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Possible Suppression of Magnetorotational Instability by Rapid Radial Flow

100%

Abramowicz M. A. , Horák J. , Kluźniak W.

Acta Astronomica

2013

tom Vol. 63, No. 2

267--273

By investigating the magnetorotational instability (MRI) in a region of rapidly varying accretion-disk thickness, e.g., in the ISCO region, we find a limit to the applicability of a recent finding by Balbus (2012) that the MRI is present also in the region of Rayleigh-unstable flow.

Binary Coalescence of a Strange Star with a Black Hole: Newtonian Results

100%

Lee W. H. , Kluźniak W. , Nix J.

Acta Astronomica

2001

tom Vol. 51, No. 4

331--346

We present Newtonian three-dimensional hydrodynamical simulations of the merger of quark stars with black holes. The initial conditions correspond to non-spinning stars in Keplerian orbits, the code includes gravitational radiation reaction in the quadrupole approximation for point masses. We find that the quark star is disrupted, forming transient accretion structures around the black hole, but 0.03 of the original stellar mass survives the initial encounter and remains in an elongated orbit as a rapidly rotating quark starlet, in all cases. No resolvable amount of mass is dynamically ejected during the encounters - the black hole eventually accretes 99.99%±0.01% of the quark matter initially present.

Modulation of the Neutron Star Boundary Layer Luminosity by Disk Oscillations

100%

Abramowicz M. A. , Horák J. , Kluźniak W.

tom Vol. 57, No. 1

1--10

Paczyński (1987) pointed out that any modulation of the rate at which mass is accreted by a neutron star from the innermost part of a relativistic accretion disk will lead to a modulation of the luminosity of the boundary layer. Following this reasoning, we demonstrate that variability of the boundary layer X-ray flux must necessarily show frequencies of certain global accretion disk oscillations. This theoretical paradigm - clock in the disk, modulation at the boundary layer - resolves one puzzling paradox in the neutron star quasi periodic oscillation (QPO) data.

Radiative Corrections to the Neutron Star Mass Inferred from QPO Frequencies

100%

Abramowicz M. A. , Kluźniak W. , Yu W.

Acta Astronomica

2011

tom Vol. 61, No. 4

319--324

The frequencies of kHz QPOs are widely interpreted as being indicative of the values of characteristic frequencies related to orbital motion around neutron stars, e.g., the radial epicyclic frequency. In regions directly exposed to the radiation from the luminous neutron star these frequencies change with the luminosity. Including radiative corrections will change the neutron star mass value inferred from the QPO frequencies. Radiative forces may also be behind the puzzling phenomenon of parallel tracks.

Distribution of Kilohertz QPO Frequencies and Their Ratios in the Atoll Source 4U 1636-53

63%

Török G. , Abramowicz M. A. , Bakala P. , Bursa M. , Horák J. , Kluźniak W. , Rebusco P. , Stuchlík Z.

2008

tom Vol. 58, No. 1

15--21

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.