Seminars 2017
22.06.2017 14:00
Abigail Stevens
Anton Pannekoek Institute for Astronomy, University of Amsterdam, The Netherlands
Comparing origins of low-frequency quasi-periodic oscillations with spectral-timing
Abstract
Abigail Stevens
Comparing origins of low-frequency quasi-periodic oscillations with spectral-timing
X-ray spectral-timing is a new field that seeks to investigate how matter behaves in strong gravitational fields. Observations suggest that different types of low-frequency quasi-periodic oscillations (QPOs) are associated with different emitting-region geometries (e.g., disk-like or jet-like) in the innermost part of the X-ray binary, close to the neutron star or black hole. We developed a technique for phase-resolved spectroscopy of QPOs, and are applying it to Type B and Type C low-frequency QPOs from the black hole X-ray binary GX 339-4. On the QPO time-scale, we find that the energy spectrum changes not only in normalization, but also in spectral shape. We can quantify how the spectral shape changes as a function of QPO phase, and the two different QPOs show markedly different spectral changes. In our previous work, we inferred that the Type B QPO could be caused by a large-scale-height (i.e., jet-like) precessing region illuminating and heating overlapping azimuthal regions of the inner accretion disk. Preliminary results of the Type C QPO indicate that a small-scale-height (disk-like) precessing region may be responsible for the observed spectral changes. This talk will feature results from this paper: https://ui.adsabs.harvard.edu/#abs/2016MNRAS.460.2796S/abstract
20.03.2017 14:00
Bruno Leibundgut
ESO, Garching-bei-München, Germany
Observing with ESO telescopes
Abstract
Bruno Leibundgut
Observing with ESO telescopes
ESO offers a wide range of telescopes and instruments. Selecting the right combination for astronomical observations can be tricky and decisive for the project. I will present the ESO process to provide a better understanding of what is required by the proposer. The talk covers proposal preparation, selection, scheduling, observations and data products as returned from ESO. It will also will touch on related topics, like available instrumentation and their science capabilities, science archive, student- and fellowship programmes. Open discussion on all topics is encouraged.
09.03.2017 14:00
Vladimír Domček
Anton Pannekoek Institute / GRAPPA, University of Amsterdam
X-ray reflection from black-hole accretion discs with a radially-stratified ionisation
Abstract
Vladimír Domček
X-ray reflection from black-hole accretion discs with a radially-stratified ionisation
Reflection spectra from the accretion discs of both the active galactic nuclei and the X-ray binaries can provide useful information about the geometry of the innermost regions in these systems. Several numerical codes were developed to explain spectra employing the relativistic effects prevailing in the vicinity of the black-holes. Using these codes as models for fitting of the sources with strong reflection component showed a steep radial emissivity measurements in number of them.
I will discuss the role that ionisation of the disc can play in these models and how it can affect the measured values of the radial emissivity parameter. By the end of the talk I will also briefly discuss the topic of the Supernova Remnants as the new field of my research.
27.02.2017 14:00
Robert Klement
Astronomical Institute, Faculty of Mathematics and Physics, Prague
Revealing the structure of the outer disks of classical Be stars - are Be disks circumbinary?
Abstract
Robert Klement
Revealing the structure of the outer disks of classical Be stars - are Be disks circumbinary?
Classical Be stars possess self-ejected gaseous circumstellar disks governed by viscous forces. The structure of the inner parts (<20 stellar radii) of these disks is well explained by the viscous decretion disk model (VDD), which is able to reproduce multi-technique observable properties of most of the so-far studied objects. Due to the nature of the emission mechanism responsible for the IR and radio continuum excess (free-free emission), the outer parts of the disks are observable at radio wavelengths only. A steepening of the spectral slope somewhere between infrared and radio wavelengths was reported for the handful of Be stars that were observed in radio, but the physical reason for this feature remained mostly unknown.
I will present results from the multi-technique modeling of beta CMi, for which we obtained new sub-mm data from the APEX telescope. The SED turndown observed in beta CMi could be reproduced only when assuming a truncated disk. The most plausible explanation for the truncation is the presence of a faint companion, which has been just independently confirmed as predicted via RV analysis of the H-alpha line. Results from the SED modeling of 5 additional stars, 4 of which have new multiband VLA data, will be presented. All the studied disks are found to be truncated, while only one of the objects is a previously known binary. The detailed structure of the radio SED revealed by the VLA observations allow for studying the exact nature of the disk truncation. The truncation is clearly not as sharp as expected, and certain features indicate that the disks may extend beyond the orbits of the companions, thus offering a possibility that Be disks are actually circumbinary disks. Impacts of the suspected high binarity fraction of Be stars on their possible common evolutionary scenario will be discussed.
04.01.2017 14:00
Michal Zajaček
I. Institute of Physics, University of Cologne, Germany
What have we learned from the observations of the NIR-excess source DSO/G2 in the Galactic centre?
Abstract
Michal Zajaček
What have we learned from the observations of the NIR-excess source DSO/G2 in the Galactic centre?
The Dusty S-cluster Object (DSO/G2) orbiting the supermassive black hole (Sgr A*) in the Galactic centre has been monitored in both near-infrared continuum and line emission. There has been a continued discussion about the character of the object: interpreting it as either a core-less gas cloud or a dust-enshrouded star. A recent analysis of polarimetry data in K_s-band (2.2 μm) by Shahzamanian et al. (2016) allows us to put further constraints on the geometry of the DSO. I will present our recent results of the radiative transfer modelling, including NIR polarized continuum, and discuss possible models of the DSO/G2 in the context of Galactic centre stellar populations.
