Chin-Shin Chang
PhD Thesis: Active Galactic Nuclei throughout the Spectrum: M 87, PKS 2052-47, and the MOJAVE sample
Active galactic nuclei (AGN) are one of the many mysteries in the Universe. A powerful jet is definitely the highlight of AGN. Astronomers have been studying AGN jets for the past decades by performing observations from the radio to the gamma-ray band, applying physical jet models to the observed properties, and trying to put the pieces into the whole picture.
In this project, we have been using different approaches.
With the VLBI technique, we are able to trace bright features being ejected from the center moving along the jet. In this thesis, we use the VLBI technique to detect a bright knot 80 pc from the center of the radio galaxy M 87. 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.
Studying multiband flares is one of the best ways to distinguish different mechanisms taken place in AGN. Current jet models suggest that time delays between multiband flares have different features while the emission mechanism differs. In this thesis, 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 TANAMI program are included in order to trace any change of jet morphology, if any; the millimeter radio flux density was measured by the APEX telescope; 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).
The broadband SED reflects the emission properties of AGN, and it shows the energy output in different wavebands. By simulating physical jet models and reconstructing the broadband SED, one can probe the emission mechanism, the jet composition, the magnetic field, the bulk velocity in jet, and the structure of AGN. 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. We constructed a broadband SED catalog from the radio to the gamma-ray band of the MOJAVE sample, and we applied polynomial 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.
About Me: I joined the VLBI group in the Max Planck Institute for Radio Astronomy for my doctoral research, under the supervision of Prof. Eduardo Ros and Prof. Anton Zensus. My research focused on understanding the dynamics and the physical properties of AGN jets. One of the main projects of my PhD was the study the inner jet of M87 using VLBA observations. I studied the dynamics and the evolution of the bright feature HST-1 located in the jet, 80 pc away from the M87 core. I analyzed 15 epochs of the 2cm VLBA data and performed wide-field imaging, which allowed me to detect HST-1 for the first time at 15GHz using VLBI observations. I am also interested in the multi-wavelength properties of AGNs, and I worked on the broadband spectral energy distribution (SED) properties of MOJAVE, which is a complete sample of blazars. This was done collecting data from the the radio to the gamma-ray band for 135 AGN, and combining the results of the SED fitting with their VLBI properties. We found that AGNs with higher jet speed also have higher frequency peaks in their broadband SEDs.
Dissertation Active Galactic Nuclei throughout the Spectrum: M 87, PKS 2052-47, and the MOJAVE sample