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Marek A. Abramowicz

Publications and source records attributed to Marek A. Abramowicz.

At least 55 records · Page 3Linked to original sources
arXiv2004

Super-Eddington black hole accretion: Polish doughnuts and slim disks

The theory of highly super-Eddington black hole accretion was developed in the 1980s in Warsaw by Paczynski and his collaborators in terms of "Polish doughnuts", i.e. low viscosity, rotating accretion flows that are optically thick, radiation pressure supported, cooled by advection, and radiatively very inefficient. Polish doughnuts resemble fat tori with two narrow funnels along rotation axis. The funnels collimate radiation into beams with highly super-Eddington luminosities. "Slim disks" introduced later by Abramowicz, Lasota and collaborators, have only moderately super-Eddington accretion rates, rather disk-like shapes, and almost thermal spectra.

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arXiv2004

Resonance in Forced Oscillations of an Accretion Disk and Kilohertz Quasi Periodic Oscillations

We have performed numerical simulations of a radially perturbed "accretion" torus around a black hole or neutron star and find that the torus performs radial and vertical motions at the appropriate epicyclic frequencies. We find clear evidence that vertical motions are excited in a non-linear resonance when the applied perturbation is periodic in time. The strongest resonant response occurs when the frequency difference of the two oscillations is equal to one-half the forcing frequency, precisely as recently observed in the accreting pulsar, SAX J1808.4-3658, where the observed kHz QPO peak separation is half the spin frequency of 401 Hz.

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arXiv2004

High-frequency QPOs as a problem in physics: non-linear resonance

The presence of a kHz frequency in LMXBs has been expected from scaling laws, by analogy with the QPO phenomenon in HMXB X-ray pulsars. Interpretation of the two kHz frequencies, observed in accreting neutron stars, in terms of non-linear resonance in strong-field gravity led to the prediction of twin QPOs in black hole systems, in a definite frequency ratio (such as 2/3). The imprint of a subharmonic of the 401 Hz rotation rate in the frequencies of the QPOs detected in the accreting millisecond pulsar is at once a signature of non-linear resonance and of coupling between accretion disk modes and the neutron star spin.

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arXiv2004

The importance of discovering a 3:2 twin-peak QPO in a ULX or how to solve the puzzle of intermediate mass black holes

Recently, twin-peak 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 relativisitic 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 ~1000 solar mass) radiating very near the Eddington limit and (2) a conventional black hole (M ~ 10 solar mass) accreting at a highly super-Eddington rate with its emission beamed along the rotation axis. We suggest that it may be possible to distinguish 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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arXiv2004

The orbital resonance model for twin peak kHz QPOs

Many Galactic black hole and neutron star sources in low X-ray mass binaries show QPOs (quasi periodic oscillations) in their observed X-ray fluxes, i.e. peaks in the Fourier variability power spectra. Pairs of twin peaks are observed, and in black-hole systems their frequencies (upp), (down) are in rational ratios. For example, in all four microquasars with twin peaks observed, (upp):(down) = 3:2. The rational ratios have been postulated in a model that explained twin peak QPOs as a non-linear resonance between modes of accretion disk oscillations. For those microquasars where the mass of the X-ray source is known, we determine the black-hole spin, following from the observed QPO frequencies within various models of resonance.

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arXiv2003

Interpreting black hole QPOs

In all the microquasars with two hHz QPOs, the ratio of the frequencies is 3:2, supporting our suggestion that a non-linear resonance between two modes of oscillation in the accretion disk plays a role in exciting the observed modulations of the X-ray flux. We discuss the evidence in favor of this interpretation, and we relate the black hole spin to the frequencies expected for various types of resonances that may occur in nearly Keplerian disks in strong gravity. For those microquasars where the mass of the central X-ray source is known, the black hole spin can be deduced from a comparison of the observed and expected frequencies.

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arXiv2003

Magnetically Arrested Disk: An Energetically Efficient Accretion Flow

We consider an accretion flow model originally proposed by Bisnovatyi-Kogan & Ruzmaikin (1974), which has been confirmed in recent 3D MHD simulations. In the model, the accreting gas drags in a strong poloidal magnetic field to the center such that the accumulated field disrupts the axisymmetric accretion flow at a relatively large radius. Inside the disruption radius, the gas accretes as discrete blobs or streams with a velocity much less than the free-fall velocity. Almost the entire rest mass energy of the gas is released as heat, radiation and mechanical/magnetic energy. Even for a non-rotating black hole, the efficiency of converting mass to energy is of order 50% or higher. The model is thus a practical analog of an idealized engine proposed by Geroch and Bekenstein.

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arXiv2003

Non-linear resonance in the accretion disk of a millisecond pulsar

Twin quasi-periodic millisecond modulations of the X-ray flux (kHz QPOs) have recently been reported from an accreting 2.5 ms X-ray pulsar. We identify modes of disk oscillations whose frequencies are in agreement with the observed ones when the rotating neutron star is modeled with realistic equations of state. The frequency difference of the twin QPOs, equal to about one half of the neutron-star spin rate, clearly indicates that resonant oscillations of the accretion disk have been observed. Similar non-linear resonances may also be spontaneously excited in the accretion disk. The two QPO frequencies in the pulsar system are close to a 5:7 ratio and this suggests a link with the QPOs in black hole systems, where frequency ratios of 2:3 and 3:5 have been reported.

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arXiv2003

Three-dimensional MHD Simulations of Radiatively Inefficient Accretion Flows

We present three-dimensional MHD simulations of rotating radiatively inefficient accretion flows onto black holes. In the simulations, we continuously inject magnetized matter into the computational domain near the outer boundary, and we run the calculations long enough for the resulting accretion flow to reach a quasi-steady state. We have studied two limiting cases for the geometry of the injected magnetic field: pure toroidal field and pure poloidal field. In the case of toroidal field injection, the accreting matter forms a nearly axisymmetric, geometrically-thick, turbulent accretion disk. The disk resembles in many respects the convection-dominated accretion flows found in previous numerical and analytical investigations of viscous hydrodynamic flows. Models with poloidal field injection evolve through two distinct phases. In an initial transient phase, the flow forms a relatively flattened, quasi-Keplerian disk with a hot corona and a bipolar outflow. However, when the flow later achieves steady state, it changes in character completely. The magnetized accreting gas becomes two-phase, with most of the volume being dominated by a strong dipolar magnetic field from which a thermal low-density wind flows out. Accretion occurs mainly via narrow slowly-rotating radial streams which `diffuse' through the magnetic field with the help of magnetic reconnection events.

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arXiv2002

No observational proof of the black-hole event-horizon

Recently, several ways of obtaining observational proof of the existence of black-hole horizons have been proposed. We argue here that such proof is fundamentally impossible: observations can provide arguments, sometimes very strong ones, in favour of the existence of the event horizon, but they cannot prove it. This applies also to future observations, which will trace very accurately the details of the spacetime metric of a body suspected of being a black hole.

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arXiv2002

Epicyclic orbital oscillations in Newton's and Einstein's dynamics

We apply Feynman's principle, ``The same equations have the same solutions'', to Kepler's problem and show that Newton's dynamics in a properly curved 3-D space is identical with that described by Einstein's theory in the 3-D optical geometry of Schwarzschild's spacetime. For this reason, rather unexpectedly, Newton's formulae for Kepler's problem, in the case of nearly circular motion in a static, spherically spherical gravitational potential accurately describe strong field general relativistic effects, in particular vanishing of the radial epicyclic frequency at the marginally stable orbit.

gr-qc↗
arXiv2002

The Magnetohydrodynamics of Convection-Dominated Accretion Flows

Radiatively inefficient accretion flows onto black holes are unstable due to both an outwardly decreasing entropy (`convection') and an outwardly decreasing rotation rate (the `magnetorotational instability'; MRI). Using a linear magnetohydrodynamic stability analysis, we show that long-wavelength modes are primarily destabilized by the entropy gradient and that such `convective' modes transport angular momentum inwards. Moreover, the stability criteria for the convective modes are the standard Hoiland criteria of hydrodynamics. By contrast, shorter wavelength modes are primarily destabilized by magnetic tension and differential rotation. These `MRI' modes transport angular momentum outwards. The convection-dominated accretion flow (CDAF) model, which has been proposed for radiatively inefficient accretion onto a black hole, posits that inward angular momentum transport and outward energy transport by long-wavelength convective fluctuations are crucial for determining the structure of the accretion flow. Our analysis suggests that the CDAF model is applicable to a magnetohydrodynamic accretion flow provided the magnetic field saturates at a sufficiently sub-equipartition value (plasma beta >> 1), so that long-wavelength convective fluctuations can fit inside the accretion disk. Numerical magnetohydrodynamic simulations are required to determine whether such a sub-equipartition field is in fact obtained.

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arXiv2001

General Relativistic Rossby-Haurwitz waves of a slowly and differentially rotating fluid shell

We show that, at first order in the angular velocity, the general relativistic description of Rossby-Haurwitz waves (the analogues of r-waves on a thin shell) can be obtained from the corresponding Newtonian one after a coordinate transformation. As an application, we show that the results recently obtained by Rezzolla and Yoshida (2001) in the analysis of Newtonian Rossby-Haurwitz waves of a slowly and differentially rotating, fluid shell apply also in General Relativity, at first order in the angular velocity.

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arXiv2001

How Dim Accreting Black Holes Could Be?

Recent hydrodynamical simulations of radiatively inefficient black hole accretion flows with low viscosity have demonstrated that these flows differ significantly from those described by an advection-dominated model. The black hole flows are advection-dominated only in their inner parts, but convectively dominated at radii R>100R_g. In such flows, the radiative output comes mostly from the convection part, and the radiative efficiency is independent of accretion rate and equals ~0.001. This value gives a limit for how dim an accreting black hole could be. It agrees with recent Chandra observations which indicate that accreting black holes in low-mass X-ray binaries are by factor about 100 dimmer that neutron stars accreting with the same accretion rates.

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arXiv2001

Convection in radiatively inefficient black hole accretion flows

Recent numerical simulations of radiatively inefficient accretion flows onto compact objects have shown that convection is a general feature in such flows. Dissipation of rotational and gravitational energies in the accretion flows results in inward increase of entropy and development of efficient convective motions. Convection-dominated accretion flows (CDAFs) have a structure that is modified significantly in comparison with the canonical advection-dominated and Bondi-like accretion flows. The flows are characterized by the flattened radial density profiles, ~R^{-1/2}, and have reduced mass accretion rates. Convection transports outward a significant amount of the released binding energy of the accretion flow. We discuss basic dynamical and observational properties of ADAFs using numerical models and self-similar analytical solutions.

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arXiv2000

Variability Model for the Microquasar GRS 1915+105

We analyse the radiation pressure instability of the standard, optically thisc disc and propose it as an explanation of the obseved variability of the superluminal microquasar GRS 1915+105. The quantitave agreement with the observed outburst amplitudes is obtained under the assumption that substantial part of the energy is carried away by the jet.

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arXiv2000

Accretion Disks Phase Transitions: 2-D or not 2-D?

We argue that the proper way to treat thin-thick accretion-disk transitions should take into account the 2-D nature of the problem. We illustrate the physical inconsistency of the 1-D vertically integrated approach by discussing a particular example of the convective transport of energy.

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