Lecture Day: Annual reports from the Center for Theoretical Astrophysics 2008
Thermal disc emission from a rotating black hole: X-ray polarization signatures
Thermal emission from the accretion disc around a black hole can be polarized, due to Thomson scattering in a disc atmosphere. General Relativity effects strongly modify the polarization properties of the thermal radiation as observed at infinity. Among these effects, the rotation of the polarization angle with energy is particularly useful as a diagnostic tool. We assume the two cases of a Schwarzschild and an extreme Kerr black hole with a standard thin disc and a scattering atmosphere. We compute the expected polarization degree and the angle as functions of the energy as they could be measured for different inclinations of the observer, optical thickness of the atmosphere and different values of the black hole spin. We assume the thermal emission dominates the X-ray band. Using the flux level of the microquasar GRS 1915+105 in the thermal state, we calculate the observed polarization.
Relations between high-frequency QPOs in neutron stars
We review properties of high-frequency quasi-periodic oscillations (QPOs) in neutron star sources. In particular, we report on the recent study of occurrences and mutual relation of twin-peak QPOs in the source 4U 1636. We have found several prominent points in the frequency-frequency correlation where the two QPO peaks exchange their dominance (in terms of rms amplitudes and quality factors). Both QPOs are detected mainly in the vicinity of these points. As the two QPO frequencies are nearly commensurable there, these transitions likely correspond to internal nonlinear resonances between the two QPO modes. We confirm this idea in a simple toy-model consisting of two harmonic oscillator coupled by a general nonlinear force.
Relating QPOs and EOS
Constraints on neutron star properties following from the modelling of quasiperiodic oscillations (QPOs) have been often discussed but not much systematically treated. We consider a specific, relativistic precession, QPO model and the data of atoll source 4U 1636-53. We show that, in difference to the commonly accepted view, the model implies a mass-angular-momentum relation rather than a single combination of neutron star mass and angular momentum. We discuss similar relations for two other models. We confront these relations with predictions of miscellaneous neutron star equations of state (EOS).
Equation of state of dense matter and rotating neutron stars
We present some properties of slowly rotating neutron stars and how these properties are affected by equation of state of dense matter.
Analysis of white dwarfs in post-common envelope binaries
Analysis of FUSE spectra of white dwarfs in post-common envelope binaries will be presented. By using available models, the temperature, surface gravity and chemical composition have been determined. Also, the observed photospheric lines have been used to trace the orbital path of the white dwarf, and hence place constraints on the binary parameters.
ModelovÃ¡nÃ power spekter zÃ¡Å™enÃ horkÃ½ch skvrn na povrchu neutronovÃ½ch hvÄ›zd a v tenkÃ½ch akrečnÃch discÃch
Toky rentgenovÃ©ho zÃ¡Å™enÃ horkÃ½ch skvrn na povrchu neutronovÃ½ch hvÄ›zd a v jejich akrečnÃch discÃch jsou vÃ½znamnÄ› ovlivnÄ›ny gravitačnÃm lensingem v silnÃ©m relativistickÃ©m poli tÄ›chto kompaktnÃch objektÅ¯. SvÄ›telnÃ© kÅ™ivky i power spektra tohoto zÃ¡Å™enÃ nesou tedy vÃ½raznÃ½ podpis geometrie systÃ©mu zdrojâ€“pozorovatel i parametrÅ¯ centrÃ¡lnÃho objektu. Modulace epicyklickÃ½mi frekvencemi orbitalnÃho pohybu v akrečnÃch discÃch je takÃ© často uvaÅ¾ovÃ¡na jako jeden z moÅ¾nÃ½ch mechanismÅ¯ vzniku QPOs â€“ kvaziperiodickÃ½ch oscilacÃ pozorovanÃ½ch v power spektrech binÃ¡rnÃch systÃ©mÅ¯ s neutronovou hvÄ›zdou či černou dÃrou. Vyvinuli jsme softwarovÃ½ plnÄ› relativistickÃ½ simulÃ¡tor modelujÃcÃ svÄ›telnÃ© kÅ™ivky a power spektra zÃ¡Å™enÃ horkÃ½ch skvrn jak na povrchu neutronovÃ½ch hvÄ›zd, tak i v akrečnÃch discÃch včetnÄ› modulace epicyklickÃ½mi frekvencemi.
Off-equatorial orbits in strong gravitational fields
Near a black hole or an ultracompact star, the motion of particles is governed by a strong gravitational field. Electrically charged particles also feel the electromagnetic force arising due to currents inside the star or plasma circling around. We study the possibility that the interplay between gravitational and electromagnetic actions may allow for the stable, energetically bound off-equatorial motion of charged particles. This would represent the well-known generalized StÃ¶rmer's 'halo orbits', which have been discussed in connection with the motion of dust grains in planetary magnetospheres. We demonstrate that such orbits exist and can be astrophysically relevant when a compact star or a black hole is endowed with a dipole-type magnetic field. In the case of the Kerr Newman solution, numerical analysis shows that the mutually connected gravitational and electromagnetic fields do not allow the existence of stable halo orbits above the outer horizon of black holes. Such orbits are either hidden under the inner black-hole horizon, or they require the presence of a naked singularity.
Integral-field spectroscopy of narrow-line regions in Seyfert galaxies
Our spatially-resolved spectroscopic study of narrow-line emission regions in a sample of 16 nearby Seyfert galaxies reveals strong departures from circular rotation, due to non-axisymmetric gravitational potentials and/or outflows of matter. We also study the gas density and ionization structure, and the composition of underlying stellar populations.
Ram pressure stripping of galaxies in clusters can yield gas deficient disks. Previous numerical simulations based on various approaches suggested that except for near edge-on disk orientations, the amount of stripping depends very little on the inclination angle. Following our previous numerical and analytical study we extend the set of parameters with the disk tilt angle and study the stripping of
the interstellar content (ISM) of galaxies in various environments of clusters with compact or large distributions of the intra-cluster medium (ICM). A grid of numerical simulations with varying parameters is performed using tree/SPH code GADGET with a modified method for calculating the ISM-ICM interaction. The simulations confirm the general trend of less stripping at orientations close to edge-on case. Although various hydrodynamical effects are present in the ISM-ICM interaction, the main quantitative stripping results appear to be roughly consistent with a simple scenario of the momentum transfer from the encountered ICM. We propose a fitting formula to reproduce the numerical results. The dependence on the disk tilt angle and the ICM column density is superior to that on the peak ram pressure. Similar behaviour can also be found in previous simulations.
ObecnÃ¡ relativita a jejÃ NewtonovskÃ¡ limita â€“ prvnÃ ilustrace na prostoročasech s pohybujÃcÃmi se zdroji
NewtonovskÃ¡ limita obecnÄ› relativistickÃ½ch Å™eÅ¡enÃ je vÃ½znamnÃ¡ jak z obecnÃ©ho hlediska srovnÃ¡nÃ rÅ¯znÃ½ch teoriÃ tak zvlÃ¡Å¡tÄ› pro pochopenÃ fyzikÃ¡lnÃch vlastnostÃ často matematicky velmi komplikovanÃ½ch Å™eÅ¡enÃ rovnic pro gravitačnÃ pole v obecnÃ© relativitÄ›. NastÃnÃme, jak Newtonovskou limitu provÃ©st pro gravitačnÃ pole rovnomÄ›rnÄ› urychlenÃ½ch čÃ¡stic či černÃ½ch dÄ›r a poukÃ¡Å¾eme na nÄ›kterÃ© podobnosti s Newtonovskou limitou "klasickÃ©ho" Schwarzschildova Å™eÅ¡enÃ. ZÃ¡vÄ›rem zmÃnÃme nÄ›kterÃ© moÅ¾nosti zobecnÄ›nÃ, napÅ™Ãklad pÅ™idÃ¡nÃ elektrickÃ©ho nÃ¡boje a odpovÃdajÃcÃho elektromagnetickÃ©ho pole.
ICD â€“ ultrafast decay pathway of ionized clusters
Core ionization of atoms and molecules produces ions in their highly excited states, lying well above the second ionization threshold. Such states can decay by electron emission. This fundamental process is known as Auger effect. Contrary to the core ionization, ionization of electrons from the inner-valence subshell produces much less excited species making the slow photon emission the only possible decay mode as long as the ion is isolated. The situation differs radically in clusters. Due to the possibility to distribute the positive charges over two or more cluster subunits the double ionization thresholds of the clusters lie much lower than those of the isolated atoms or molecules. This brings about an interatomic (or intermolecular) decay process in which the excess energy of the ion with a hole in the inner-valence subshell is utilized to ionize a neutral neighbour. The two positive charges distributed over the cluster usually lead to the explosion (break-up) of the cluster into two singly-charged ions. The decay process occurs via the Coulomb interaction between the electrons of the two cluster subunits and is thus called an inter-atomic Coulombic decay (ICD).