| 1. |
- 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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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- Abramowicz, Marek A, 1945, et al.
(författare)
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Epicyclic frequencies derived from the effective potential: simple and practical formulae
- 2005
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Ingår i: Astrophys. Space Sci.. - : Springer Science and Business Media LLC. - 0004-640X. ; 300:1-3, s. 127-136
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Tidskriftsartikel (refereegranskat)abstract
- We present and discuss a short and simple derivation of orbital epicyclic frequencies for circular geodesic orbits in stationary and axially symmetric spacetimes. Such spacetimes include as special cases analytically known black hole Kerr and Schwarzschild spacetimes, as well as the analytic Hartle-Thorne spacetime and all numerically constructed spacetimes relevant for rotating neutron stars. Our derivation follows directly from energy and angular momentum conservation and it uses the concept of the effective potential. It has never been published, except for a few special cases, but it has already become a part of the common knowledge in the field.
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| 7. |
- Abramowicz, Marek A, 1945, et al.
(författare)
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Epicyclic oscillations of fluid bodies: II. Strong gravity
- 2006
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Ingår i: Classical and Quantum Gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 23:5, s. 1689-1696
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Tidskriftsartikel (refereegranskat)abstract
- Fluids in external gravity may oscillate with frequencies characteristic of the epicyclic motions of test particles. We explicitly demonstrate that global oscillations of a slender, perfect fluid torus around a Kerr black hole admit incompressible vertical and radial epicyclic modes. Our results may be directly relevant to one of the most puzzling astrophysical phenomena-high (hundreds of hertz) frequency quasiperiodic oscillations (QPOs) detected in x-ray fluxes from several black hole sources. Such QPOs are pairs of stable frequencies in the 3/2 ratio. It seems that they originate a few gravitational radii away from the black hole and thus their observations have the potential to become an accurate probe of super-strong gravity.
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| 8. |
- Abramowicz, Marek A, 1945
(författare)
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Five Ideas on Black Hole Accretion Disks
- 2009
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Ingår i: ASP Conference Series. ; 403
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Konferensbidrag (refereegranskat)abstract
- I review five of Bohdan Paczyński's ideas on black hole accretion disk theory. They formed my understanding of the subject and often guided intuition in my research. They are fundamentally profound, rich in physical consequences, mathematically elegant and clever, and in addition are useful in several technically difficult practical applications.
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| 9. |
- Abramowicz, Marek A, 1945, et al.
(författare)
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Introduction
- 2008
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Ingår i: New Astronomy Reviews. - : Elsevier BV. - 1387-6473. ; 51
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 10. |
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