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

Discovery of Gas Bulk Motion in the Galaxy Cluster Abell 2256 with Suzaku

The results from Suzaku observations of the galaxy cluster Abell2256 are presented. This cluster is a prototypical and well-studied merging system, exhibiting substructures both in the X-ray surface brightness and in the radial velocity distribution of member galaxies. There are main and sub components separating by 3'.5 in the sky and by about 2000 km s$^{-1}$ in radial velocity peaks of member galaxies. In order to measure Doppler shifts of iron K-shell lines from the two gas components by the Suzaku XIS, the energy scale of the instrument was evaluated carefully and found to be calibrated well. A significant shift of the radial velocity of the sub component gas with respect to that of the main cluster was detected. All three XIS sensors show the shift independently and consistently among the three. The difference is found to be 1500 $\pm 300$ (statistical) $\pm 300$ (systematic) km s$^{-1}$. The X-ray determined absolute redshifts of and hence the difference between the main and sub components are consistent with those of member galaxies in optical. The observation indicates robustly that the X-ray emitting gas is moving together with galaxies as a substructure within the cluster. These results along with other X-ray observations of gas bulk motions in merging clusters are discussed.Comment: Accepted for publication in PASJ in 2011-03-2

Detection of Very-High Energy Gamma-Rays from the BL Lac Object PG 1553+113 with the MAGIC Telescope

The MAGIC telescope has observed very-high energy gamma-ray emission from the BL Lac object PG 1553+113 in 2005 and 2006 at an overall significance is 8.8 sigma. The light curve shows no significant flux variations on a daily timescale. The flux level during 2005 was, however, significantly higher as compared to 2006. The differential energy spectrum between approx. 90 GeV and 500 GeV is well described by a power law with a spectral index of -4.2+-0.3. The photon energy spectrum and spectral modeling allow to pose upper limits of z=0.74 and z=0.56, respectively, on the yet undetermined redshift of PG 1553+113. Recent VLT observations of this blazar show featureless spectra in the near-IR, thus no direct redshift could be determined from these measurements.Comment: 4 pages, 4 figures, to appear in the proceedings of the 30th International Cosmic Ray Conference, Merida, July 200

Prototyping Hexagonal Light Concentrators Using High-Reflectance Specular Films for the Large-Sized Telescopes of the Cherenkov Telescope Array

We have developed a prototype hexagonal light concentrator for the Large-Sized Telescopes of the Cherenkov Telescope Array. To maximize the photodetection efficiency of the focal-plane camera pixels for atmospheric Cherenkov photons and to lower the energy threshold, a specular film with a very high reflectance of 92-99% has been developed to cover the inner surfaces of the light concentrators. The prototype has a relative anode sensitivity (which can be roughly regarded as collection efficiency) of about 95 to 105% at the most important angles of incidence. The design, simulation, production procedure, and performance measurements of the light-concentrator prototype are reported.Comment: 21 pages, 14 figures, accepted for publication in JINS