Rauch, Christoph (2016). High frequency VLBI observations of galactic and extragalactic sources. PhD thesis, Universität zu Köln.


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The radio source Sagittarius A* (Sgr A*), which is commonly associated with the super-massive black hole in the Galactic Center, offers the largest angular dimension of an active galactic nucleus and, therefore, the best opportunity to study the underlying physics of these objects. While orbital parameters of nearby stellar sources have proven its existence beyond reasonable doubt, its energy production mechanism is still uncertain. Sgr A* shows intensity outbursts, referred to as flares, which occur on time scales ranging from 7-10 minutes (sub-flares) to 1-2 hours (main-flares). Several models are currently under investigation, that can explain the observable properties of the quiescent and flaring state of Sgr A*. The observed amplitudes and time scales of near-Infrared (NIR) and X-ray flares are consistent with expected values of an expansion with an adiabatic cooling mechanism. Alternatively, a hot spot model, is also capable of reproducing the observed parameters. Such a feature is, in analogy to the solar coronal mass ejection, formed by flux ropes, which are capable of rapidly expelling material, if their equilibrium stability is lost. Other models explain these flux excesses by a temporary accretion disc with a short time jet or a mixed Jet-advection-dominated accretion flow. A set of observable parameters, that can be used to discriminate between the currently proposed models, are the full-width at half max (FWHM) Gaussian size of the emission region, the absolute position of Sgr A*, its symmetry, as well as time delays and time scales of its flares. Relativistic magnetohydrodynamic simulations predict an almost constant size and shape of the emission region. Therefore, a change of its FWHM size, position or symmetry would point towards a jet or hot spot model. The expected changing source size and/or asymmetry are caused by an adiabatically expanding, orbiting or spatially constant, off-center component. In addition to this, the flares of Sgr A* show correlated flux excesses at different wavelengths ranging from NIR/X-ray to radio frequencies. The time delay found between these frequencies offers the opportunity to analyze physical properties that arise at different radii, because individual observing bands measure distinct radial regions. The subject of this thesis is to discriminate between the described flaring models of Sgr A*. Three six-hour Very Long Baseline Interferometry (VLBI) observations at 43 GHz have been performed on May 16 - 18 2012 in parallel to NIR observations carried out at the Very Large Telescope (VLT). A NIR flare observed on May 17 triggered the VLBI observation of this date, which shows a flare peaking at 1.41 Jy, delayed by (4.5±0.5) h. This increasing flux density is apparently accompanied by a changing source structure, that can best be modeled by a central emission region with a weak secondary component of 0.02 Jy at about 1.5 mas under 140° (east of north). Different cleaning methods have been tested, which lead to consistent maps of Sgr A* showing an off-center component in both polarizations. Symmetry-tests, based on analysis of closure phases, residual noise maps and simulated artificial data sets, improved the robustness of this two component model. Furthermore, positions acquired from phase referencing analysis offer large error limits and have to be considered as constant. The observed position of the secondary component, which is still affected by interstellar scattering, offers speeds of (0.4 ± 0.3) c for the presented time delay of (4:5 ± 0:5) h between the NIR and 7mm flare. This time lag represents the velocity between different radial regions around the black hole mapped by the individual frequencies. Relating this and several other delays, provided by the current literature, to theoretical intrinsic source sizes and interpolating them to a common NIR/X-ray center, results in a radial velocity profile of Sgr A* that appears to be accelerating towards the outermost regions. The complexity of required calibration and technical observing inaccuracies does not allow to completely exclude weather or other random observational effects to be the cause of the extended source structure, but we have shown, that with most careful cleaning efforts, this two component structure stays robust and is most probable a real data feature. The complete quantity of indications of a secondary component during the flaring states of Sgr A* are within one to two sigma. The consistent set of hints towards a two component structure excludes the hot spot model, because of its spatial dimension, and points towards the existence of a temporary jet anchored at Sgr A*.

Item Type: Thesis (PhD thesis)
CreatorsEmailORCIDORCID Put Code
URN: urn:nbn:de:hbz:38-68715
Date: 22 February 2016
Language: English
Faculty: Faculty of Mathematics and Natural Sciences
Divisions: Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics I
Subjects: Physics
Uncontrolled Keywords:
Active Galactic NucleiEnglish
Galactic CenterEnglish
Date of oral exam: 21 April 2016
NameAcademic Title
Eckart, AndreasProf. Dr.
Refereed: Yes
URI: http://kups.ub.uni-koeln.de/id/eprint/6871


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