Sebastian Kiehlmann

PhD Thesis: Origin of the gamma-ray emission in AGN jets - A multi-wavelength photometry and polarimetry data analysis of the quasar 3C279
Sebastian Kiehlmann

PhD Supervisor: Dr. Tuomas Savolainen

Collaborators: I. Agudo, M.F. Aller, H.D. Aller, L. Berdnikov, V. Chavushyan, L. Fuhrmann, Y.
Fukazawa, M. Gurwell, J. Heidt, S. G. Jorstad , O.M. Kurtanidze, A. Lahteenmaki, A. Marscher,
K. Nilsson, Sang-Sung Lee, F. K. Schinzel, K.V. Sokolovsky, J.L. Tavares, J.A. Zensus

PhD Examination: June 26, 2015

The project  One of the main topics regarding the physics of AGN jets is the origin of the γ-ray emission. he favoured model explaining the production of high energy radiation in blazars is inverse Compton scattering. Though numerically and empirically successfully tested, two major questions remain topics of substantial discussion: First, where is the seed photon field coming from? Does it originate in the jet itself (synchrotron self-Compton (SSC)) or in the accretion disc, the dust torus or the broad line region (external Compton (EC))? And second, where in the jet does the inverse Compton scattering take place?

My thesis aimed to locate the γ-ray emission site in the archetypical blazar 3C 279 based on the multi-frequency photometry data provided by the Quasar Movie Project. This data set includes 140 light curves at more than twenty bands, providing dense sampling in frequency and time domain over more than two years. These data allow us to analyse the variability of the light curves and to perform cross-correlation analysis over a large range of frequencies. We estimate the variability power spectra at 26 frequencies. We find similar indices of a power-law spectrum at sub-mm bands and X-rays on the one hand, and at ultraviolet and γ rays on the other hand. Additionally, we find a strong correlation between X-rays and the 1 mm light curve at short variability time scales. We can infer that the X-ray emission site is located at the mm~VLBI core and that X-rays are produced either by synchrotron self-Compton scattering of mm-wavelength synchrotron photons or by external Compton scattering of photons originating from the cosmic microwave background. The correlation between X rays, γ rays, and optical bands exhibits complex behaviour. Time lags between the bands change over time, indicating probably different emission sites and different physical conditions. But we find some indication that the γ-ray emission site is located, at least occasionally, at the mm VLBI core. Thus, it is located beyond the broad line region, where infrared photons either from the jet itself or from the dust torus may serve as seed photons for the inverse Compton scattering to GeV energies.

Photometry and polarimetry of 3C279 from May 2008 to September 2012. From top to bottom: gamma-ray light curve (first panel), x-ray light curve (second panel), ultraviolett, optical and infrared light curves (third panel), sub-mm and mm light curves (fourth panel), cm light curves (fifth panel), optical and mm electric vector position angle (bottom panel).

The combined optical polarimetry data provided by the Quasar Movie Project yields an unprecedentedly well sampled polarization curve of 3C 279 which shows strong variability with rotations of the polarization angle in both directions with different rotation rates and amplitudes. We introduce various, new methods to analyse the polarization variability. We test different classes of stochastic models against the observed data and come to the conclusion that the polarization variability of 3C 279 is following two different processes. During a low brightness state the polarization is consistent with a stochastic process. During a flaring state the variability is dominated by a different process. The preferred model is that of an emission feature on a helical path in a helical magnetic field.

About me  I was born in Moers, Germany. I have studied physics in Göttingen and received my diploma in October 2010 for studying the distribution of supermassive black hole masses, luminosities and the Eddington ratio of active galactic nuclei at different redshifts at the Institute for Astrophysics Göttingen. I stayed at the University of Göttingen being involved in the e-learning portal and supervising physics and astrophysics lab courses, before moving to Bonn in May 2012, joining the IMPRS for Astronomy and Astrophysics at the Max Plack Institute for Radio Astronomy. After finishing my doctoral period in Bonn I moved to Aalto University as postdoctoral researcher.  My future prospects are to continue my research especially in the science field of active galactic nuclei at a research institute or university and to engage myself in the educational branch of academia.

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