arXiv subjects

Marek A. Abramowicz

Publications and source records attributed to Marek A. Abramowicz.

At least 37 records · Page 2Linked to original sources

arXiv2009

The Paczynski-Wiita potential: A step-by-step "derivation"

Paczynski realized that a properly chosen gravitational potential may accurately model (in a "pseudo Newtonian" theory) general relativistic effects that determine motion of matter near a non-rotating black hole. Paczynski's choice, known today as the "Paczynski-Wiita potential", proved to be very practical. It was used by numerous researchers in the black hole accretion theory, and became a standard tool in relativistic astrophysics. The model is an example of Paczynski's admired ability to invent "out of nowhere" simple ideas that were brilliant, deep and useful. Paczynski has guessed intuitively the form of the potential. However, it could be also derived by a a step-by-step formal procedure. I show the derivation here. My derivation is based on a standard definition of the relativistic "effective potential" in the Schwarzschild spacetime. The relativistic effective potential may be uniquely divided into its "gravitational" and "centrifugal" part. The gravitational part differs from the Paczynski-Wiita potential only by a constant.

astro-ph.HE↗

arXiv2008

Five ideas on black hole accretion disks

I review five of Bohdan Paczynski'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.

astro-ph↗

arXiv2008

A short answer to critics of our article "Eppur si espande" [arXiv:astro-ph/0612155]

Recently [arXiv:astro-ph/0612155] 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.

astro-ph↗

arXiv2008

On the origin of clustering of frequency ratios in the atoll source 4U 1636-53

A long discussion has been devoted to the issue of clustering of the kHz QPO frequency ratios in the neutron star sources. While the distribution of ratios inferred from an occurrence of a single QPO seems to be consistent with a random walk, the distribution based on simultaneous detections of both peaks indicates a preference of ratios of small integers. Based on the public RXTE data we further investigate this issue for the source 4U 1636-53. Quality factors and rms amplitudes of both the QPOs nearly equal to the points where the frequencies are commensurable, and where the twin QPO detections cluster. We discuss a connection of the clustering with the varying properties of the two QPO modes. Assuming approximative relations for the observed correlations of the QPO properties, we attempt to reproduce the frequency and ratio distributions using a simple model of a random-walk evolution along the observed frequency-frequency correlation. We obtain results which are in qualitative agreement with the observed distributions.

astro-ph↗

arXiv2008

Distribution of kilohertz QPO frequencies and their ratios in the atoll source 4U 1636-53

A recently published study on long term evolution of the frequencies of the kilohertz quasi-periodic oscillations (QPOs) in the atoll source 4U 1636-53 concluded that there is no preferred frequency ratio in a distribution of twin QPOs that was inferred from the distribution of a single frequency alone. However, we find that the distribution of the ratio of actually observed pairs of kHz QPO frequencies is peaked close to the 3/2 value, and possibly also close to the 5/4 ratio. To resolve the apparent contradiction between the two studies, we examine in detail the frequency distributions of the lower kHz QPO and the upper kHz QPO detected in our data set. We demonstrate that for each of the two kHz QPOs (the lower or the upper), the frequency distribution in all detections of a QPO differs from the distribution of frequency of the same QPO in the subset of observations where both the kHz QPOs are detected. We conclude that detections of individual QPOs alone should not be used for calculation of the distribution of the frequency ratios.

astro-ph↗

arXiv2008

Oscillations of tori in the pseudo-Newtonian potential

Context. The high-frequency quasi-periodic oscillations (HF QPOs) in neutron star and stellar-mass black hole X-ray binaries may be the result of a resonance between the radial and vertical epicyclic oscillations in strong gravity. Aims. In this paper we investigate the resonant coupling between the epicyclic modes in a torus in a strong gravitational field. Methods. We perform numerical simulations of axisymmetric constant angular momentum tori in the pseudo-Newtonian potential. The epicyclic motion is excited by adding a constant radial velocity to the torus. Results. We verify that slender tori perform epicyclic motions at the frequencies of free particles, but the epicyclic frequencies decrease as the tori grow thicker. More importantly, and in contrast to previous numerical studies, we do not find a coupling between the radial and vertical epicyclic motions. The appearance of other modes than the radial epicyclic motion in our simulations is rather due to small numerical deviations from exact equilibrium in the initial state of our torus. Conclusions. We find that there is no pressure coupling between the two axisymmetric epicyclic modes as long as the torus is symmetric with respect to the equatorial plane. However we also find that there are other modes in the disc that may be more attractive for explaining the HF QPOs.

astro-ph↗

arXiv2007

Epicyclic Oscillations of Fluid Bodies: Newtonian Nonslender Torus

We study epicyclic oscillations of fluid tori around black holes (in the Paczynski-Wiita potential) and derive exact analytic expressions for their radial and vertical eigenfrequencies nu_r and nu_z to second-order accuracy in the width of the torus. We prove that pressure effects make the eigenfrequencies smaller than those for free particles. However, the particular ratio nu_z/nu_r=3/2, which is important for the theory of high-frequency quasi-periodic oscillations (QPOs), occurs when the fluid tori epicyclic frequencies nu_r and nu_z are about 15% higher than the ones corresponding to free particles. Our results therefore suggest that previous estimates of black hole spins from QPOs have produced values that are too high.

astro-ph↗

arXiv2007

Eppur si espande

The rather wide-spread belief that cosmological expansion of a flat 3D--space (with spatial curvature k=0) cannot be observationally distinguished from a kinematics of galaxies moving in a flat and non-expanding space is erroneous. We suggest that the error may have its source in a non relativistic intuition that imagines the Universe not as a spacetime but separates space from time and pictures the cosmological expansion as space evolving in time. The physical reality, however, is fundamentally different -- the expanding Universe is necessarily a curved spacetime. We show here that the fact that the spacetime is curved implies that the interpretation of the observed cosmological redshift as being due to the expansion of the cosmological 3D--space is observationally verifiable. Thus it is impossible to mimic the true cosmological redshift by a Doppler effect caused by motion of galaxies in a non-expanding 3D-space, flat or curved. We summarize our points in simple spacetime diagrams that illustrate a gedanken experiment distinguishing between expansion of space and pure kinematics. We also provide all relevant mathematics. None of the previously published discussions of the issue, including a recent popular Scientific American article, offered a similarly clear way out of the confusion.

astro-ph↗

arXiv2006

The twin paradox on the photon sphere

We consider a new version of the twin paradox. The twins move along the same circular free photon path around the Schwarzschild center. In this case, despite their different velocities, all twins have the same non-zero acceleration. On the circular photon path, the symmetry between the twins situations is broken not by acceleration (as it is in the case of the classic twin paradox), but by the existence of an absolute standard of rest (timelike Killing vector). The twin with the higher velocity with respect to the standard of rest is younger on reunion. This closely resembles the case of periodic motions in compact (non-trivial topology) 3-D space recently considered in the context of the twin paradox by Barrow and Levin, except that there accelerations of all twins were equal to zero, and that in the case considered here, the 3-D space has trivial topology.

astro-ph↗

arXiv2006

QPOs in microquasars: the spin problem

In the Galactic microquasars with double peak kHz quasi-periodic oscillations (QPOs), the ratio of the two frequencies is 3:2. This supports the suggestion that double peak kHz QPOs are due to a non-linear resonance between two modes of accretion disk oscillations. For the microquasars with known mass, we briefly compare the black hole spin estimates based on the orbital resonance model with the recently reported spin predictions obtained by fitting the spectral continua. Results of these two approaches are not in good agreement. We stress that if the spectral fit estimates are accurate and can be taken as referential (which is still questionable), the disagreement between the predicted and referential values would represent a rather generic problem for any relativistic QPO model, as no spin influence would appear in the observed 1/M scaling of the QPO frequencies. The epicyclic frequencies relevant in these models are often considered to be equal to those of a test particle motion. However modifications of the frequencies due to the disc pressure or other non-geodesic effects may play an important role, and the inaccuracy introduced in the spin estimates by the test particle approximation could be crucial.

astro-ph↗

arXiv2006

Wavelet analysis of MCG-6-30-15 and NGC 4051: a possible discovery of QPOs in 2:1 and 3:2 resonance

Following our previous work of Lachowicz & Czerny (2005), we explore further the application of the continuous wavelet transform to X-ray astronomical signals. Using the public archive of the XMM-Newton satellite, we analyze all available EPIC-pn observations for nearby Seyfert 1 galaxies MCG-6-30-15 and NGC 4051. We confine our analysis to 0.002-0.007 Hz frequency band in which, on the way of theoretically motivated premises, some quasi-periodic oscillations (QPOs) are expected to be found. We find that indeed wavelet power histogram analysis reveals such QPOs centered at two frequencies of ~2.5E-3 Hz and 4-6E-3 Hz, respectively. We show that these quasi-periodic features can be disentangled from the Poisson noise contamination level what is hardly to achieve with the standard Fourier analysis. Interestingly, we find some of them to be in 2:1 or 3:2 ratio. If real, our finding may be considered as a link between QPOs observed in AGN and kHz QPOs seen in X-ray binary systems.

astro-ph↗

arXiv2006

A non-linear resonance model for the black hole and neutron star QPOs: theory supported by observations

Kilohertz Quasi-Periodic Oscillations (QPOs) have been detected in many accreting X-ray binaries. It has been suggested that the highest QPO frequencies observed in the modulation of the X-ray flux reflect a non-linear resonance between two modes of accreting disk oscillation. This hypothesis implies certain very general predictions, several of which have been borne out by observations. Some of these follow from properties of non-linear oscillators, while the others are specific to oscillations of fluid in strong gravity. A 3:2 resonant ratio of frequencies can be clearly recognized in the black-hole as well as in the neutron-star QPO data.

astro-ph↗

arXiv2006

Optical geometry analysis of the electromagnetic self-force

We present an analysis of the behaviour of the electromagnetic self-force for charged particles in a conformally static spacetime, interpreting the results with the help of optical geometry. Some conditions for the vanishing of the local terms in the self-force are derived and discussed.

gr-qc↗

arXiv2006

QPOs: Einstein's gravity non-linear resonances

There is strong evidence that the observed kHz Quasi Periodic Oscillations (QPOs) in the X-ray flux of neutron star and black hole sources in LMXRBs are linked to Einstein's General Relativity. Abramowicz&Kluźniak (2001) suggested a non-linear resonance model to explain the QPOs origin: here we summarize their idea and the development of a mathematical toy-model which begins to throw light on the nature of Einstein's gravity non-linear oscillations.

astro-ph↗

arXiv2005

QPO as the Rosetta Stone for understanding black hole accretion

Quasi periodic oscillations (QPO) seen in the X-ray fluxes of individual neutron stars and black hole sources are one of most intriguing phenomena in today's astrophysics. The QPO nature is visibly determined by super-strong Einstein's gravity. I argue here that it also profoundly depends on the MRI turbulence in accretion flows. Understanding the QPO physics may therefore guide accretion theory out of its present state of confusion.

astro-ph↗

arXiv2005

Epicyclic oscillations of fluid bodies Paper II. Strong gravity

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 observations of them have the potential to become an accurate probe of super-strong gravity.

astro-ph↗

arXiv2005

QPOs in Cataclysmic Variables and in X-ray Binaries

Recent observations, reported by Warner and Woudt, of Dwarf Nova Oscillations (DNOs) exhibiting frequency drift, period doubling, and 1:2:3 harmonic structure, can be understood as disc oscillations that are excited by perturbations at the spin frequency of the white dwarf or of its equatorial layers. Similar quasi-periodic disc oscillations in black hole low-mass X-ray binary (LMXB) transients in a 2:3 frequency ratio show no evidence of frequency drift and correspond to two separate modes of disc oscillation excited by an internal resonance. Just as no effects of general relativity play a role in white dwarf DNOs, no stellar surface or magnetic field effects need be invoked to explain the black hole QPOs.

astro-ph↗

arXiv2004

Rayleigh and Solberg criteria reversal near black holes: the optical geometry explanation

The familiar Newtonian version of the Rayleigh criterion demands that for dynamical stability the specific angular momentum should increase with the increasing circumferential radius of circular trajectories of matter. However, sufficiently close to a black hole the Rayleigh criterion reverses: in stable fluids, the angular momentum must decrease with the increasing circumferential radius. The geometrical reason for this reversal is that the space is so very strongly warped that it turns inside-out.

astro-ph↗