Seminars in 2018


Michal Křížek

Local Hubble Expansion

Life on Earth has existed continually for at least 3.5 Gyr and this requires relatively stable conditions during this very long time period. However, since the luminosity of the Sun increases, the Earth should recede from the Sun. We present several examples indicating that the Solar System expands by a speed comparable to the Hubble constant. This guarantees that the Earth received almost constant solar flux during the last 3.5 Gyr. We give three independent arguments showing that the average Earth-Sun distance increases about 5 m/yr due to the finite speed of gravitational interaction. Such a large recession speed cannot be explained by solar wind, tidal forces, plasma outbursts from the Sun, or by the decrease of the Solar mass due to nuclear reactions. Models based on Newtonian mechanics can explain only a few cm per year. The measured average increase in the Earth-Moon distance is 3.84 cm/yr, while Newtonian mechanics is able to explain only 2.1 cm/yr. We claim that this difference is also caused by the finite speed of gravitational interaction. Mars was much closer to the Sun as well, otherwise it could not have had rivers 3.5 Gyr ago, when the Sun’s luminosity was only 75 % of its present value, see [1] for details. References [1] M. Křížek, L. Somer, Manifestations of dark energy in the Solar system, Grav. Cosmol. 21 (2015), 58–71.


Christopher Jacobs

VLBI and its application for building reference frames for spacecraft navigation

Christopher Jacobs is a senior deep space navigation engineer at NASA’s Jet Propulsion Laboratory (JPL) of the California Institute of Technology. Jacobs holds a degree in Applied Physics from Caltech. He joined JPL in 1983 and has taken on roles of increasing responsibility in the area of deep space tracking specializing in the area of celestial and terrestrial reference frames. He has served as the Reference Frame Calibration task manager for 25 years in which role he has been responsible for delivering the reference frames used to navigate NASA missions such as MSL to planetary targets. In this talk he will give a brief overview of the Very Long Baseline Interferometry (VLBI) technique and show how it is applied to building reference frames for spacecraft navigation.


Alet de Witt

Imaging of extragalactic radio sources at K-band with VLBI

Alet de Witt is an operations astronomer at the Hartebeesthoek Radio Astronomy Observatory (HartRAO) in South Africa. She will introduce the K-band imaging project where she is the principal investigator. The VLBI data set at K-band has world-class spatial resolution (few parsecs) coupled with a temporal resolution from a 0.5 to 2 months’ cadence of observation for a given radio source. What can one do with such a data set? Few ideas will be presented such as searching for periodicity as a sign of binary black holes, jet precession, or optical vs. radio offsets. This data set should produce results of interest to the relativistic astrophysics groups theoretical working on black holes and accretions disks and to the relativistic astrophysics group’s studies of spatially resolved AGN which can build a basis for mutual cooperation.


Joerg Dabringhausen

The curiously high mass-to-light ratios of ultra-compact dwarf galaxies

While ultra-compact dwarf galaxies (UCDs) might just be the most massive globular clusters (GCs), they have also a few properties other than their mass and luminosity that set them apart from more conventional GCs. Among these are their dynamical mass-to-light ratios, which are rather high, and seem in fact inconsistent with the premise of a non-varying stellar initial mass-function (IMF). It was therefore proposed that the IMF in UCDs is top-heavy. I will discuss how this hypothesis relates to the high mass-to-light ratios of UCDs, their populations of neutron stars, and the possible presence of super-massive black holes in them.


James Fuller

Saturn Ring Seismology: Complex Interactions Between the Planet, the Rings, and the Moons

The rich dynamics of the Saturn ring and moon systems offer unique opportunities to study the evolution of the planet and its surrounding bodies. For instance, seismology of Saturn is made possible by the gravitational interaction between Saturn and its rings, in which density waves in the rings are excited at Lindblad resonances with Saturn's oscillation modes. The seismic signatures in the rings suggest the existence of stable stratification in the deep interior of the planet, likely created by composition gradients between the core and envelope due to helium sedimentation and/or core erosion. These structures within the planet influence the tidal interactions which drive the outward migration of Saturn's inner satellites. Rapid migration can occur when moons become locked in resonance with Saturn's oscillation modes, driving the moons outward on a planetary evolution timescale.


Hana Krásná

Galactic aberration estimated from VLBI geodetic data

Very Long Baseline Interferometry (VLBI) is a space-geodetic technique directly connecting the Terrestrial Reference Frame realized by positions of Earth-based stations with the Celestial Reference Frame (CRF) defined by a set of extragalactic radio sources (quasars) well distributed throughout the sky. Due to the rotation of the Solar System Barycentre (SSB) around the centre of Milky Way galaxy, the arising acceleration of the SSB induces an apparent proper motion of the extragalactic objects observed by VLBI, i.e., a change in the apparent source positions over time. The aberration amplitude estimates (5 - 7 microas/year) from geodetic VLBI are close to the independent estimates derived from astrometric measurements of proper motions and parallaxes of masers, and it is not negligible in terms of the upcoming ICRF3 catalogue anymore.


Juan Matías Loiseau (TUTE)

Astronomy and cartoons -- Argentine graphic humor

Juan Matías Loiseau, better known as TUTE, is a recognized Argentine humorist, cartoonist, poet, film director, TV presenter and songwriter. As the author himself remarks, a frequent topic in his jokes is the relationship between women and men. TUTE will also present his last book in the "Prague 24th International Book Fair and Literary Festival" (Interpreted: Czech, Spanish - Velvyslanectví Argentinské republiky),


Tania Regimbau

The quest for a GW stochastic background with LIGO and Virgo

A primary target for gravitational wave astronomy is the detection of a stochastic background formed by the superposition of many unresolved independent sources at different stages of the evolution of the Universe. After the first observations of a gravitational wave from the merger of two black holes (BHs) or two neutron stars (NSs), the next big milestone could be the observation of the stochastic background created by the superposition of all the unresolved compact binary coalescences (CBCs). The observation of this background will be the opportunity to study the population of NSs and BHs at high redshift, complementing individual detections at close distances. In this talk, I will give an overview of the different sources and will present the data analysis methods used in the LIGO/Virgo collaboration to measure the GW stochastic background. I will also discuss how the future generation of detectors can be used to remove the astrophysical contribution in order to observe the signal of cosmological origin.


Constantinos Skordis

A survey of theories of gravity and their testing with cosmology

In this talk, I will give a brief overview of the dark sector in cosmology: dark matter and dark energy. I will discuss possibilities that the existence of either of both of these (unknown) substances could be made redundant by using a theory other than General Relativity (GR) as the basis for gravity. I will briefly discuss various ongoing efforts to test alternatives to GR with cosmological observations.


Lýdia Štofanová

Relativistic corrections in hard X-ray spectra of accreting black holes

Hard X-ray spectra of accreting black holes in active galactic nuclei and X–ray binaries are characterized by a power-law shape with an exponential cut-off energy at several tens up to few hundreds of keV. The value of the cut-off energy is related to the temperature of a hot corona that reprocesses and inversely Comptonizes thermal emission from the accretion disc. The exact geometry of the corona is still unknown. Several observations suggest it to be very compact and in a close proximity to the black hole. This implies strong relativistic effects such as gravitational redshift, Doppler shift, light bending and beaming to shape the resulting spectra. However, the relativistic effects on primary X–ray emission are often neglected in the data spectral fitting. In this work, we investigate how large uncertainty is introduced by neglecting these relativistic effects. To this purpose, we performed simulations of X–ray spectra for different coronal geometries, and compared the intrinsic and observed values of the cutoff energy. We re-analyzed NuSTAR observations of an active galactic nucleus 1H0419-577 and X–ray binary GRS 1915+105. We found that the extremely low coronal temperatures observed in these sources may be explained by the gravitational redshift due to the proximity of the compact corona to the black hole. We also claim that it is incorrect to link the cutoff energy in the reflection and primary power-law model, which is an often used assumption that can lead to wrong ionization and black-hole spin measurements.


Romana Mikušincová

Accreting black holes via X-ray polarimetry

In this talk I will summarize the results of my Master's Thesis for which I performed polarization simulations of the X-ray binary GRS 1915+105. The aim of this work is to put independent constraints on a black hole spin and inclination of the system via X-ray polarimetry. To simulate polarization spectra, we used a multicolor blackbody emission model accounting for thermal radiation from the disk accretion. Finally, we fit these data to estimate the precision of constraints on black hole spin and inclination.


Boris Deshev

Effects of dynamic environment on galaxy evolution

Mergers of structures on scales of galaxy groups or larger are common in LCDM universe where small structures form first and then grow primarily through mergers with other structures. We use data from the LoCuSS survey to investigate the effects these highly energetic events have on the galaxies residing within the merging units. We probe a range of mass scales from accretion of large groups onto clusters with mass ratio ~1:10 to a massive cluster mergers like A520 with a mass ratio ~1:1. We employ Lick index measurements and full spectrum fitting to find the star-formation history of the galaxies and correlate that with the recent history of the environment in which they are embedded.


Michal Zajaček

Precession of accretion flows and jets in blazars

I will describe the precession model of an accretion disc/jet system and its connection to the observed radio variability of blazars. In particular, I will describe the application to OJ287, which has been intensively monitored in optical and radio domains. The precession can be either caused by the secondary black hole in a binary system or by the Lense-Thirring precession of the disc/jet system.


Alexei Moiseev

3D spectroscopy of galaxies at the 6-m telescope

Panoramic spectroscopy at the Russian 6-m telescope was developed in two directions: integral-field spectrographs based on microlenses+fibers arrays and scanning Fabry-Perot interferometers. Now both techniques are available as different modes in the prime focus of the SCORPIO-2 multimode reducer. In this talk, we briefly review recent results in studying nearby galaxies, obtained with our 3D-spectroscopic facilities: gas and stellar kinematics in active and interacting galaxies, multi-spin systems, etc. Special attention is given to star formation feedback in dwarf galaxies and formation of kpc-sized shells of neutral and ionized gas.


Valeriia Tynianskaia

Energy balance between disk and corona in active galactic nuclei

The presence of X-ray radiation that comes from the innermost regions of Active Galactic Nuclei indicates a presence of a hot gas component located close to the central black hole. The exact location and geometry of this so called corona is not known and various configurations are being considered in the literature. Another key question is how is the corona energized, i.e. where does it take energy from. Assuming the disk is the only source of energy in an accreting system and that its internal energy is partly radiated and partly used to support magnetic fields, we evaluate geometrical constraints on the corona from the energy conservation condition. Lastly, we try to investigate the total emitted spectrum of a system consisting of a central black hole, a thin accretion disk and a slab corona.


Sante Carloni

A first-order secular theory for the post-Newtonian two-body problem with spin: the restricted case

We revisit the relativistic restricted two-body problem with spin employing a perturbation scheme based on Lie Series. Starting from a post-Newtonian expansion of the field equations, we develop a first-order secular theory that reproduces well-known relativistic effects such as the precession of the pericentre and the Lense-Thirring and geodetic effects. Additionally, our theory takes into full account the complex interplay between the various relativistic effects, and provides a new explicit solution of the averaged equations of motion in terms of elliptic functions. Our analysis reveals the presence of particular configurations for which non-periodical behaviour can arise. The application of our results to real astrodynamical systems (such as Mercury-like and pulsar planets) highlights the contribution of relativistic effects to the long-term evolution of the spin and orbit of the secondary body.


Giovanni Acquaviva

A gravitational energy-momentum and the thermodynamic description of gravity

A proposal for the gravitational energy-momentum tensor, known in the literature as the square root of Bel-Robinson tensor (SQBR), is analyzed in detail. Being constructed exclusively from the Weyl part of the Riemann tensor, such tensor encapsulates the geometric properties of free gravitational fields in terms of optical scalars of null congruences: making use of the general decomposition of any energy–momentum tensor, we explore the thermodynamic interpretation of such geometric quantities. While the matter energy-momentum is identically conserved due to Einstein’s field equations, the SQBR is not necessarily conserved and dissipative terms could arise in its vacuum continuity equation. We discuss the possible physical interpretations of such mathematical properties.


Aleksandra Kotek

Evaporating Primordial Black Holes as sources of Ultra High Energy Cosmic Rays

My presentation will focus on the current state of knowledge of cosmic rays with energies range 10^18-10^19 eV. Due to the small cross-section, the rarity of detection and the influence of the Earth's atmosphere, direct observation of the cosmic ray flux requires the detectors of these particles to be elevated to high altitudes. An alternative method is the observation of secondary particles. Currently, the origin of these particles is unknown, and one of the greatest riddles of modern astrophysics is to find their origin. I will present the arguments of various theories for the most probable candidates and in addition, the evaporation of Primordial Black Holes as candidates for the Ultra High Energy Cosmic Rays source and I will show the results of my studies.


Christos Efthymiopoulos

Manifold Dynamics in the Solar System and in Galaxies

Manifold dynamics started as a concept allowing to unveil how the dynamical behavior of the trajectories close to unstable equilibrium points or unstable periodic orbits can be exploited in trajectory design in astrodynamics (e.g. the design of spacecraft orbits in the framework of the restricted three body problem). Nowadays, manifold dynamics serves also to interpret a variety of natural phenomena in gravitational systems, from planetesimal accretion, and planet or asteroid migration within our Solar System, to the spiral arms and the tidal streams and tails in galaxies. The seminar will present the basics of manifold dynamics from the viewpoint of dynamical astronomy. Then, we will refer to results obtained so far from manifold computations (analytical and numerical) in conjunction with N-body simulations.


Vyacheslav Lukin

Magnetic Reconnection in a Weakly Ionized Plasma of the Solar Chromosphere

This talk will describe efforts to self-consistently model magnetic reconnection processes in weakly ionized plasmas, with a focus on the solar chromosphere. The solar chromosphere is a complex and dynamic boundary layer of the solar atmosphere where interdependence of the magnetic field evolution, radiation transport, plasma reactivity, and dissipation mechanisms make it a particularly difficult system to model and understand. Past studies have focused on the micro-physics of multi-fluid magnetic reconnection at magnetic nulls^[1] . Recently, the previous work was extended by considering a range of spatial scales and plasma β values in a configuration with component magnetic reconnection^[2] . I will show that in all cases the non-equilibrium reactivity of a weakly ionized plasma is important for determining the properties of a reconnection region, explore current sheet stability to secondary instabilities, and speculate as to the possible observables of magnetic reconnection in the lower solar atmosphere. [1] Leake, et al, ApJ 760 (2012); Leake, et al, PoP 20 (2013); Murphy & Lukin, ApJ 805 (2015). [2] L. Ni, et al, ApJ 852 (2018); L. Ni, et al, PoP 25 (2018).


Rocio Isabel Paez

Describing Trojan dynamics: resonant structure, stability and long-term diffusion

Trojan motion around the triangular Lagrangian equilibria L4 and L5 is a classical subject of Celestial Mechanics, with a revived current interest motivated by the search for Trojan exoplanets. In this talk, we will discuss the basic features of Trojan dynamics in both an analytical and numerical context. We will focus on the intricate structures generated in phase space due to secondary resonances. A new analytical formalism allows to characterize the web of resonances as they appear in the space of proper elements (action variables). The main analytical result regards the possibility to define a domain of practical stability for trojan bodies, delimited by the separatrices of the most important secondary resonances. The analytical results are then compared with numerical ones, obtained in the framework of the elliptic (restricted and full) three body problem, by means of i) FLI stability maps, ii) study of ensembles of orbits, and iii) normal form computations.


Federico Garcia

Prospecting the wind accretion in the sgHMXB IGR J16320-4751

IGR J16320-4751 is a High Mass X-ray Binary (HMXB) formed by a neutron star (NS) spinning with a period of ~1300 s which orbits around a supergiant companion in about ~9 days accreting material from its powerful wind. It is a highly-obscured system characterized by a typical intrinsic absorption column density of 10^{23} cm^{-2}, an order of magnitude higher than the Galactic column density along the line of sight. In this talk, I will present our recent results arising from an orbital monitoring of this source performed with XMM-Newton and Swift/BAT X-ray satellites. By means of the hard X-ray data from Swift/BAT we re-calculated the ephemeris of the source, updating its reported orbital period. Based on the soft X-ray light curves of nine XMM-Newton observations performed at different orbital phases we generated time-resolved spectra that we fitted using an intrinsically-absorbed comptonization model for the continuum emission and three narrow gaussians Fe lines. We found that the spectral evolution is mainly governed by variations in the absorption column which correlate with the intensity of the Fe lines. Moreover, we also found that the absorption column peaks before the maximum of the Swift/BAT light curve is reached. Using the information gathered from this X-ray monitoring we propose a simple model assuming a typical wind profile for the supergiant companion, which enables us are to simultaneously fit the evolution of the hard X-ray light curve and the orbital evolution of the absorption column and to constrain the eccentricity and inclination of the binary system.


Martin Scholtz

Mathematical aspects of the black holes Meissner effect

Meissner effect is a well-known property describing the expulsion of external electromagnetic fields from the horizon of extremal axially symmetric black holes. This phenomenon has been "observed theoretically" first for test fields in the Kerr spacetime, but today many examples of exact solutions exhibiting the Meissner effect exist, e.g. black hole immersed in Melvin magnetic universe. Nevertheless, whether the Meissner effect is a generic property of black holes remained unclear. We provide a general proof showing that Meissner effect holds for any axially symmetric horizon representing a black hole in equilibrium. In this talk we present the mathematical background of our proof, namely the Ashtekar's formalism of isolated horizons and show the intimate relation between the presence of the symmetries and the Meissner effect. Then we show that the presence of the Meissner effect is independent of the deformations of the horizon induced by an external matter. We demonstrate the transition from under-extremal to extremal case on a distorted Kerr black hole for which a new coordinate system and tetrad had to be developed. We briefly discuss possible implications of the Meissner effect on the efficiency of the Blandford-Znajek process. We extend our original proof also to the case of electrically charged black holes which requires a discussion on the actual meaning of the Meissner effect in such a case, since the non-linearity of general relativity makes it difficult to disentangle contributions of the external fields and the field induced by the black hole itself. Finally, we relate the Meissner effect to the uniqueness of extremal horizons and generalize previous uniqueness theorem by Lewandowski and Pawlowski to the case of black holes admitting conical singularities, showing that the Meissner effect holds also for C-metric black holes with possibly non-vanishing NUT parameter.


Vojtěch Witzany

Finite-size effects in extreme mass ratio inspirals

Extreme mass ratio inspirals are gravitational-wave inspirals of compact, stellar-mass objects into supermassive black holes that are expected to be detected by the space-based mission LISA. The motion of the compact object is modeled as the motion of a particle in the space-time of the supermassive black hole perturbed by the particle itself. However, additional corrections due to the finite size of the compact object must be included in an accurate waveform model. I will first discuss the scaling of these contributions to the orbital motion and show that it seems that we only need first order finite-size corrections to meet the LISA precision requirements. I will then proceed to present recent results on the analytical solution of the orbital motion under the first-order corrections.


Rhys Taylor

Simulating the evolution of optically dark H I clouds in the Virgo cluster: will no one rid me of this turbulent sphere?

Most detected neutral atomic hydrogen (H I) at low redshift is associated with optically bright galaxies. However, a handful of H I clouds are known which appear to be optically dark and have no nearby potential progenitor galaxies, making tidal debris an unlikely explanation. In particular, 6 clouds identified by the Arecibo Galaxy Environment Survey are interesting due to the combination of their small size, isolation, and especially their broad line widths atypical of other such clouds. A recent suggestion is that these clouds exist in pressure equilibrium with the intracluster medium, with the line width arising from turbulent internal motions. Here, we explore that possibility by using the FLASH code to perform a series of 3D hydro simulations. Our clouds are modelled using spherical Gaussian density profiles, embedded in a hot, low-density gas representing the intracluster medium. The simulations account for heating and cooling of the gas, and we vary the structure and strength of their internal motions. We create synthetic H I spectra, and find that none of our simulations reproduce the observed cloud parameters for longer than ˜100 Myr: the clouds either collapse, disperse, or experience rapid heating which would cause ionization and render them undetectable to H I surveys. While the turbulent motions required to explain the high line widths generate structures which appear to be inherently unstable, making this an unlikely explanation for the observed clouds, these simulations demonstrate the importance of including the intracluster medium in any model seeking to explain the existence of these objects.


Marek Abramowicz

Collisions of primordial black holes with neutron stars may explain the phenomenon of fast radio bursts

If primordial black holes constitute a (small) fraction of the dark matter halos of galaxies, their collisions with galactic neutron stars explain the puzzling fast radio burst (FRB) phenomenon, in particular the FRB high occurrence rate, large luminosity and short duration. In these collisions 1.5 M_Sun black holes are formed.


Wlodek Kluzniak

Radiative GRMHD simulations of accretion disks and the ULX problem

Inclusion of radiation in GRMHD simulations has allowed us to re-examine the question of stability of black hole accretion disks. Both the thermal instability and the viscous instability are apparent in our global simulations of thin accretion disks in the radiation-pressure dominated regime. We examine the context of Ultraluminous X-ray sources (ULX).

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