Piotr Wielgórski (CAMK PAN, Warsaw)
Radially pulsating stars, like Cepheids (Classical, Type II) and RR Lyrae are widely used as precise distance indicators thanks to their period-luminosity relations which serve as a standard candle. However, there is another, semi-geometrical method which can be used to derive distances of such stars, known as the Baade-Wesselink or parallax-of-pulsation technique. In this method, change of the angular diameter of the star, which is calculated from photometric or interferometric measurements, is compared to the physical radius displacement obtained from integrating the pulsational velocity curve (velocity of the atmosphere of the star during pulsations). Pulsational velocity can be obtained from radial velocity measured from doppler shift of absorption lines in spectra once the so-called projection factor (p-factor) is known. The value of this parameter is primarily the result of the geometrical projection of the pulsational velocity of different parts of the stellar disk onto the line of sight, but factors like limb darkening, velocity gradient in the atmosphere etc. modify the value of p-factor. Precision of the Baade-Wesselink method is currently limited to ~5-10% due to poor knowledge of this parameter. During my talk I will present the efforts of the Araucaria group in calibrating the projection factor.
Oliver Porth (University of Amsterdam)
The time-variable (in particular pulsed-) emission from black holes and neutron stars holds key information on the nature of curved spacetime and ultra dense nuclear matter. However, due to the complex plasma dynamics at play, the interpretation of the observed signals is difficult. I will present some of our recent work on modeling general relativistic magnetohydrodynamic processes around black holes and accreting neutron stars. I will first discuss the dynamics of strongly perturbed black hole magnetospheres as they can occur for example during binary neutron star merger events. The simulations have revealed an intriguing rapid alignment of the magnetic moment and black hole spin which can uniquely imprint the high energy emission of the current sheet. I will further discuss the dynamics of multipolar black hole magnetospheres (Selvi et al. in prep). Switching gears, I will then discuss our recent 3D simulations of accreting neutron stars with oblique magnetospheres, focusing on the questions of jet formation and pulsed emission. The obtained X-ray pulse profiles are consistent in amplitude and harmonic content with the observations of AMXPs, however they also exhibit significant and rapid variability in hotspot temperature and shape due to the turbulent accretion flow.
Arkadiusz Orłowski (Department of Artificial Intelligence, Institute of Information Technology, SGGW, Warsaw)
