Seminars
Our seminars take place in the lecture room of the building at
Praha–Sporilov.
Next seminar:
11.05.2015 15:00
Pavel Kroupa
Helmholtz-Institut für Strahlen- und Kernphysik, Bonn
Testing the standard cold dark matter cosmological model
Abstract
Pavel Kroupa
Testing the standard cold dark matter cosmological model
The dual-dwarf-galaxy theorem, according to which two types of galaxies must exist and which must be true in the standard model of cosmology, appears to be ruled out by astronomical data: both types of dwarf galaxy, those with putative exotic dark matter and those known to not contain dark matter even if it were to exist, cannot be distinguished by observation. Furthermore, the arrangement of satellite galaxies in rotating disk-like vast near-polar structures around the Milky Way and Andromeda galaxies and the frequent occurrence of anisotropic flattened satellite populations around major galaxies, seem to very strongly support the conclusion that only one type of satellite dwarf galaxy exists, namely the type without dark matter. Also, the orbital decay implied by dynamical friction on the putative dark matter halos is not evident in interacting galaxies. Dynamically relevant cold or warm dark matter therefore seems not to be present. Instead and as suggested by Milgrom, scale-invariant dynamics is showing a new direction for understanding the astrophysics of galaxies. Galaxies are observed to be simple systems following laws that result from scale-invariant dynamics which do not emanate from the haphazard merging history of halos of exotic dark matter. As a result, the present-day cosmological description of galaxy formation and evolution appears to need major revision.
25.05.2015 14:00
Q. Daniel Wang
Astronomy Department, University of Massachusetts, Amherst
A close-up of the nearest supermasive black hole – Sgr A*
Abstract
Q. Daniel Wang
A close-up of the nearest supermasive black hole – Sgr A*
Supermassive black holes (SMBHs) are typically low in radiation luminosity. But the collective mechanical feedback from such radiatively inefficient SMBHs could rival or even exceed that from active galactic nuclei and could hence strongly affect galaxy evolution. Our own Galaxy's SMBH, Sgr A*, provides an excellent laboratory to study this radiatively inefficient accretion state. A close-up view of its x-ray emission based on recent 3 megaseconds of Chandra observations is providing new insights into the interplay of the SMBH with its environment. The X-ray spectrum of Sgr A* shows multiple emission lines of highly ionized ions. This, together with the lack of the neutral iron K-alpha line, suggests that the accretion of the SMBH is truly in a hot mode. A spectral modeling further suggests that the accretion must be accompanied by an outflow, which likely removes more than 99% of the material initially captured by the SMBH. This conclusion is confirmed in a fitting of the spatially-resolved X-ray emission structure with accretion/outflow simulations, which also enables us to determine its angular momentum. Both the fitted position and inclination angles of the flow are consistent with those of the surrounding disk of massive stars, confirming that their stellar winds are feeding the SMBH. If time allows, I will discuss what we are learning from a systematic time variability analysis and ongoing multi-wavelength monitoring of Sgr A*, as well as a comparison with other nearby low-luminosity SMBHs.
03.06.2015 15:00
Nick Kylafis
University of Crete, Greece
Accretion and ejection in black-hole X-ray transients
Abstract
Nick Kylafis
Accretion and ejection in black-hole X-ray transients
Black-hole X-ray transients (BHTs) often exhibit during their outbursts a characteristic q-shaped curve in a hardness-luminosity diagram. A rich phenomenology has been accumulated over the years regarding this diagram. It is desirable to have a physical picture of BHTs over the entire q-shaped curve, which hopefully will have predictive power. Such a physical picture is proposed here and it relies on two assumptions, easily justifiable. The first is that the mass-accretion rate to the black hole in a BHT outburst has a generic “bell-shaped” form. This is guaranteed by the observational fact that all BHTs start their outburst and end it at the quiescent state, i.e., at very low accretion rates. The mass-accretion rate increases, reaches values close to the Eddington rate, and decreases again. The second assumption is that at low accretion rates the accretion disk is geometrically thick, ADAF-like, while at high accretion rates it is thin. This assumption is generally accepted. Unlike phenomenological pictures typically invoked for BHTs, our physical picture explains a) the difference between type C and type B QPOs, b) the formation and the destruction of jets, and c) why BHTs traverse the q-shaped curve always in the counterclockwise direction and that no BHT is expected to ever traverse the entire q-curve in the clockwise direction. Our physical picture explains the q-shaped curve and its associated phenomenology with only one parameter, the accretion rate.
Previous seminar:
27.04.2015 14:00
Prashanth Mohan
ARIES (Aryabhatta Research Institute of Observational Sciences), Nainital, India
Radiatively driven relativistic jets
Abstract
Prashanth Mohan
Radiatively driven relativistic jets
We present a general relativistic (GR) model of jet variability in active galactic nuclei due to orbiting blobs in helical motion along a funnel or cone shaped magnetic surface anchored to the accretion disk near the black hole. Considering a radiation pressure driven flow in the inner region, we find that it stabilizes the flow yielding Lorentz factors ranging between 1.1-7 at small radii for reasonable initial conditions. Assuming these as inputs, simulated light curves (LCs) for the funnel model include Doppler and gravitational shifts, aberration, light bending and time delay. These LCs are studied for quasi-periodic oscillations (QPOs) and the power spectral density (PSD) shape and yield an increased amplitude (~12%); a beamed portion and a systematic phase shift with respect to that from a previous special relativistic model. The results strongly justify implementing a realistic magnetic surface geometry in a GR framework to describe effects on emission from orbital features in the jet close to the horizon radius. Accepted for publication in The Astrophysical Journal (arXiv:1503.06551).
If you would like to give a seminar in our group, please contact Vladimir Karas or Jaroslav Hamersky.
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