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Hayashida, Masaaki (2008): Observation of Very-High-Energy Gamma-Rays from Blazars with the MAGIC Telescope. Dissertation, LMU München: Fakultät für Physik



Blazars, a subclass of active galactic nuclei in which the jet is aligned very close to our line of sight, can accelerate charged particles to relativistic energies in the jet. Electromagnetic emission from this class of sources can be observed from radio up to TeV energies. The MAGIC telescope is an Imaging Atmospheric Cherenkov Telescope with a 17-m diameter dish, located on the Canary Island of La Palma, in operation for exploring a new window of very high energy (VHE) gamma-ray bands, above 50 GeV. Searching for new VHE gamma-ray blazars, BL Lacertae was observed with the MAGIC telescope in 2005 and 2006. A VHE gamma-ray signal was discovered with a 5.1 sigma excess in the 2005 data. This discovery established a new class of VHE gamma-ray emitters, "low-frequency peaked BL Lac objects". On the other hand, the 2006 data showed no significant excess. This drop in flux followed the observed trend in the optical activity. The MAGIC telescope continuously observed the bright known blazars Mkn501, 1ES1959+650 and Mkn421. In particular, extensive simultaneous multiwavelength observations with the MAGIC telescope and the X-ray Satellite Suzaku were carried out for Mkn501 in July 2006 and for 1ES1959+650 in May 2006. VHE gamma-ray signals from about 100 GeV to a few TeV were clearly detected. For the first time, the VHE gamma-ray spectra were simultaneously obtained with the X-ray spectra during their low states of activity. Long term observations of Mkn421 in 2006 showed a strong variability in VHE gamma-ray emission. The spectral energy distributions (SEDs) of these four blazars could be well explained by a homogeneous one-zone synchrotron self-Compton model. This model suggests that the variation of the injected electron population in the jet is responsible for observed variations of the SEDs of the blazars. For all sources, the derived magnetic field strength in the jet and the Doppler beaming factor showed similar values. A contribution on the hardware sector is also presented in this thesis. For further lowering energy threshold in the MAGIC project, a new type of photosensor, "HPDs with an 18-mm diameter GaAsP photocathode", were developed. A quantum efficiency of the photocathode could reach over 50 %. Compared to the PMTs currently used in MAGIC, the new photosensors would improve the overall Cherenkov photon conversion efficiency by a factor of 2. Other performance values including lifetime also fulfilled the requirements of photosensors to be used in the MAGIC telescope.