possibility that it can be mapped in the near future.
We begin by reviewing the case for the presence of accretion discs in many Active Galactic Nuclei (AGN), such as the Seyfert 1 galaxies. Here we are concentrating on the inner disc within radii R ≲ 100Rs, where Rs is the Schwarzschild radius of the central object (assumed here to be a black hole). Such discs were first detected from the UV excess and in particular by the variable soft X-ray emission that they produce. Further rapid progress has been hindered by the unfortunate coincidence that most of the direct thermal radiation produced by accretion discs around massive objects is emitted in the EUV, where photoelectric absorption by the interstellar medium of our Galaxy is strong.
The presence of iron lines and high energy excesses in the X-ray spectra of Seyfert galaxies has been firmly established by Ginga (e.g. Nandra & Pounds 1993 and references therein). These features are generally interpreted as signatures of the reprocessing of the primary X-rays by matter in the neighbourhood of the central black hole, probably distributed in an accretion disc (Lightman & White 1988, George & Fabian 1991, Matt, Perola & Piro 1991).
The properties of hot gaseous haloes in massive early-type galaxies are briefly reviewed. Gas flows in such haloes are complex yet so large-scale that they may guide us in the understanding of flows around disk galaxies. The intracluster medium is discussed as a further illustration of the properties of diffuse hot gas trapped in a gravtational well. Finally, the possibility of the existence of a significant diffuse medium in the Local Group, and in groups in general, is revived. Such a medium would generate a substantial disk-halo interaction with our Galaxy.
The evidence for cooling flows in clusters of galaxies is discussed. Peaked X-ray surface brightness profiles and the presence of soft X-ray spectral components are characteristic signatures of cooling flows. The best available data are consistent with gas cooling at a rate of ~ 100M⊙yr-1 and depositing cooled matter over a radius of 100 – 200 kpc. The same rate is obtained from observations of gas cooling over a range of temperatures and thus a range of cooling times covering less than 3 × 107 yr to more than 3 × 109 yr. It appears that cooling flows are both steady and long-lived. Some indirect optical evidence for distant cooling flows (0.5 < z < 1) is presented.
The clustering of the background-contributing X-ray sources is reflected in the excess (i.e., non-Poisson) fluctuations in the X-ray background (XRB). Observational limits on ΔI/I can therefore be used to constrain either the clumpiness of X-ray sources or their contribution to the XRB if their clustering properties are known (see Barcons & Fabian 1987 for details).
X-ray observations have shown that early-type galaxies contain a hot interstellar medium. This implies that the galaxies have a) a low supernova rate; b) high total gravitational binding masses and c) continuous star formation. Much of the gas in isolated galaxies is probably due to stellar mass-loss. The details of its behaviour are complex.
Recent X-ray observations of active galactic nuclei and Seyfert galaxies in particular are briefly reviewed. The application of the efficiency limit to rapidly varying luminous sources such as NGC 6814 is discussed. It is argued that the variability and probable MeV spectral turnover imply that most of the electrons which radiate the observed flux are only mildly relativistic. A possible link between the steep soft X-ray spectra and featureless optical continua of BL Lac objects is considered.
A periodic modulation in the optical flux from Nova Cygni 1975 has been observed since shortly after outburst (Tempesti 1975) and the period is known to have varied by at least a few percent (Semeniuk et al. 1976). We account for the modulation in terms of a simple geometrical model in which the wind emanating from the nova is shadowed by its binary companion (see Fabian and Pringle 1977 for a fuller account). This produces an azimuthal variation in the radius of the surface of last scattering, RS, which an observer sees as a periodic modulation of the. continuum with period roughly equal to the orbital period. Variation of the observed period is accounted for in terms of variation of the size of RS.
We consider spherically symmetric accretion flow onto a strongly magnetized neutron star. We show that, under certain conditions, the flow is intermittent and that the resultant accretion luminosity (X-rays) from the stellar surface is akin to that observed in the bursting X-ray sources. We investigate the properties of such burst flows under a variety of conditions, in the hopes of providing a basic theoretical framework on which realistic models of the observed bursting sources can be built. This work was supported in part by NSF Grant PHY75-08790.
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