Observations of variability of TeV gamma-ray blazars

Abstract

The boom in ground-based gamma-ray astronomy since the beginning of the 21st century has enabled a new probe of the universe using very-high-energy photons. The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is an array of four 12-m imaging Cherenkov telescopes that is sensitive to gamma rays in the energy range between ~100 GeV and ~30 TeV. Among all known TeV sources, blazars, a particular type of active galactic nuclei, have shown exceptional variabilities over a wide range of timescales and energies. The observations of such variabilities have been previously limited at lower energies, ranging from radio to X-ray. However, the superior sensitivity of VERITAS has made the detection of fast TeV gamma-ray variability of blazars possible. The studies of their gamma-ray variability can, in a relatively model-independent way, shed significant light on the emitting regions and production mechanisms in blazars. This thesis describes my work on blazar variability, based primarily on the VERITAS observations but are interpreted in a multi-wavelength context. One of the most exceptional phenomena observed in blazars with VERITAS is the fast variability of the TeV gamma rays. The short duration of these flares strongly constrains the size of the emitting region, and provides insights to the kinetics and location of the emitting region. We describe the fast TeV flare of BL Lacertae as an example, and discuss the connection between TeV flares and multi-wavelength observations that may help localize the TeV emitting region. To study the persistent variability of TeV blazars, we examine a variety of statistical properties in the time and frequency domains. We study both local properties of time series, e.g. time lags between different energy bands and spectral hysteresis during flares, and global properties, e.g. variability amplitude and power spectrum. These properties are connected to the physical processes in blazars, although they are also limited by the time resolution and sampling of the observations. We also test the statistical methods to obtain these properties with simulated light curves to study their effectiveness under different circumstances