Seminars in 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.


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.


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".


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.


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.


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.


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.


Dr. 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.


Dr. Ruth Daly

Black Hole Spin and Accretion Disk Properties obtained with a New Method for 750+ Sources

Four types of black hole system will be discussed including stellar-mass and supermassive black holes. All of the black hole systems are active and have powerful collimated outflows. Empirically determined relationships between the outflow beam power, accretion disk luminosity, and black hole mass indicate that the outflows are spin powered. The general equation that describes a spin powered outflow is rewritten in dimensionless separable form allowing a determination of the black hole spin. Accretion disk properties such as the disk magnetic field strength and mass accretion rate can also be determined for each source. Good agreement is found between spin values obtained with the new method and published values obtained with well- established methods. The spin values obtained range from about (0.4 - 1), and are found to be independent of source type. Accretion disk magnetic field strengths for the three AGN samples have similar distributions and mean values of about 10 kG, while those for stellar-mass GBHs have mean values of about 100 MG. The new method is independent of a specific accretion disk model; thus the results can be applied to study and constrain disk models.


Noemie Globus

Chirality, Life and Cosmic Rays

While we have not yet reached a consensus on the definition of life, biological homochirality seems to be part of the definition as a necessary step for life’s emergence. The unraveling of its origin require interdisciplinary research, by exploring each of fundamental physics, modern chemistry, astrophysics and biology. In this talk, I will focus on the origin of biological homochirality in the context of astrophysics and particle physics. The weak force, one of the fundamental forces operating in nature, is parity-violating, and has been implicated in biological homochirality since over half a century. Cosmic rays, high energy particles coming from outer space, induce showers of billions of secondary particles when they interact with atoms in the atmosphere. On Earth, at ground level, most of our cosmic radiation dose comes from polarized muons formed in a decay involving the weak force. I will show how the spin-polarization is transmitted in cosmic showers in different environments, how it can induce a chiral preference in the early biological life forms and I will discuss the implications for the search of life in other worlds.


Dr. Krista Lynne Smith

Through the Kaleidoscope: Applying Temporal and Multiwavelength Datasets to Supermassive Black Hole Phenomenology

Active galactic nuclei provide unique laboratories for violent physical processes like stellar tidal disruption, highly relativistic jets, and turbulent accretion flows. A thorough understanding of these objects and their profound effect on galaxy evolution now depends upon synthesizing observations across many wavelengths and understanding their time evolution. In particular, high-resolution radio imaging surveys of outflows and star formation and timing observations with breakthrough instruments like the Kepler and TESS exoplanet-hunting missions have provided new intersectional insights and promise a fertile new temporal phase space for exploring the detailed phenomenology and cosmological implications of AGN. In my talk, I will discuss the 22 GHz radio survey of 268 radio-quiet BAT AGN and implications for both the origin of radio emission in non-jetted objects and galaxy-wide star formation suppression. I will also discuss time domain results in accretion physics and binary AGN from the Kepler and TESS AGN samples, as well as theoretical implications for observed properties of quasi-periodic oscillations.


Mislav Baloković

Enhancing Broadband X-ray Spectroscopy as a Probe of AGN Structure

Obscuration of the innermost parts of active galactic nuclei (AGN) is observed in the majority of the population both in the nearby universe and at high redshift. Recent advances in broadband X-ray spectroscopy suggest that the covering factor of the "obscuring torus" is maximal around the typical Seyfert X-ray luminosity and decreases toward both lower and higher luminosities. However, the picture of this obscuring structure is far from clear as even cutting-edge instruments in the radio and infrared bands do not have the angular resolution sufficient to resolve emission from AGN on the relevant physical scales (broadly, ~pc), except in a few isolated cases. In this talk I will focus on measurements of AGN structural parameters using spatially-unresolved, single-epoch, broadband X-ray spectroscopy and discuss some of their limitations. I will then show how some of those limitations can be surpassed using novel spectral models, multi-epoch, multi-wavelength, and/or spatially resolved data already available today. This approach can help us build a more self-consistent picture of the various AGN types in advance of the anticipated future observations at high spectral and spatial resolution with the potential to directly probe the key physical scales connecting AGN to their galactic environment.


Dr. David Garofalo

Active black holes across space and time

Beginning in the 1970’s and with the aid of decades of numerical simulations, the ‘spin paradigm’ today purports to explain black hole accretion and jet formation across space and time. Unfortunately, there is substantial tension between predictions and observations. I show how opening up counterrotation between black holes and accretion disks as a possibility, resolves the tension and leads to a coarse-grained understanding of a number of unresolved issues in high energy astrophysics including the redshift distribution of active galactic nuclei, the radio loud/radio quiet dichotomy, the association of excitation class with radio morphology, the cluster richness dependence of AGN, and the behavior of microquasars, among others. Including input from observations of jet/ISM coupling into the model, I will also describe recent insights into the AGN-star formation connection over cosmic time.


Dr. Dorota Kozieł-Wierzbowska

Morphological and physical classification of radio sources

Based on the ROGUE I catalogue, I will discuss the methods of identification and classification of radio sources in the large radio surveys. The catalogue was created by cross-matching galaxies from the SDSS with the FIRST and NVSS catalogues. It contains more than 32,000 galaxies with a FIRST identification within 3arcsec of the optical position, and is the largest, handmade catalogue of this kind. However, as most catalogues with a low flux density limit, it contains the mixture of radio sources. The separation of radio AGNs from the radio emission arising in the star-forming regions is a challenging task. In the recent papers (e.g. Best & Heckman 2012, Sabater et al. 2019) separation of these two populations was made based on several different diagrams. This procedure is complicated and requires the knowledge of many different parameters, like optical emission line ratios or galaxy mass. Therefore, during my talk I will also discuss diagrams that can be used to distinguish radio emission produced by jets from the one emitted in star-forming regions.


Dr. Ross Turner

RAiSE HD: Lagrangian particle based model for extended AGNs at radio and X-ray wavelengths

Radio-frequency surveys using new generation telescopes are providing observations of millions of previously undiscovered galaxies by probing ever deeper into the universe. However, at most a few percent of active galactic nuclei (AGNs) that will be detected in these surveys will have optical spectroscopy or photometry. Turner & Shabala (2019) showed the distance to nearby AGNs can be accurately measured using only radio-frequency observations by making use of the Radio AGNs in Semi-analytic Environments (RAiSE) astrophysical fluid dynamics model (Turner & Shabala 2015). Turner et al. (2020b) used this new approach to constrain the distances for a modest sample of 15 AGNs up to 11.9 million light-years away and found excellent agreement (<10% error) with spectroscopic redshifts. However, the existing RAiSE code will (in a few months) be superseded by a Lagrangian particle based analytic model that considers the evolutionary histories of individual packets of synchrotron-emitting electrons. With the extra precision of the new model, the measured distances to AGNs with known spectroscopic redshifts can instead be used to constrain cosmological constants; with good coverage to high redshifts (z > 5) this method may help resolve the current tension in Hubble constant measurements. This research may also benefit from X-ray observations to find candidate high-redshift extended AGNs. Turner et al. (2020a) previously found a factor of ten more remnants can be detected using X-ray wavelengths over radio frequencies at z > 2.2, increasing to a factor of 100 for redshifts z > 3.1.



Joint Journal Club


Iossif Papadakis

From the X-rays to the optical/UV: clues about the nature of the central source in AGN.

Active Galactic Nuclei are powered by accretion of matter to the supermassive black holes that reside at their center. The optical/UV "Big Blue Bump" component in their spectra is commonly interpreted as optically thick thermal radiation which arises in the region of steeply falling potential region near the black hole, and strongly supports the "accretion-powered" model. AGN are also strong X-ray emitters, but standard accretion disc models cannot account for them. The X-ray source must be powered by the mass accretion to the black hole, but we still do not know how or where the X-ray source is located. Monitoring X-ray/UV/optical observations the last few decades (as well as by the study of the reflection signatures in the X-ray spectra) have provided important clues regarding the disc/X-ray source geometry in AGN. I will discuss recent results from intense X-ray/UV/optical monitoring campaigns of bright AGN the last few years, and I will argue that proper treatment of the X-ray illumination of a "standard" accretion disc can explain them (contrary to what was believed in the past). I will also address the implication of X-ray illuminated disc models to the results from other challenging observations (like the "disc-size" problem implied by microlensing observations of lensed quasars etc).

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