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Outcomes of Birmingham Hip Resurfacing: A Systematic Review
Steve Karas
Asian journal of Sports Medicine , 2012,
Abstract: Purpose: We performed a systematic review to assess the functional outcomes of Birmingham Hip Resurfacing as reported in peer-reviewed literature.Methods: We performed a computerized search on the data sources up to February 2011. The following text and key words were searched: “Birmingham hip”, “Birmingham hip resurfacing” and “Hip resurfacing”. Each of these key words was again searched with “outcomes” following them. We also hand searched the bibliographies of the retrieved articles and our own files to identify specifically relevant articles.Results: Fourteen retrospective studies and three prospective studies were included for review. Each of these studies was evaluated by the criteria given by Sackett and AACPDM. The design, patient criteria, intervention, outcomes, duration of follow up and results of the research were reported.Conclusions: Although the technique of BHR does allow the femur to be spared, claims that it may allow patients to be more active need to be further investigated.
Fourier-phase analysis of the orbiting bright-spot model for AGN variability
V. Karas
Physics , 1997, DOI: 10.1093/mnras/288.1.12
Abstract: Fourier power spectra and phases of a signal from a large number of radiating sources orbiting around a black hole are investigated. It is assumed that the individual sources (bright spots) are located in an accretion disc and their lifetime exceeds the corresponding orbital period. This model is relevant for the short-time X-ray variability of active galactic nuclei. Previous works on this subject were mostly concentrated on temporal characteristics and power spectra of observed light curves. In our present contribution, Fourier phases are brought into consideration and studied systematically for a broad range of input parameters. In particular, conditions for the phase coherence are discussed. It is shown that one can discriminate between the two classes of models which are currently under consideration--orbital motion of a large number of sources versus short-lived independent flares--although parameters of the model are not completely arbitrary. It is also shown that predicted power spectra depend rather strongly on the spot distribution across the disk surface. We conclude that the orbital motion of the spots cannot be the only reason for the source fluctuations, but it certainly influences observational properties of the source intrinsic variability.
Retro-lensing light curves near a black hole
V. Karas
Physics , 2009,
Abstract: We model the light-curves from radiation-driven clouds near an accreting black hole. Taking into account the multiple images due to strong gravitational lensing, we find that sharp spikes can significantly enhance the observed flux. Following our previous work (Horak & Karas 2006a,b) we assume that scattering of ambient light takes place in a cloud that is in radial motion under a combined influence of black hole gravity and the radiation field. The retro-lensed photons give rise to peaks in the observed signal that follow with a characteristic time lag after the direct-image photons. Duration of these features is very short and the predicted polarization varies abruptly on the time-scale comparable with the light-crossing time of the system -- a signature of the photon orbit. We also consider the polarization properties of scattered light.
Strong-gravity effects acting on polarization from orbiting spots
V. Karas
Physics , 2009,
Abstract: Accretion onto black holes often proceeds via an accretion disc or a temporary disc-like pattern. Variability features, observed in the light curves of such objects, and theoretical models of accretion flows suggest that accretion discs are inhomogeneous and non-axisymmetric. Fast orbital motion of the individual clumps can modulate the observed signal. If the emission from these clumps is partially polarized, which is likely the case, then rapid polarization changes of the observed signal are expected as a result of general relativity effects.
Light curve of a source orbiting around a black hole: A fitting-formula
V. Karas
Physics , 1996, DOI: 10.1086/177904
Abstract: A simple, analytical fitting-formula for a photometric light curve of a source of light orbiting around a black hole is presented. The formula is applicable for sources on a circular orbit with radius smaller than 45 gravitational radii from the black hole. This range of radii requires gravitational focusation of light rays and the Doppler effect to be taken into account with care. The fitting-formula is therefore useful for modelling the X-ray variability of inner regions in active galactic nuclei.
Stationary electro-vacuum fields around black holes
Vladimir Karas
Physics , 2014,
Abstract: This is the second lecture of `RAGtime' series on electrodynamical effects near black holes. We will summarize the basic equations of relativistic electrodynamics in terms of spin-coefficient (Newman-Penrose) formalism. The aim of the lecture is to present important relations that hold for exact electro-vacuum solutions and to exhibit, in a pedagogical manner, some illustrative solutions and useful approximation approaches. First, we concentrate on weak electromagnetic fields and we illustrate their structure by constructing the magnetic and electric lines of force. Gravitational field of the black hole assumes axial symmetry, whereas the electromagnetic field may or may not share the same symmetry. With these solutions we can investigate the frame-dragging effects acting on electromagnetic fields near a rotating black hole. These fields develop magnetic null points and current sheets. Their structure suggests that magnetic reconnection takes place near the rotating black hole horizon. Finally, the last section will be devoted to the transition from test-field solution to exact solutions of coupled Einstein-Maxwell equations. New effects emerge within the framework of exact solutions: the expulsion of the magnetic flux out of the black-hole horizon depends on the intensity of the imposed magnetic field.
Tame automorphisms of C^3 with multidegree of the form (3,d_2,d_3)
Marek Karas
Mathematics , 2009,
Abstract: In this note we prove that the sequence (3,d_2,d_3), where d_3>= d_2>= 3, is the multidegee of some tame automorphism of C^3, if and only if 3|d_2 or d_3 is a linaer combination of 3 and d_2 with coefficients in N.
There is no tame automorphism of C^3 with multidegree (4,5,6)
Marek Karas
Mathematics , 2011,
Abstract: It is known that not each triple (d_1,d_2},d_3) of positive integers is a multidegree of a tame automorphism of C^3. In this paper we show that there is no tame automorphism of C^3 with multidegree (4,5,6). To do this we show that there is no pair of polynomials P,Q in C[x,y,z] with certain properties. These properties do not seem particularly restrictive so the non-existence result can be interesting in its own right.
Sudska praksa o policijskom postupanju: razlikovanje obavijes-nog razgovora od izjava izre？enih u po？injenju kaznenog djela, izjave privilegiranih svjedoka izvan kaznenog postupka
Karas, ？eljko
- , 2011,
Abstract: Hr？ak ID: 7922
Sudska praksa o policijskom postupanju: poticanje od strane prikrivenog istra？itelja
Karas, ？eljko
- , 2010,
Abstract: Sa？etak Analiza slu？aja i presuda Vrhovnog suda Republike Hrvatsk
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