Jets are among the most energetic cosmic phenomena - The power it emits is extreme,that you would have to collect more than a billion suns to compete. The dynamic scale is also hysteric, as large at that the light will take 3 million years to travel. But what also excites us physicists is that they are unusually efficient particle accelerators (of course in space, that is).
This extreme jet power is most likely generated near the central massive black hole, probably via the conversion of gravitational energy from accreting matter. The fundamental questions are; how is the accretion energy channeled into the jets? and by what mechanisms are the particles accelerated to such high energies? Blazars are Active Galactic Nuclei(AGN) possessing jets aligned close to the line of sight, and thus the observed emission is dominated by the Doppler-enhanced jet component. This makes blazars ideal targets for studyings jet physics.
If I were to pick one word to describe the recent progress in the blazar study, it will be 'multi-wavelength'. Prompted by the discovery of γ-rays from over 60 blazars in the 1990s, many multi-wavelength campaigns have been held to measure the overall spectral energy distribution-ranging over 20 orders of magnitude in frequency. Many more theoretical modeling of the overall emission has been continued to keep up with the updated observations, and now we are starting to understand quite a lot about the radiation processes, especially concerning the average properties of blazars.
The approach taken in this thesis is to focus on the variability in blazars. Provided the new knowledge about the radiation processes, we can now step forward to explore the actual dynamics of the particles that are accelerated in the jets. X-rays are ideal for studying the variability, as this is where the electrons that are accelerated to the highest energy contribute, and accordingly the variability is most rapid. Furthermore, the observational advantage in the X-rays is the ability for continuous monitoring with stable statistics. In particular for the brightest sources (such as the TeV blazars discussed in this thesis), the coverage of a rapid flare is far better than in any other frequency range. Little contamination from emission sources other than the jet (e.g., galaxy or nuclear emission) is also a big advantage in X-ray observations.
Chapter 1 gives a brief introduction to blazars, including the definition of the class. I will also describe what has been solved in the recent studies of blazars, which serves as the background of the study. Chapter 2 includes the description of the instrumentation used for the observations. The data screening and reduction will also be described here.
The following 2 chapters describes the observational results. Chapter 3 describes the results from the long-look X-ray observations we performed with ASCA, mainly concerning the light curves, and Chapter 4 will focus on the X-ray spectrum. In these two chapters, we try to generalize the variability properties observed from TeV blazars.
Chapter 6 and 7 includes the interpretation and discussion on the observed variability in blazars. In Chapter 7, we consider the full pattern of the light curve. This is the first attempt to explain the variability as a series of flares. In particular, we show that the internal shock scenario can explain all the observed variability properties naturally, with only one condition - that the initial fluctuation of the bulk Lorentz factors of the ejecting shells, are small. A brief summary will given at the end.