| 1. |
- Feroci, M., et al.
(författare)
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The Large Observatory for X-ray Timing (LOFT)
- 2012
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Ingår i: Experimental Astronomy. - : Springer Science and Business Media LLC. - 0922-6435 .- 1572-9508. ; 34:2, s. 415-444
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Tidskriftsartikel (refereegranskat)abstract
- High-time-resolution X-ray observations of compact objects provide direct access to strong-field gravity, to the equation of state of ultradense matter and to black hole masses and spins. A 10 m(2)-class instrument in combination with good spectral resolution is required to exploit the relevant diagnostics and answer two of the fundamental questions of the European Space Agency (ESA) Cosmic Vision Theme "Matter under extreme conditions", namely: does matter orbiting close to the event horizon follow the predictions of general relativity? What is the equation of state of matter in neutron stars? The Large Observatory For X-ray Timing (LOFT), selected by ESA as one of the four Cosmic Vision M3 candidate missions to undergo an assessment phase, will revolutionise the study of collapsed objects in our galaxy and of the brightest supermassive black holes in active galactic nuclei. Thanks to an innovative design and the development of large-area monolithic silicon drift detectors, the Large Area Detector (LAD) on board LOFT will achieve an effective area of similar to 12 m(2) (more than an order of magnitude larger than any spaceborne predecessor) in the 2-30 keV range (up to 50 keV in expanded mode), yet still fits a conventional platform and small/medium-class launcher. With this large area and a spectral resolution of < 260 eV, LOFT will yield unprecedented information on strongly curved spacetimes and matter under extreme conditions of pressure and magnetic field strength.
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| 2. |
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| 3. |
- Ciesielski, A., et al.
(författare)
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Stability of radiation-pressure dominated disks: I. the dispersion relation for a delayed heating α-viscosity prescription
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 538
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Tidskriftsartikel (refereegranskat)abstract
- We derive and investigate the dispersion relation for accretion disks with retarded or advanced heating. We follow the α-prescription but allow for a time offset τ between heating and pressure perturbations, as well as for a diminished response of heating to pressure variations. We study in detail solutions of the dispersion relation for disks with radiation-pressure fraction, 1-β, and ξ, the ratio of viscous stress response to pressure perturbations. For τ < 0 (advanced heating) the number and sign of real solutions for the growth rate depend on the values of τ, and ξ: if the magnitude of τ is larger than a critical value (e.g., more than twice the thermal time,-τ > 2 τ th, for β = 0 and ξ = 1) two real solutions exist, which are both negative. These results imply that radiation-pressure dominated accretion disks may be stabilized when there is a time delay between stress fluctuations and fluctuations in heating. © 2012 ESO.
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| 4. |
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| 5. |
- Sadowski, A., et al.
(författare)
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Vertical dissipation profiles and the photosphere location in thin and slim accretion disks
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 502:1, s. 7-13
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Tidskriftsartikel (refereegranskat)abstract
- As several authors in the past, we calculate optically thick but geometrically thin (and slim) accretion disk models and perform a ray-tracing of photons in the Kerr geometry to calculate the observed disk continuum spectra. Previously, it was common practice to ray-trace photons assuming that they are emitted from the Kerr geometry equatorial plane, z = 0. We show that the continuum spectra calculated with this assumption differ from these calculated under the assumption that photons are emitted from the actual surface of the disc, z = H(r). This implies that a knowledge of the location of the thin disk effective photosphere is relevant for calculating the continuum emission. In this paper we investigate, in terms of a simple model, a possible influence of the (unknown, and therefore assumed ad hoc) vertical dissipation profiles on the vertical structure of the disk and thus on the location of the effective photosphere, and on the observed continuum spectra. For disks with moderate and high mass accretion rates (dot m > 0.01 dot m_C), we find that the photosphere location in the inner, radiation pressure dominated, disk region (where most of the radiation comes from) does not depend on the dissipation profile and therefore emerging disk spectra are insensitive to the choice of the dissipation function. For lower accretion rates, the photosphere location depends on the assumed vertical dissipation profile down to the disk inner edge, but the dependence is very weak and thus of minor importance. We conclude that the continuum spectra of optically thick accretion disks around black holes should be calculated with ray-tracing from the effective photosphere and that, fortunately, the choice of a particular vertical dissipation profile does not substantially influence the calculated emission.
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| 6. |
- Vincent, F. H., et al.
(författare)
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A magnetized torus for modeling Sagittarius A* millimeter images and spectra
- 2015
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 574
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Tidskriftsartikel (refereegranskat)abstract
- Context. The supermassive black hole, Sagittarius (Sgr) A*, in the centre of our Galaxy has the largest angular size in the sky among all astrophysical black holes. Its shadow, assuming no rotation, spans similar to 50 mu as. Resolving such dimensions has long been out of reach for astronomical instruments until a new generation of interferometers being operational during this decade. Of particular interest is the Event Horizon Telescope (EHT) with resolution similar to 20 mu as in the millimeter- wavelength range 0.87 mm- 1.3 mm. Aims. We investigate the ability of the fully general relativistic Komissarov (2006, MNRAS, 368, 993) analytical magnetized torus model to account for observable constraints at Sgr A* in the centimeter and millimeter domains. The impact of the magnetic field geometry on the observables is also studied. Methods. We calculate ray-traced centimeter- and millimeter-wavelength synchrotron spectra and images of a magnetized accretion torus surrounding the central black hole in Sgr A*. We assume stationarity, axial symmetry, constant specific angular momentum and polytropic equation of state. A hybrid population of thermal and non-thermal electrons is considered. Results. We show that the torus model is capable of reproducing spectral constraints in the millimeter domain, and in particular in the observable domain of the EHT. However, the torus model is not yet able to fit the centimeter spectrum. 1.3 mm images at high inclinations are in agreement with observable constraints. Conclusions. The ability of the torus model to account for observations of Sgr A* in the millimeter domain is interesting in the perspective of the future EHT. Such an analytical model allows very fast computations. It will thus be a suitable test bed for investigating large domains of physical parameters, as well as non-black-hole compact object candidates and alternative theories of gravity.
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| 7. |
- Vincent, F. H., et al.
(författare)
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Multi-wavelength torus-jet model for Sagittarius A*
- 2019
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 1432-0746 .- 0004-6361. ; 624
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Tidskriftsartikel (refereegranskat)abstract
- Context. The properties of the accretion/ejection flow surrounding the supermassive central black hole of the Galaxy Sgr A* will be scrutinized by the new-generation instrument GRAVITY and the Event Horizon Telescope (EHT). Developing fast, robust, and simple models of such flows is therefore important and very timely. Aims. We want to model the quiescent emission of Sgr A* from radio to mid-infrared wavelengths, using thermal and nonthermal synchrotron. The radiation is emitted by the overlay of a magnetized compact torus close to the black hole, and a large-scale magnetized jet. We compare model spectra and images to the multi-wavelength observable constraints available to date. We simulate EHT observations at 1.3 mm of the best-fit model for different inclinations. Methods. We use a simple analytic description for the geometry of the torus and jet. We model their emission by thermal synchrotron and kappa-distribution synchrotron, respectively. We use relativistic ray tracing to compute simulated spectra and images, restricting our analysis to the Schwarzschild (zero spin) case. A best-fit is found by adjusting the simulated spectra to the latest observed data, and we check the consistency of our spectral best fits with the radio-image sizes and infrared spectral index constraints. We use the open-source eht-imaging library to generate EHT-reconstructed images. Results. We find perfect spectral fit (chi(2 )(red)approximate to 1)both for nearly face-on and nearly edge-on views. These best fits give parameter values very close to those found by the most recent numerical simulations, which are much more complex than our model. The intrinsic radio size of Sgr A* is found to be in reasonable agreement with the centimetric observed constraints. Our best-fit infrared spectral index is in perfect agreement with the latest constraints. Our emission region at 1.3 mm, although larger than the early-EHT Gaussian best fit, does contain bright features at the less than or similar to 40 mu as scale. EHT-reconstructed images show that torus/jet-specific features persist after the reconstruction procedure, and that these features are sensitive to inclination. Conclusions. The main interest of our model is to provide a simple and fast model of the quiescent state of Sgr A*, which gives extremely similar results to those of state-of-the-art numerical simulations. Our model is easy to use and we publish all the material necessary to reproduce our spectra and images, meaning that anyone interested can use our results relatively straightforwardly. We hope that such a public tool will be useful in the context of the recent and near-future GRAVITY and EHT results.
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| 8. |
- Abramowicz, Marek A, 1945, et al.
(författare)
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A Robust Test of the Existence of Primordial Black Holes in Galactic Dark Matter Halos
- 2022
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 935:2
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Tidskriftsartikel (refereegranskat)abstract
- If very low mass primordial black holes (PBH) within the asteroid/moon-mass range indeed reside in galactic dark matter halos, they must necessarily collide with galactic neutron stars (NSs). These collisions must, again necessarily, form light black holes (LBHs) with masses of typical NSs, M (LBH) approximate to 1-2 M (circle dot). LBHs may be behind events already detected by ground-based gravitational-wave detectors (GW170817, GW190425, and others such as a mixed stellar black hole-NS-mass event GW191219_163120), and most recently by microlensing (OGLE-BLG-2011-0462). Although the status of these observations as containing LBHs is not confirmed, there is no question that gravitational-wave detectors and microlensing are in principle and in practice capable of detecting LBHs. We have calculated the creation rate of LBHs resulting from these light primordial black hole (PBH) collisions with NSs. On this basis, we claim that if improved gravitational-wave detectors and microlensing statistics of the LBH events would indicate that the number of LBHs is significantly lower that what follows from the calculated creation rate, then this would be an unambiguous proof that there is no significant light PBH contribution to the galactic dark matter halos. Otherwise, if observed and calculated numbers of LBHs roughly agree, then the hypothesis of primordial black hole existence gets strong observational support, and in addition their collisions with NSs may be considered a natural creation channel for the LBHs, solving the problem of their origin, as it is known that they cannot be a product of standard stellar evolution.
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| 9. |
- Abramowicz, Marek A, 1945, et al.
(författare)
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A Short Answer to Critics of Our Article "Eppur si Espande"
- 2009
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Ingår i: Acta Astronomica. ; 59:1, s. 131-139
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Tidskriftsartikel (refereegranskat)abstract
- Recently we presented a formal mathematical proof that, contrary to a widespread misconception, cosmological expansion cannot be understood as the motion of galaxies in non-expanding space. We showed that the cosmological redshift must be physically interpreted as the expansion of space. Although our proof was generally accepted, a few authors disagreed. We rebut their criticism in this Note.
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| 10. |
- Abramowicz, Marek A, 1945, et al.
(författare)
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And yet it's expanding
- 2007
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Ingår i: Acta Astronomica. - 0001-5237. ; 57:2, s. 139-148
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Tidskriftsartikel (refereegranskat)
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| 11. |
- Abramowicz, Marek A, 1945, et al.
(författare)
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The importance of discovering A 3:2 twin-peak quasi-periodic oscillation in an ultraluminous X-ray source, or how to solve the puzzle of intermediate-mass black holes
- 2004
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Ingår i: Astrophysical Journal, Letters. - 0067-0049. ; 609:2
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Tidskriftsartikel (refereegranskat)abstract
- Recently, twin-peak quasi-periodic oscillations (QPOs) have been observed in a 3:2 ratio for three Galactic black hole microquasars with frequencies that have been shown to scale as 1/M, as expected for general relativistic motion near a black hole. It may be possible to extend this result to distinguish between the following two disparate models that have been proposed for the puzzling ultraluminous X-ray sources (ULXs): (1) an intermediate-mass black hole (M~103 M⊙) radiating very near the Eddington limit and (2) a conventional black hole (M~10 M⊙) accreting at a highly super-Eddington rate with its emission beamed along the rotation axis. We suggest that one could discriminate between these models by detecting the counterpart of a Galactic twin-peak QPO in a ULX: the expected frequency for the intermediate-mass black hole model is only about 1 Hz, whereas for the conventional black hole model the expected frequency would be the ~100 Hz value observed for the Galactic microquasars.
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| 12. |
- Bursa, M., et al.
(författare)
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The timescale of encircling light
- 2007
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Ingår i: Silesian University in Opava. ; , s. 21-25
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Konferensbidrag (refereegranskat)abstract
- We show that in some astronomical sources containing black holes, the signature of the circular photon orbit may be detected by searching for the shortest timescale in the variability data. A positive detection would provide the direct empirical support for Einstein's general relativity in its super-strong field limit, relevant to black holes.
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| 13. |
- Kotrlova, A., et al.
(författare)
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Models of high-frequency quasi-periodic oscillations and black hole spin estimates in Galactic microquasars
- 2020
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 643
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Tidskriftsartikel (refereegranskat)abstract
- We explore the influence of nongeodesic pressure forces present in an accretion disc on the frequencies of its axisymmetric and nonaxisymmetric epicyclic oscillation modes. We discuss its implications for models of high-frequency quasi-periodic oscillations (QPOs), which have been observed in the X-ray flux of accreting black holes (BHs) in the three Galactic microquasars, GRS 1915+105, GRO J1655-40, and XTE J1550-564. We focus on previously considered QPO models that deal with low-azimuthal-number epicyclic modes, |m| <= 2, and outline the consequences for the estimations of BH spin, a is an element of [0, 1]. For four out of six examined models, we find only small, rather insignificant changes compared to the geodesic case. For the other two models, on the other hand, there is a significant increase of the estimated upper limit on the spin. Regarding the falsifiability of the QPO models, we find that one particular model from the examined set is incompatible with the data. If the spectral spin estimates for the microquasars that point to a> 0.65 were fully confirmed, two more QPO models would be ruled out. Moreover, if two very different values of the spin, such as a approximate to 0.65 in GRO J1655-40 and a approximate to 1 in GRS 1915+105, were confirmed, all the models except one would remain unsupported by our results. Finally, we discuss the implications for a model that was recently proposed in the context of neutron star (NS) QPOs as a disc-oscillation-based modification of the relativistic precession model. This model provides overall better fits of the NS data and predicts more realistic values of the NS mass compared to the relativistic precession model. We conclude that it also implies a significantly higher upper limit on the microquasar's BH spin (a similar to 0.75 vs. a similar to 0.55).
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| 14. |
- Lančová, D., et al.
(författare)
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Puffy Accretion Disks: Sub-Eddington, Optically Thick, and Stable
- 2019
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 884:2
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Tidskriftsartikel (refereegranskat)abstract
- We report on a new class of solutions of black hole accretion disks that we have found through three-dimensional, global, radiative magnetohydrodynamic simulations in general relativity. It combines features of the canonical thin, slim, and thick disk models but differs in crucial respects from each of them. We expect these new solutions to provide a more realistic description of black hole disks than the slim disk model. We are presenting a disk solution for a nonspinning black hole at a sub-Eddington mass accretion rate,. By the density scale-height measure the disk appears to be thin, having a high density core near the equatorial plane of height, but most of the inflow occurs through a highly advective, turbulent, optically thick, Keplerian region that sandwiches the core and has a substantial geometrical thickness comparable to the radius, H ∼ r. The accreting fluid is supported above the midplane in large part by the magnetic field, with the gas and radiation to magnetic pressure ratio β ∼ 1, this makes the disk thermally stable, even though the radiation pressure strongly dominates over gas pressure. A significant part of the radiation emerging from the disk is captured by the black hole, so the disk is less luminous than a thin disk would be at the same accretion rate. © 2019. The American Astronomical Society. All rights reserved..
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| 15. |
- Lasota, J. P., et al.
(författare)
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Extracting black-hole rotational energy: The generalized Penrose process
- 2014
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Ingår i: Physical Review D. - 1550-7998. ; 89:2
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Tidskriftsartikel (refereegranskat)abstract
- In the case involving particles, the necessary and sufficient condition for the Penrose process to extract energy from a rotating black hole is absorption of particles with negative energies and angular momenta. No torque at the black-hole horizon occurs. In this article we consider the case of arbitrary fields or matter described by an unspecified, general energy-momentum tensor T-mu nu and show that the necessary and sufficient condition for extraction of a black hole's rotational energy is analogous to that in the mechanical Penrose process: absorption of negative energy and negative angular momentum. We also show that a necessary condition for the Penrose process to occur is for the Noether current (the conserved energy-momentum density vector) to be spacelike or past directed (timelike or null) on some part of the horizon. In the particle case, our general criterion for the occurrence of a Penrose process reproduces the standard result. In the case of relativistic jet-producing "magnetically arrested disks," we show that the negative energy and angular-momentum absorption condition is obeyed when the Blandford-Znajek mechanism is at work, and hence the high energy extraction efficiency up to similar to 300% found in recent numerical simulations of such accretion flows results from tapping the black hole's rotational energy through the Penrose process. We show how black-hole rotational energy extraction works in this case by describing the Penrose process in terms of the Noether current.
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| 16. |
- Lasota, J. P., et al.
(författare)
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The slimming effect of advection on black-hole accretion flows
- 2016
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 1432-0746 .- 0004-6361. ; 587
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Tidskriftsartikel (refereegranskat)abstract
- Context. At super-Eddington rates accretion flows onto black holes have been described as slim (aspect ratio H/R less than or similar to 1) or thick (H/R > 1) discs, also known as tori or (Polish) doughnuts. The relation between the two descriptions has never been established, but it was commonly believed that at sufficiently high accretion rates slim discs inflate, becoming thick. Aims. We wish to establish under what conditions slim accretion flows become thick. Methods. We use analytical equations, numerical 1 + 1 schemes, and numerical radiative MHD codes to describe and compare various accretion flow models at very high accretion rates. Results. We find that the dominant effect of advection at high accretion rates precludes slim discs becoming thick. Conclusions. At super-Eddington rates accretion flows around black holes can always be considered slim rather than thick.
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| 17. |
- Sadowski, A., et al.
(författare)
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Energy flows in thick accretion discs and their consequences for black hole feedback
- 2016
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 456:4, s. 3915-3928
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Tidskriftsartikel (refereegranskat)abstract
- We study energy flows in geometrically thick accretion discs, both optically thick and thin, using general relativistic, three-dimensional simulations of black hole accretion flows. We find that for non-rotating black holes the efficiency of the total feedback from thick accretion discs is 3 per cent - roughly half of the thin disc efficiency. This amount of energy is ultimately distributed between outflow and radiation, the latter scaling weakly with the accretion rate for super-critical accretion rates, and returned to the interstellar medium. Accretion on to rotating black holes is more efficient because of the additional extraction of rotational energy. However, the jet component is collimated and likely to interact only weakly with the environment, whereas the outflow and radiation components cover a wide solid angle.
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| 18. |
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| 19. |
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| 20. |
- Wielgus, M., et al.
(författare)
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Observational properties of puffy discs: radiative GRMHD spectra of mildly sub-Eddington accretion
- 2022
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 514:1, s. 780-789
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Tidskriftsartikel (refereegranskat)abstract
- Numerical general relativistic radiative magnetohydrodynamic simulations of accretion discs around a stellar-mass black hole with a luminosity above 0.5 of the Eddington value reveal their stratified, elevated vertical structure. We refer to these thermally stable numerical solutions as puffy discs. Above a dense and geometrically thin core of dimensionless thickness h/r similar to 0.1, crudely resembling a classic thin accretion disc, a puffed-up, geometrically thick layer of lower density is formed. This puffy layer corresponds to h/r similar to 1.0, with a very limited dependence of the dimensionless thickness on the mass accretion rate. We discuss the observational properties of puffy discs, particularly the geometrical obscuration of the inner disc by the elevated puffy region at higher observing inclinations, and collimation of the radiation along the accretion disc spin axis, which may explain the apparent super-Eddington luminosity of some X-ray objects. We also present synthetic spectra of puffy discs, and show that they are qualitatively similar to those of a Comptonized thin disc. We demonstrate that the existing xspec spectral fitting models provide good fits to synthetic observations of puffy discs, but cannot correctly recover the input black hole spin. The puffy region remains optically thick to scattering; in its spectral properties, the puffy disc roughly resembles that of a warm corona sandwiching the disc core. We suggest that puffy discs may correspond to X-ray binary systems of luminosities above 0.3 of the Eddington luminosity in the intermediate spectral states.
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| 21. |
- Wielgus, M., et al.
(författare)
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Stable, levitating, optically thin atmospheres of Eddington-luminosity neutron stars
- 2015
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 454:4, s. 3766-3770
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Tidskriftsartikel (refereegranskat)abstract
- In general relativity, static gaseous atmospheres may be in hydrostatic balance in the absence of a supporting stellar surface, provided that the luminosity is close to the Eddington value. We construct analytic models of optically thin, spherically symmetric shells supported by the radiation pressure of a luminous central body in the Schwarzschild metric. Opacity is assumed to be dominated by Thomson scattering. The inner parts of the atmospheres, where the luminosity locally has supercritical values, are characterized by a density and pressure inversion. The atmospheres are convectively and Rayleigh-Taylor stable, and there is no outflow of gas. RAMOWICZ MA, 1990, ASTROPHYSICAL JOURNAL, V361, P470
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| 22. |
- Xue, L, et al.
(författare)
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STUDIES OF THERMALLY UNSTABLE ACCRETION DISKS AROUND BLACK HOLES WITH ADAPTIVE PSEUDOSPECTRAL DOMAIN DECOMPOSITION METHOD. II. LIMIT-CYCLE BEHAVIOR IN ACCRETION DISKS AROUND KERR BLACK HOLES
- 2011
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Ingår i: ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES. - 0067-0049. ; 195:1
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Tidskriftsartikel (refereegranskat)abstract
- For the first time ever, we derive equations governing the time-evolution of fully relativistic slim accretion disks in the Kerr metric, and numerically construct their detailed non-stationary models. We discuss applications of these general results to a possible limit-cycle behavior of thermally unstable disks. Our equations and numerical method are applicable in a wide class of possible viscosity prescriptions, but in this paper we use a diffusive form of the “standard alpha prescription” that assumes the viscous torque is proportional to the total pressure. In this particular case, we find that the parameters which dominate the limit-cycle properties are the mass-supply rate and the value of the alphaviscosity parameter. Although the duration of the cycle (or the outburst) does not exhibit any clear dependence on the black hole spin, the maximal outburst luminosity (in the Eddington units) is positively correlated with the spin value. We suggest a simple method for a rough estimate of the black hole spin based on the maximal luminosity and the ratio of outburst to cycle durations. We also discuss a temperature-luminosity relation for the Kerr black hole accretion discs limit-cycle. Based on these results we discuss the limit-cycle behavior observed in microquasar GRS 1915+105. We also extend this study to several non-standard viscosity prescriptions, including a “delayed heating” prescription recently stimulated by the recent MHD simulations of accretion disks.
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| 23. |
- Abramowicz, Marek A, 1945, et al.
(författare)
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A Galactic centre gravitational-wave Messenger
- 2020
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 10:1
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Tidskriftsartikel (refereegranskat)abstract
- Our existence in the Universe resulted from a rare combination of circumstances. The same must hold for any highly developed extraterrestrial civilisation, and if they have ever existed in the Milky Way, they would likely be scattered over large distances in space and time. However, all technologically advanced species must be aware of the unique property of the galactic centre: it hosts Sagittarius A* (Sgr A*), the closest supermassive black hole to anyone in the Galaxy. A civilisation with sufficient technical know-how may have placed material in orbit around Sgr A* for research, energy extraction, and communication purposes. In either case, its orbital motion will necessarily be a source of gravitational waves. We show that a Jupiter-mass probe on the retrograde innermost stable circular orbit around Sgr A* emits, depending on the black hole spin, at a frequency of fGW = 0.63–1.07 mHz and with a power of PGW = 2.7 × 1036–2.0 × 1037 erg/s. We discuss that the energy output of a single star is sufficient to stabilise the location of an orbiting probe for a billion years against gravitational wave induced orbital decay. Placing and sustaining a device near Sgr A* is therefore astrophysically possible. Such a probe will emit an unambiguously artificial continuous gravitational wave signal that is observable with LISA-type detectors. © 2020, The Author(s).
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| 24. |
- Abramowicz, Marek A, 1945, et al.
(författare)
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A note on the slope-shift anticorrelation in the neutron star kHz QPOs data
- 2005
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Ingår i: n Proceedings of RAGtime 6/7: Workshops on black holes and neutron stars. - 807248334X
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Konferensbidrag (refereegranskat)abstract
- Observations show that the upper νU and lower νL of the ``twin peak'' high frequency QPOs in neutron star sources vary along lines νU = AνL + B in a frequency-frequency plot, and that their ratios νU/νL cluster near the value 3/2. This behaviour is well consistent with the predictions of the non-linear resonance model for QPOs. In this Note, we further explore our recent finding that the coefficients A, B of the frequency-frequency lines for individual sources are anticorrelated. In the (A,B) plane, they occupy rather a narrow region along the line A = 3/2 - B/600 Hz. We show that this observational property of QPOs also follows from the resonance model
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| 25. |
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