Tomasz Bulik (OA UW and CAMK)
Ananda Deepika Bollimpalli (CAMK, Warsaw)
Neutron stars are excellent astrophysical laboratories for studying matter at very high nuclear densities. To understand the equation of state (EoS) of such dense material, it is very important to know the mass and radius of the neutron star. There is plenty of observational evidence that some neutron star systems reach super-Eddington luminosities, either through the accretion of matter onto the stellar surface or by thermonuclear burning. Stars with such high luminosities are shown to harbor levitating atmospheres, supported by the radiation pressure from the star at a certain height above the stellar surface. We study the oscillations of these radiation-supported atmospheres to find a family of relativistic eigenmodes and eigenfrequencies of the radial oscillations. The frequency of these oscillations depends on the stellar parameters and varies with the stellar luminosity/ flux. We also find that damping due to radiation drag limits the frequency of these oscillations introducing a characteristic maximum in the frequencies. The main focus of this talk will be to discuss how this maximum frequency and the frequency variation with the flux can be used to determine the mass and radius of the neutron star. In addition to the stellar parameters, observation of the variation of the oscillation with flux would allow us to estimate the stellar luminosity and therefore the distance to the source with an accuracy of a few percents.
Paweł Haensel (NCAC, Warsaw)
Rainer Spurzem (Frankfurt Institute for Advanced Study)
Black Holes are difficult to detect directly; indirect evidence for them comes e.g. from stellar motions or from tidal disruption events; stars getting on their orbit too close to the black hole are destroyed due to strong tidal forces, and the debris forms a transient disk with electromagnetic signature. Another interesting topic is the possible presence of a central gaseous disk in active galactic nuclei (AGN); stars interact with such disks and tidal disruption processes are dynamically distinct from the standard case. Tidal disruption events also may provide clues to the dynamical state of galaxies, for example whether there has been a recent merger. We study with computer simulations based on direct N-body simulations with Post-Newtonian general relativity (if needed) how nuclear star clusters, surrounding supermassive black holes, evolve, and look for evidence of galaxy mergers and central disks in the dynamical characteristics of tidal disruption events. We will also briefly touch the issue of gravitational wave emission in case of compact objects, which are too compact to be tidally disrupted, but will be just accreted to the central black hole under emission of gravitational waves. The computer simulations are quite challenging and can only be done due to the use of high performance GPU accelerated supercomputers. Some fresh benchmarks using the newest Volta V100 GPU hardware are shown.
