Other
Scientific paper
Oct 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010phdt........66c&link_type=abstract
PhD Thesis, University of Cologne, 280 pages
Other
Scientific paper
The aim of this thesis is to study the extragalactic jets by using different approaches. We investigate: (i) a bright feature in the jet of the radio galaxy M87 as observed using high-resolution VLBI technique; (ii) a multiband flare of the blazar PKS 2052-47 by conducting a multifrequency campaign; (iii) the broadband spectral energy distribution (SED) of a radio-selected, statistically-complete sample of the most conspicuous AGN in the Northern sky: the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE) sample.
We use the VLBI technique to detect a bright knot 80 pc from the center of the radio galaxy M87. It was suggested that the HST-1 knot was the site where a TeV flare of M87 was generated from. By analyzing the VLBI data at 15 GHz from 2000 to 2009, we detected HST-1 during 2003 and 2007. We studied the apparent speed, the flux variability, and the spectral properties of HST-1. Although we found that the light curve of the compact HST-1 region at VLBA 15 GHz peaked in 2005, we saw that the HST-1 region was very extended and had a steep spectrum; the projected apparent speed of HST-1 was subluminal. Therefore, our results do not completely support the hypothesis.
We studied the flat spectrum radio quasar PKS 2052-47, which experienced an optical flare followed by a gamma-ray flare in July 2009. We arranged a multiwavelength campaign from radio to gamma-ray after the blazar's flare. In the campaign, VLBI observations by the Tracking Active galactic Nuclei with Austral Milliarcsecond Interferometry (TANAMI) program are included in order to trace any change of jet morphology, if any; the millimeter radio flux density was measured by the Atacama Pathfinder EXperiment (APEX); the optical and the X-ray band observations were obtained by the Swift UV/Optical telescope (UVOT) and X-ray telescope (XRT); the gamma-ray observations were obtained by the Fermi Large Area Telescope (LAT). We do not see significant change in the X-ray photon indices and fluxes during the 5-day Swift monitoring of the source.
We study the broadband SED from the radio to the gamma-ray band of a radio-selected sample. The MOJAVE program has been monitoring 135 radio-bright, statistically-complete sample of AGN using the Very Long Baseline Array (VLBA) at 15 GHz for a decade, and the MOJAVE sample is consisted of mostly blazars, which are AGN as seen jet-on. The typical broadband SED of a blazar has a double-hump profile. It is believed that the lower-energy hump is caused by the synchrotron emission from the AGN jet, and the higher-energy hump is produced by the inverse-Compton (IC) up-scattering of the seed photons from the jet or other external sources. We constructed a broadband SED catalog of the MOJAVE sample, and we applied polpolynomial fits to the observed data at the low and the high energy humps of the SED as a first approach to understand the properties of the sources. The peak positions of the two humps can be derived from the polynomial fit for many of the MOJAVE sources except for some sources having deviated SED profiles.
In this thesis, we try to understand the AGN puzzles by using different approaches. We contributed in understanding the emission nature of AGN with novel techniques and instruments, and our efforts help to make a step forward to reveal the whole picture.
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