Observation of Very-High-Energy Gamma-Rays from Blazars with the MAGIC Telescope

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.