Maciek Wielgus

Radiative, two-temperature simulations of low luminosity black hole accretion flows in general relativity

We present a numerical method which evolves a two-temperature, magnetized, radiative, accretion flow around a black hole, within the framework of general relativistic radiation magnetohydrodynamics. As implemented in the code KORAL, the gas consists of two sub-components -- ions and electrons -- which share the same dynamics but experience independent, relativistically consistent, thermodynamical evolution. The electrons and ions are heated independently according to a standard prescription from the literature for magnetohydrodynamical turbulent dissipation. Energy exchange between the particle species via Coulomb collisions is included. In addition, electrons gain and lose energy and momentum by absorbing and emitting synchrotron and bremsstrahlung radiation, and through Compton scattering. All evolution equations are handled within a fully covariant framework in the relativistic fixed-metric spacetime of the black hole. Numerical results are presented for five models of low luminosity black hole accretion. In the case of a model with a mass accretion rate

Stability of radiation-pressure dominated disks - I. The dispersion relation for a delayed heating α-viscosity prescription

\dot{M}\sim10^{-8} \dot M_{\rm Edd}

Continuous phase estimation from noisy fringe patterns based on the implicit smoothing splines.

, we find that radiation has a negligible effect on either the dynamics or the thermodynamics of the accreting gas. In contrast, a model with a larger

Levitating atmospheres of Eddington-luminosity neutron stars

\dot{M}\sim 4\times 10^{-4} \dot M_{\rm Edd}

Stable, levitating, optically thin atmospheres of Eddington-luminosity neutron stars

behaves very differently. The accreting gas is much cooler and the flow is geometrically less thick, though it is not quite a thin accretion disk.

The perihelion of Mercury advance and the light bending calculated in (enhanced) Newton's theory

We derive and investigate the dispersion relation for accretion disks with retarded or advanced heating. We follow the α-prescription but allow for a time offset τ between heating and pressure perturbations, as well as for a diminished response of heating to pressure variations. We study in detail solutions of the dispersion relation for disks with radiation-pressure fraction, 1 − β ,a ndξ, the ratio of viscous stress response to pressure perturbations. For τ 2 τth ,f orβ = 0a ndξ = 1) two real solutions exist, which are both negative. These results imply that radiation-pressure dominated accretion disks may be stabilized when there is a time delay between stress fluctuations and fluctuations in heating.

Evaluation of optical parameters of quasi-parallel plates with single-frame interferogram analysis methods and eliminating the influence of camera parasitic fringes

We introduce the algorithm for the direct phase estimation from the single noisy interferometric pattern. The method, named implicit smoothing spline (ISS), can be regarded as a formal generalization of the smoothing spline interpolation for the case when the interpolated data is given implicitly. We derive the necessary equations, discuss the properties of the method and address its application for the direct estimation of the continuous phase in both classical interferometry and digital speckle pattern interferometry (DSPI). The numerical illustrations of the algorithm performance are provided to corroborate the high quality of the results.

Evaluation of the implicit smoothing splines algorithm for the interferometric fringe pattern phase retrieval

We construct models of static, spherically symmetric shells supported by the radiation flux of a luminous neutron star in the Schwarzschild metric. The atmospheres are disconnected from the star and levitate above its surface. Gas pressure and density inversion appear in the inner region of these atmospheres, which is a purely relativistic phenomenon. We account for the scattering opacity dependence on temperature and utilize the relativistic M1 closure scheme for the radiation tensor, hence allowing for a fully GR-consistent treatment of the photon flux and radiation tensor anisotropy. In this way we are able to address atmospheres of both large and moderate/low optical depths with the same set of equations. We discuss properties of the levitating atmospheres and find that they may indeed be optically thick, with the distance between star surface and the photosphere expanding as luminosity increases. These results may be relevant for the photosphereric radius expansion X-ray bursts.

Nanocrystallic thin films statistical structural analysis by the automatic image processing

In general relativity static gaseous atmospheres may be in hydrostatic balance in the absence of a supporting stellar surface, provided that the luminosity is close to the Eddington value. We construct analytic models of optically thin, spherically symmetric shells supported by the radiation pressure of a luminous central body in the Schwarzschild metric. Opacity is assumed to be dominated by Thomson scattering. The inner parts of the atmospheres, where the luminosity locally has supercritical values, are characterized by a density and pressure inversion. The atmospheres are convectively and Rayleigh-Taylor stable, and there is no outflow of gas.

Two-frame tilt-shift error estimation and phase demodulation algorithm

We show that results of a simple dynamical gedanken experiment interpreted according to standard Newton’s gravitational theory, may reveal that three-dimensional space is curved. The experiment may be used to reconstruct the curved geometry of space, i.e. its non-Euclidean metric