Radiation pressure supported stars in Einstein gravity: eternally collapsing objects     

A. Mitra † 《Monthly notices of the Royal Astronomical Society》2006,369(1):492-496

Even when we consider Newtonian stars, that is, stars with surface gravitational redshift   z ≪ 1  , it is well known that, theoretically, it is possible to have stars supported against self-gravity almost entirely by radiation pressure. However, such Newtonian stars must necessarily be supermassive. We point out that this requirement for excessively large M in the Newtonian case is a consequence of the occurrence of low   z ≪ 1  . However, if we remove such restrictions, and allow for the possible occurrence of a highly general relativistic regime,   z ≫ 1  , we show that it is possible to have radiation pressure supported stars (RPSSs) at an arbitrary value of M . Since RPSSs necessarily radiate at the Eddington limit, in Einstein gravity, they are never in strict hydrodynamical equilibrium. Further, it is believed that sufficiently massive or dense objects undergo continued gravitational collapse to the black hole (BH) stage characterized by   z =∞  . Thus, late stages of BH formation, by definition, would have   z ≫ 1  , and hence would be examples of quasi-stable general relativistic RPSSs. It is shown that the observed duration of such Eddington limited radiation pressure dominated states is   t ≈ 5 × 10⁸ (1 + z ) yr  . Thus,   t →∞  as BH formation  ( z →∞)  takes place. Consequently, such radiation pressure dominated extreme general relativistic stars become eternally collapsing objects (ECOs) and the BH state is preceded by such an ECO phase. This result is also supported by our previous finding that trapped surfaces are not formed in gravitational collapse and the value of the integration constant in the vacuum Schwarzschild solution is zero. Hence the supposed observed BHs are actually ECOs.  相似文献   

Modelling of sediment dynamics in a laboratory‐scale experimental catchment     

G. R. Hancock  J. Nuake  S. G. Fityus 《水文研究》2006,20(1):67-84

The variability of hillslope form and function is examined experimentally using a simple model catchment in which most landscape development parameters are either known or controlled. It is demonstrated that there is considerable variability in sediment output from similar catchments, subjected to the same hydrological processes, and for which the initial hillslope profiles are the same. The results demonstrate that, in the case of catchments with a linear initial hillslope profile, the sediment output is initially high but reduces through time, whereas for a concave initial profile the sediment output was smaller and relatively constant. Concave hillslope profiles also displayed reduced sediment output when compared with linear slopes with the same overall slope. Using this experimental model catchment data, the SIBERIA landscape evolution model was tested for its ability to predict temporal sediment transport. When calibrated for the rainfall and erodible material, SIBERIA is able to simulate mean temporal sediment output for the experimental catchment over a range of hillslope profiles and rainfall intensities. SIBERIA is also able to match the hillslope profile of the experimental catchments. The results of the study provide confidence in the ability of SIBERIA to predict temporal sediment output. The experimental and modelling data also demonstrate that, even with all geomorphic and hydrological variables being known and/or controlled, there is still a need for long‐term stream gauging to obtain reliable assessments of field catchment hydrology and sediment transport. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

Broad Iron Lines in AGN and X-Ray Binaries     

A. C. Fabian 《Astrophysics and Space Science》2005,300(1-3):97-105

Several AGN and black hole X-ray binaries show a clear very broad iron line, which is strong evidence that the black holes are rapidly spinning. Detailed analysis of these objects shows that the emission line is not significantly affected by absorption and that the source variability is principally due to variation in amplitude of a power-law. Underlying this is a much less variable, relativistically-smeared, reflection-dominated, component which carries the imprint of strong gravity at a few gravitational radii. The strong gravitational light bending in these regions then explains the power-law variability as due to changes in height of the primary X-ray source above the disc. The reflection component, in particular its variability and the profile of the iron line, enables us to study the innermost regions around an accreting, spinning, black hole.  相似文献   

RBS 1032: a dwarf-nucleated spheroidal galaxy with an intermediate-mass black hole hosted in a globular cluster     

Kajal K. Ghosh  Valera Suleymanov  Ilfan Bikmaev  Slava Shimansky  Nail Sakhibullin 《Monthly notices of the Royal Astronomical Society》2006,371(4):1587-1593

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Black hole spin in GRS 1915+105   总被引：2，自引：0，他引：2  

Matthew Middleton  Chris Done  Marek Gierli&#;ski  Shane W. Davis 《Monthly notices of the Royal Astronomical Society》2006,373(3):1004-1012

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On the deep minimum state in the Seyfert galaxy MCG−6-30-15     

Christopher S. Reynolds  Jörn Wilms  Mitchell C. Begelman  Rüdiger Staubert  Eckhard Kendziorra 《Monthly notices of the Royal Astronomical Society》2004,349(4):1153-1166

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On the X-ray spectra of luminous, inhomogeneous accretion flows   总被引：1，自引：0，他引：1  

A. Merloni  J. Malzac  A. C. Fabian  R. R. Ross 《Monthly notices of the Royal Astronomical Society》2006,370(4):1699-1712

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Exploring the Jet/Accretion Flow Relationship in low Disk Luminosity Sources     

Sera Markoff 《Astrophysics and Space Science》2005,300(1-3):189-195

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The star-forming environment of an ultraluminous X-ray source in NGC 4559: an optical study     

Roberto Soria  Mark Cropper  Manfred Pakull  Richard Mushotzky  Kinwah Wu 《Monthly notices of the Royal Astronomical Society》2005,356(1):12-28

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Energy-dependent variability from accretion flows     

Andrzej A. Zdziarski 《Monthly notices of the Royal Astronomical Society》2005,360(3):816-824

We develop a formalism to calculate energy-dependent fractional variability (rms) in accretion flows. We consider rms spectra resulting from radial dependences of the level of local variability (as expected from the propagation of disturbances in accretion flows) assuming the constant shape of the spectrum emitted at a given radius. We consider the cases when the variability of the flow is either coherent or incoherent between different radial zones. As an example of local emission, we consider blackbody, Wien and thermal Comptonization spectra. In addition to numerical results, we present a number of analytical formulae for the resulting rms. We also find an analytical formula for the disc Wien spectrum, which we find to be a very good approximation to the disc blackbody. We compare our results to the rms spectrum observed in an ultrasoft state of GRS 1915+105.  相似文献