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".

08.02.2021

Radek Vavřička

Tenké akreční disky s magnetickým advekčním členem

Analýza termálních spekter akrečních disků rentgenových dovjhvězd s černou dírou dlouhodobě ukazuje, že standardní model tenkého disku při akrečních rychlostech vyšších než 0.3 Mdot_Eddington přestává být schopen spávně a věrně popsat strukturu a záření disku. Novým pokusem o nalezení řešení tohoto problému, který bude na semináři prezentován, je zahrnutí advekčního členu a odhadnutí jeho příspěvku z výsledků numerických MHD simulací včetně započtení vlivu magnetického tlaku. Přitom se ukáže, že výsledné řešení překvapivě dobře vystihuje pozorované změny spekter probíhající v závislosti na akreční rychlosti.

15.02.2021

Ana Laura Müller

High-energy processes in starburst superwinds and active galactic nuclei

Starburst galaxies and active galactic nuclei are promising candidates for accelerators of ultra-high energy cosmic rays. Processes taking place in both astrophysical sources are capable of releasing large amounts of energy, which can be converted into relativistic particles in specific situations. The theoretical arguments are supported by observational evidence showing that these objects are non-thermal emitters at different wavelengths. Additionally, the Pierre Auger and the IceCube Collaborations have found hints of correlation between the arrival directions of cosmic rays and neutrinos and the position of starburst galaxies and active galactic nuclei. In this talk I will present results for the production of cosmic rays in the superwind region of starburst galaxies and the expected non-thermal emission, as well as an alternative model to explain the observed gamma radiation of non-jetted active galactic nuclei.

04.03.2021

Dr. Stacey Alberts

Toward a Complete Census of AGN at Cosmic Noon: Ultra-deep X-ray, Radio, and (coming soon!) JWST

The challenges in overcoming the selection biases inherent in AGN selection techniques - and the challenge of identifying heavily obscured and Compton Thick AGN at all - have long hindered our global understanding of AGN and their hosts. In this talk, I will present our first step toward a complete census of AGN in the GOODS-S/HUDF region at cosmic noon (z~1-3). Using new, ultra-deep, high resolution radio imaging at 3 and 6 GHz, we identify AGN in a population of faint star forming galaxies by minimizing selection bias through multiple techniques: X-ray properties via the unique 7 Ms Chandra imaging; mid-IR colors and SED fitting via extensive 3D-HST photometry; and radio properties such as radio excess or a flat radio spectrum. The AGN counts - fully half of our sub-microJy radio sources have evidence for an AGN - and properties from this sample inform future AGN surveys. Likely still missing from this census are the most heavily obscured AGN at cosmic noon, which we argue will be best revealed through their mid-infrared properties via the wavelength coverage of JWST/MIRI. In closing, I will present our Cycle 1 GTO MIRI imaging and NIRSpec spectroscopy program aimed at completing our census.

01.04.2021

Ana Maria Jimenez Gallardo

Radio galaxies, from the most powerful to the most compact in the local Universe

I will present an overview of the latest results of the 3CR Chandra Snapshot Survey, a survey dedicated to completing the X-ray coverage of the 3CR catalog with observations of the Chandra X-ray observatory. In particular, I will present an overview of the survey, focused on the results obtained for radio sources in the redshift range between 1.5 and 2.5. This survey has proven to be an invaluable tool in the study of radio sources, providing new Chandra archival observations of 122 sources. Furthermore, thanks to this survey, we discovered the presence of extended X-ray emission around FR II radio galaxies, which motivated the start of a systematic analysis aimed at (i) investigating whether it is due to Inverse Compton scattering (IC) of seed photons arising from the Cosmic Microwave Background (CMB) or to thermal emission from an intracluster medium (ICM), and (ii) at testing the impact of this extended emission on hotspot detection. I will present the final results of this work as well as the preliminary results of our study of the extended X-ray emission at scales of a few tens of kiloparsecs for the radio galaxies 3CR 318.1 and 3CR 196.1. Lastly, I will introduce the COMP2CAT catalog, a catalog of compact double radio galaxies selected from an existing catalog of radio sources based on NVSS, FIRST, and SDSS. This catalog, the fourth in a series of radio source catalogs recently created (FRICAT, FRIICAT, and WATCAT), aims at attaining a better understanding of sources with intermediate morphologies between FR IIs and FR 0s and it can potentially be used to clarify the role of double sources in the general evolutionary scheme of radio galaxies.

07.04.2021

Tim Waters

Irradiated atmospheric fragments (IAFs): the outcome of thermal instability operating in thermally driven AGN winds

AGN outflows provide a natural link between a galaxy and its central black hole. Thermally driven outflows occur when X-rays generated in the close vicinity of a supermassive black hole irradiate the distant regions of the AGN disk. Specifically, established theory places these ionized outflows near the Compton radius where escape velocities are characteristic of the warm absorbers commonly seen in AGN spectra. Such models have thus far been unable to account for the range of inferred ionization parameters, however. In this talk, I will present recent results showing that beyond about 100 Compton radii, thermal instability operates in irradiated disk atmospheres, and this results in a broader range of ionization parameters. The resulting dynamics, as revealed by numerical solutions to the equations of non-adiabatic (multiphase) gas dynamics, is the episodic formation of tsunami-like structures that we refer to as IAFs. I'll discuss the underlying principles of these new clumpy wind solutions and explain how they can inform interpretations of the absorption measure distribution.

06.05.2021

Mislav Baloković

TBC