Seminars archive

Role of emission angular directionality in spin determination of accreting black holes with a broad iron line

Jiri Svoboda

Spin of an accreting black hole can be determined by spectroscopy of the emission and absorption features produced in the inner regions of an accretion disc. In this talk we will discuss the method employing the relativistic line profiles of iron in the X-ray domain, where the emergent spectrum is blurred by general relativistic effects. We study how sensitive the spin determination is to the assumptions about the intrinsic angular distribution of the emitted photons. The uncertainty of the directional emission distribution translates to 20% uncertainty in determination of the marginally stable orbit.

The Pea and The Sun

Lubos Pick

The talk will be dedicated to the famous (or perhaps rather infamous) 1924 result of S. Banach and A. Tarski which ignited at its time a lot of controversy not only among mathematicians. This topic, whose nature puts it somewhere near the crossroads of the set theory and the measure theory, and which was later on called the Banach-Tarski Paradox, is a beautiful and amusing illustration of certain specific features of mathematics that no other science can share. The talk will be accessible to anybody, no mathematical knowledge is required.

Theory of relativity in our lives and in the stars

David Kofron

(Lecture for general public) At the beginning we touch upon the relation between physics and philosophy - or, rather, just the relation between theory and experiment. A short historical discourse about the crucial changes in the paradigms of physics follows. We mention three most influential figures of physics: Newton, Maxwell and Einstein. Then we analyze what motivated scientists (not only Einstein) to invent theories of special and general relativity. The use of these abstract theories even in our lives! is shown by examples. The second part of the lecture is devoted to astrophysical laboratories of general relativity found in the distant universe, the double pulsar PSR J0737-3039A/B being the exceptional one.

Black holes yesterday, today and tomorrow

Michal Bursa

Summary of the conference "Probing strong gravity near black holes", 15-18 February 2010

Broad emission lines for a negatively spinning black hole

Thomas Dauser

We present an extended scheme for the calculation of the profiles of emission lines from accretion discs around rotating black holes. The scheme includes discs with angular momenta which are parallel and antiparallel with respect to the black hole's angular momentum, as both configurations are assumed to be stable (King et al., 2005). We discuss line shapes for such discs and present a code for modelling observational data with this scheme in X-ray data analysis programs. Based on a Green's function approach, an arbitrary radius dependence of the disc emissivity and arbitrary limb darkening laws can be easily taken into account, while the amount of precomputed data is significantly reduced with respect to other available models.

SPH and friends: the future of numerical star formation simulations

David Hubber

Smoothed Particle Hydrodynamics (SPH) and Adaptive Mesh Refinement (AMR) codes have been used for the past 20+ years to investigate various astrophysical problems such as star formation and cosmological simulations. While often championed for their strengths, both methods have noticable weaknesses that can undermine their achievements. We discuss recent attempts in the literature to improve our understanding of how, why and when SPH and AMR agree and disagree, and how this knowledge can be used to inform us better on when to trust numerical simulations and how to improve them in the future. We explore some of the present and future algorithmic developments of these two methods, and what kinds of problems we will be able to attack in the next decade, as well as other methods that may become powerful tools for numerical astrophysics.

Dynamical evolution of globular star clusters in the galactic tidal field

Zeinab Khorrami

In this seminar the results of a large set of N-body simulation will be reported. These results include the evolution of globular clusters with both single-mass and multi-mass stellar populations in the tidal field of the galaxy. The main focus is to study the dissolution time of clusters to obtain the effect of different initial properties of the cluster, such as its total mass (Mtot), half-mass radius (Rh), and galactocentric distance (RG). The evolution of the stellar mass function will also be discussed.

A multi-wavelength approach to the study of extragalactic jets: modeling their spectral energy distribution

Giulia Migliori

The study of extragalactic jets by modeling their spectral energy distribution (SED) has become a powerful approach to investigate jet physics, due to the unprecedented wealth of multi-wavelength data. The broadband fitting allows to infer the main physical parameters of the emitting plasma and estimate the kinetic power carried by the jet. In the talk, I will first introduce a leptonic synchrotron and inverse Compton model, generalized by taking into account the main local and external radiative fields which can act as seed photons. The model is then applied to investigate two astrophysical cases: 1- the origin the X-ray emission in young and compact radio galaxies; 2- the study of the jet structure in misaligned radio sources detected in the gamma-ray band by Fermi-LAT, focusing in particular on the results obtained for the FRI radio galaxy NGC6251.

High angular resolution radio interferometry observations of Galactic Centre

Devaky Kunneriath

High angular resolution radio interferometry observations at high frequencies enable us to study the emission mechanisms of the central region of our Galactic Centre, which harbours the supermassive black hole source Sgr A*, in great detail. This talk will focus on recent results from GC observations with radio interferometer arrays such as CARMA and ATCA. I will present results from the flare modelling of Sgr A* based on our global coordinated multiwavelength observing campaigns in 2007 and 2008, and high resolution maps along with a spectral index analysis of the extended emission of the central minispiral region at multiple wavelengths.

Clustered star formation

Thomas Maschberger

Typically, during a star formation event several stars are formed, from a few dozens to millions, perhaps in star clusters, which has extensive ramifications. On a galaxy-wide scale, the star cluster population traces the star formation history of its host. I will discuss this with the example of the Large Magellanic Cloud, where a detailed star formation history from Colour-Magnitude-Diagrams has been derived and the parameters of the star cluster population have been determined. The number of bound star clusters that are formed follow the total amount of star formation, but contribute only 10-20%. This implies that within a star forming region either only a fraction of stars form in a bound star cluster, or that the transition from the embedded to the gas-free phase destroys star clusters. Hydrodynamical modelling offers insight on these early stages of a star forming event, such as evolution of substructure, (primordial) mass segregation and the build-up of the stellar mass function, and their implications for the subsequent evolution. I will report results on these topics from analysing the simulations of Bonnell et al., and discuss the statistical problems that emerge because of the amound of substructure and the small-number statistics.

Radiative transfer in the axial symmetry

Daniela Korcakova

The method for the solution of the radiative transfer equation in moving media in the assumption of the axial symmetry will be introduced. The results for the stellar wind and rapidly rotating stars will be presented. The last part of the talk will be focused mostly on accretion discs of cataclysmic variables, namely, SS Cygni and AM CVn systems.