15.01.2021

Anastasiya Yilmaz

Timing and Spectral Properties of Ultraluminous X-ray Sources: Black Holes vs. Neutron Stars

Ultraluminous X-ray Sources (ULXs) are extragalactic, off-nuclear point-like sources with exceptionally high luminosities exceeding the Eddington luminosity. The nature of these energetic systems remains heavily debated. Such high luminosities are generally accepted to be due to accretion onto a compact object although a fraction of them could be rotationally-powered. The most widely accepted interpretation of the nature of these systems initially included stellar-mass black holes accreting at super-Eddington rates with anisotropic (beamed) emission or intermediate-mass black holes accreting at much lower sub-Eddington rates. With extensive studies, neutron stars accreting at super-Eddington rates were recently introduced as a power source for ULXs after discoveries of coherent pulsations, first from the ULX system M82 X-2 and shortly after followed by NGC 7793 P13 and many others which eventually contributed to a growing group of pulsating ULXs or PULXs. In this talk, I will briefly review the evolution of observational diagnostics on the nature of ULXs focusing on widely accepted theoretical considerations for the origin of their spectral and timing properties. I will then discuss the implications of different modes used to describe the characteristics of the system, dependent on whether they harbor a black hole or a neutron star as an accretor.

20.01.2021

Petra Suková

Jan Frič Award Lecture: Chaos and nonlinear regime in the dynamics around black holes

Among the interesting X-ray sources on the sky belong the so-called microquasars or X-ray binaries, consisting of a black hole or neutron star accompanied by a usual star. Their emitted X-ray flux is highly variable and the microquasars are often found in different temporal states with very distinct behaviour. The origin of such substantial changes is still unknown. If the variations of the outgoing radiation are an outcome of a global deterministic system governed by a low number of nonlinear equations, we can reveal traces of the nonlinear behaviour from their lightcurves by means of time series analysis. I will present a novel method for finding such evidences based on the recurrence analysis of the measured time series and its comparison to an ensemble of surrogate data. In the second part of the talk, I will focus on the variability of the supermassive black holes. The close neighborhood of a supermassive black hole contains not only the accreting gas and dust but also stellar-sized objects, such as late-type and early-type stars and compact remnants that belong to the nuclear star cluster. These entities interact with the accretion flow, which is perturbed by their passages. I will present the latest GRMHD simulations of the repetitive transits of the star through a radiatively inefficient accretion flow in low luminous galactic nuclei. I will discuss the changes in the structure of the accreting torus, the influence on the accretion rate, and the emergence of outflowing blobs of gas, which are expelled by the star into the funnel region.

27.01.2021

Jonny Pierce

Characterising the full radio AGN population: the important journey towards lower radio powers

Sensitive radio surveys with broad sky coverage have revealed that the local radio AGN population is dominated by sources with lower powers than those previously studied as representative of the class. High-excitation radio galaxies (HERGs) with intermediate radio powers (10^22.5 < L_{1.4GHz} < 10^25 W/Hz) represent a particularly important subset of these lower-power objects, given their strong association with kinematic disturbances in the multiphase gas of their host galaxies that can affect subsequent star formation and black hole accretion. Study of radio-intermediate HERG sources is hence important for obtaining a more representative understanding of the role of radio AGN in galaxy evolution. Despite this, little is currently known about the typical properties of such objects. High-radio-power HERGs are found to show predominantly elliptical morphological types and frequent signatures of galaxy mergers and interactions, which are thought to provide the dominant triggering mechanism for their AGN. In this talk, I will discuss how we have used deep optical imaging observations of radio-intermediate HERGs to characterise their detailed morphologies for the first time, in order to determine the dominant host types and the importance of merger-based triggering in the intermediate radio power regime. The dependence of these properties on radio power and or optical emission line luminosity has also been investigated in detail, using similar analysis of the morphologies of powerful 3CR radio galaxies and the hosts of Type 2 quasars, alongside stellar-mass- and redshift-matched non-active control galaxies. In addition, previous results suggest that high-radio-power HERGs are predominantly associated with double-lobed, edge-brightened (FRII) extended radio structures. Such radio structures are also seen (on smaller scales) for some Seyfert galaxies at low radio powers, which has led to suggestions that these structures are connected with radiatively-efficient AGN accretion at all radio powers. I will discuss how we have also used high-resolution VLA observations of radio-intermediate HERGs to test whether this connection continues at intermediate radio powers, or whether they exhibit the highly compact morphologies found from equivalent observations of LERGs with similar radio powers -- so-called "FR0s".