Seminars in 2024

03.01.2024

Thomas González Roberts

Who is following the rules in space?

Assessing GEO satellite operators’ compliance with ITU orbital assignments

09.01.2024

Michal Zajaček

IMBHs in galactic nuclei: Potential dynamical and spectral signatures

In the local Universe as well as in more distant galaxies, detected black holes (using electromagnetic or gravitational waves) mostly fall into two categories in terms of their mass: stellar black holes (<100 Msun) and massive black holes (>10^5 Msun). The intermediate-mass black holes (IMBHs, 100 Msun < M < 10^5 Msun) appear to be rather rare, which is most likely due to several characteristics, such as the environment where they form and their subsequent accretion mode. I will start the talk by reviewing IMBH formation mechanisms, with the focus on their occurrence in galactic nuclei, which are the densest systems in galaxies. Then I will list several specific examples, where supposedly IMBHs could be present, including the Galactic center. In the rest of my talk, I will describe in more detail the extragalactic source first identified by the ASASSN survey, which exhibited quasiperiodic enhancements in the soft X-ray absorption every 8.5 days. In other words, the source has a quasiperiodic ultrafast outflow that can obscure the inner accretion flow. Using analytical as well as GRMHD simulations, we show that the source properties are best described by an IMBH orbiting around the SMBH.

09.01.2024

Maureen Henderson

Angular Momentum, a novel-in-progress: incorporating research in fiction

A decade ago I fell in love with an artist’s rendering in an astronomy textbook, an illustration of the concept of angular momentum as it occurs in binary stars. A few years later, I visited Prague for the first time. Ever since, taking angular momentum as a metaphor for human relationships, and reflecting on the tenacity of the Czechs in surviving successive authoritarian regimes, the science, the city, and the country have been inspiring the writing of a novel. Angular Momentum has become a story about the challenge of making human connections in difficult circumstances, and the importance of finding ones’s convictions in dangerous times. I’ll be talking about how I’ve conducted research in astronomy and in Czech history and culture, and the problem of selecting the specific details that illuminate character and setting while maintaining the momentum of story.

01.02.2024

Ivana Ebrová

Stellar kinematics and merger histories of early-type galaxies

Seemingly smooth and boring elliptical and lenticular galaxies (ETGs) tend to be secretive about details of their current and past life experiences. Now, in the era of spatially resolved spectroscopic surveys on one hand and deep imaging surveys on the other, we started to reveal the wealth of different features in statistically significant samples of ETGs. We use the publicly available data of the Illustris project — a set of large-scale cosmological hydrodynamical simulations — to study the origin and characteristics of galaxies with prolate rotation (i.e. rotation around the long axis) and kinematically distinct cores. We found that basically all the simulated massive prolate rotators were created in relatively recent major mergers of galaxies. Such mergers are expected to produce tidal features (tails, shells, asymmetric stellar halos) that should be visible in sufficiently deep images. We combine archival data and new observations (using the 1.4m Milankovic telescope) to assemble deep optical images of the complete sample of all known nearby massive prolate rotators and compare their frequency of tidal disturbance with a general sample of ETGs in MATLAS — a deep imaging survey. The most frequent tidal features among the prolate rotators happen to be shells. We developed methods that allow us to exploit the special kinematics of stellar shells to calculate the probable time of the merger from deep optical images. Shells will allow us to make such estimates for a large portion of the sample and compare it with the predictions of the Illustris simulation. In our current project of Marie Skłodowska-Curie Actions, we plan to expand the methods to use them on even larger samples of shell galaxies supplied by upcoming large surveys like LSST at Rubin Observatory. This will provide an unprecedented amount of statistically significant data on the recent merger history of our Universe and allow extensive investigation of impact of mergers to a wide range of other astrophysical phenomena.

21.03.2024

Gerardo Urrutia

The large-scale interaction between short GRB jets and disk wind outflows

Short GRBs are produced by relativistic jets arising from binary NS-NS or NS-BH mergers. Since the detection of the first unambiguous off-axis GRB 170817A, we learned that energy distribution in the jet plays an important role in explaining the GRB emission. The structure and dynamics are modified during the first seconds of the jet interaction with a post-merger environment. Conventional studies often assume this environment as a simple homologous and symmetrically expanding wind. However, post-merger outflows exhibit complex dynamics influenced by the accretion disc evolution. Moreover, the r-process nucleosynthesis influences the thermodynamics and properties of the post-merger neutron-rich environment. During the talk, I will show the results of numerical simulations studying the impact of realistic post-merger disc outflow over the jet dynamics at large scales. Our results are substantially different from the typical model with symmetric homologous wind. a) the impact of the r-process on initial wind pressure leads to significant changes in the jet collimation and cocoon expansion; b) the angular structure of thermal and kinetic energy components in the jets, cocoons, and winds differ concerning simple homologous models, hence it would affect the predictions of GRB afterglow emission; c) the temporal evolution of the structure reveals conversion of thermal to kinetic energy being different for each component in the system (jet, cocoon, and wind); d) post-merger environments influence energy structure and material dispersion, altering the interaction between jets and disk winds.

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