Wednesday Colloquium


"What Can Tidal Disruption Events Teach Us About Black Hole Accretion?"

Mitchell C. Begelman (JILA, Boulder, Colorado)

For a year or more after a star is tidally disrupted by a black hole, debris can fall back at a rate that greatly exceeds the Eddington limit. Both observations and theoretical arguments indicate that mass loss is unable to regulate the rate at which matter is actually swallowed by the hole, leading to black hole growth rates and energy outputs that can exceed the Eddington limit by orders of magnitude. I will explain why regulation fails in such a case, and explain how this alternate mode of black hole growth could also be crucial for gamma-ray bursts and the rapid growth of supermassive black holes during the epoch of galaxy formation. I will also suggest that hyperaccreting black holes may be associated with the fastest jets, and that these are propelled by radiation pressure instead of magnetic forces.


"Polarization as a diagnostic of the emission processes in relativistic jets"

Markus Böttcher (North-West University, Potchefstroom, RSA)

Astrophysical sources exhibiting relativistic jets, such as active galactic nuclei (AGN) and gamma-ray bursts (GRBs) are strong emitters of radiation across the electromagnetic spectrum, from radio to gamma-rays. In spite of intensive, co-ordinated observing efforts over the past several decades, the nature of the radiating particles, the location of the gamma-ray emissino region, and the mechanism leading to the emission of high-energy radiation, are still uncertain. As a new strategy to break the degeneracy of different models proposed for the origin of gamma-rays from relativistic jets, many observatories are now including measurements of optical polarization into multi-wavelength studies of relativistic jet sources, and a new generation of X-ray polarimeters is currently being developed. These studies have, in several instances, revealed large rotations of the position angle of the optical polarization, correlated with gamma-ray flaring activity in blazars, a highly beamed class of jet-dominated AGN. In this talk, I will summarize recent theoretical predictions of the high-energy (X-ray and gamma-ray) polarization from different blazar models, discuss the implications of high-energy polarization on gamma-gamma absorption of high-energy gamma-rays within the emission region, and present a new theoretical interpretation of the optical polarization-angle swings associated with gamma-ray flares in blazars.


"Feedback from deeply-embedded protostars probed with Herschel / PACS"

Agata Karska (MPE Garching / Leiden Observatory / UAM Poznań)


"Simulations of flux variability of oscillating accretion fluid tori around Kerr black holes"

Kateřina Goluchová (Slezská univerzita v Opavě)

We present a study of oscillating accretion tori orbiting in the vicinity of relativistic compact objects. The study was performed on the background of the Kerr spacetime geometry. We have demonstrated that a significant variation of the observed flux can be caused by combination of radial and vertical oscillations modes of a slender, polytropic, perfect fluid, non-selfgraviting torus with constant specific angular momentum.


"Radio polarisation – a tool for study of cosmic magnetic fields"

Richard Wielebinski (Max-Planck-Institut für Radioastronomie, Bonn)

The Zeeman Effect was the first method of remote study of cosmic magnetic fields. Using optical polarisation observations the magnetic field in the Sun (Hale, 1908) and later in stars (Babcock, 1947) were detected. The development of radio astronomy methods led to a first report of the polarisation of Solar emission in 1946 already. In a way this detection paved the way to the interpretation of the origin of radio cosmic radio waves. For a time optical polarisation was also used for studies of magnetic fields (Hiltner; Hall, 1949). However there was a long discussion in the optical community about the origin of the polarisation – scattering, aligned dust, etc. Theoretical work on the origin of radio emission came quickly to the conclusion that the intense radio waves were non-thermal in origin, generated in magnetic fields. The bold suggestion of Shklovsky (1953) that the optical (and radio) emission of the Crab Nebula had its origin from energetic particles entering the magnetic field of the Nebula quickly led to the realisation that the synchrotron emission process was responsible. Since the synchrotron emission is polarised the argument could be inverted: by measuring the radio polarisation the studies of cosmic magnetic fields became possible. At first the observations of radio polarisation were limited due to instrumental problems. In particular it was necessary to move to higher radio frequencies where the Faraday Effect was negligible. The solar community studied magnetic fields since 1946. The first reported detection of radio polarisation in a distant cosmic object was in fact of the Crab Nebula in 1957. The next cosmic object that showed polarisation was Jupiter (1960). The year 1962 was the big bonanza year for polarisation: the Milky Way and radio galaxies were shown to be polarised. The Faraday Effect became a further source of information about cosmic magnetic fields: while the linear polarisation gives information about the field normal to the line of sight the Faraday Effect gives information about the fields parallel in the line of sight. From these early times the research field of cosmic magnetic fields has grown enormously. On the one hand the Milky Way has been studied in great detail. There is a clear connection between magnetic fields and star formation. Studies of nearby galaxies have shown surprisingly well oriented magnetic fields. Going further in the Universe magnetic fields exist in jets and lobes of radio galaxies. Practically every distant cosmic object has been detected as a non-thermal radio source and hence must have magnetic fields. Magnetic fields have been detected in the intergalactic space in clusters of galaxies. In my talk I will sketch both the observational and the theoretical developments up to the present day.