Seminars archive

Local Hubble Expansion

Michal Křížek

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.

VLBI and its application for building reference frames for spacecraft navigation

Christopher Jacobs

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.

Imaging of extragalactic radio sources at K-band with VLBI

Alet de Witt

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.

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

Joerg Dabringhausen

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.

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

James Fuller

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.

Galactic aberration estimated from VLBI geodetic data

Hana Krásná

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.

Astronomy and cartoons -- Argentine graphic humor

Juan Matías Loiseau (TUTE)

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), http://sk2018.svetknihy.cz/userdata/files/2018/katalog-programu/09-katalog-program-2018-nedele.pdf

The quest for a GW stochastic background with LIGO and Virgo

Tania Regimbau

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.

A survey of theories of gravity and their testing with cosmology

Constantinos Skordis

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.

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

Lýdia Štofanová

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.

Accreting black holes via X-ray polarimetry

Romana Mikušincová

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.

TBA (LSB galaxies)

Boris Deshev