Seminars
Our seminars take place in the lecture room of the building at
Praha–Sporilov.
Next seminar:
15.04.2015 14:00
Martin C. Weisskopf, Stephen L. O'Dell
NASA Marshall Space Flight Center
The Imaging X-ray Polarimetry Explorer (IXPE): An international collaboration
Abstract
Martin C. Weisskopf, Stephen L. O'Dell
The Imaging X-ray Polarimetry Explorer (IXPE): An international collaboration
The Imaging X-ray Polarimetry Explorer (IXPE) is a mission recently proposed in response to a NASA solicitation announcement for an Astrophysics Small Explorer (SMEX). IXPE will use x-ray polarimetry to expand observation space dramatically, providing fresh insights into x-ray emission mechanisms and geometry of cosmic sources-including pulsar-wind nebulae, isolated and accreting neutron stars, and stellar-mass and supermassive black holes. The two-year mission is very low-risk, utilizing mature flight elements and combining expertise for x-ray optics at NASA Marshall Space Flight Center and for polarization-sensitive x-ray detectors at the Istituto Nazionale di Fisica Nucleare (INFN) Pisa and Istituto Nazionale di Astrofisica (INAF) Istituto di Astrofisica e Planetologia Spaziali (IAPS). The science team is comprised of many interested researchers throughout the world. The expected performance and scientific capability of IXPE will be described.
On behalf of the IXPE collaboration.
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:
09.04.2015 15:00
Michal Zajaček
I. Institute of Physics, University of Cologne, Germany
Monitoring the Dusty S-cluster object (DSO/G2) around the Galactic center black hole: core-less cloud vs. enshrouded star scenario
Abstract
Michal Zajaček
Monitoring the Dusty S-cluster object (DSO/G2) around the Galactic center black hole: core-less cloud vs. enshrouded star scenario
We have monitored the Dusty S-cluster object (DSO/G2) during its closest approach to the Galactic Center supermassive black hole in 2014 with integral spectroscopy imager ESO VLT/SINFONI (Valencia-S. et al., 2015). We report on our findings, i.e. ionized-hydrogen emission from the DSO that remains spatially compact before and after the peribothron passage. Before 2014 May, we detect red-shifted Br-gamma and Pa-alpha emission lines at about 40 mas east of Sgr A*, indicating a (pre-peribothron) LOS velocity of +2700+/-60 km/s. Then, no blueshifted emission above the noise level is detected at the position of Sgr A* or upstream the presumed highly-elliptical orbit. The analysis of data after May 2014 shows a spatially compact blue-shifted Br-gamma emission line with (post-peribothron) LOS velocity of -3320+/-60 km/s and no significant red-shifted component.
The detection of DSO/G2 object as a compact single-peak emission line source is in contradiction with the original hypothesis of a core-less cloud that is necessarily tidally stretched, hence producing double-peak emission line profile around the pericentre passage. Therefore we suggest that the DSO/G2 source is a dust-enshrouded star, presumably young, rather than a core-less gas cloud.
I will discuss details of the model, specifically how the accretion of material onto the stellar surface from the circumstellar disk can contribute significantly to the line emission and the observed large line width of the order of 10 angstrom. Given the high eccentricity of the DSO/G2 orbit, the Roche lobe radius shrinks to approx. 1 AU at the pericentre, which can cause an extended circumstellar disk/envelope to be tidally perturbed and finally detached from the putative star. We will discuss whether and when this material can reach the immediate accretion zone of Sgr A*, potentially causing the increase of its activity in the future.
I will comment on the recent ESO press release, http://www.eso.org/public/news/eso1512/ .
If you would like to give a seminar in our group, please contact Vladimir Karas or Jaroslav Hamersky.
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