T. Schweizer

Observations of 54 Active Galactic Nuclei with the HEGRA system of Cherenkov telescopes

A sample of 54 selected Active Galactic Nuclei (AGN) has been observed with the HEGRA stereoscopic system of Cherenkov Telescopes between 1996 and 2002 in the TeV energy regime. The observations were motivated by the positive results obtained for Mkn 421 and Mkn 501. The distances of the selected objects vary over a large range of redshifts between z = 0.004 and z = 0.7. Among the observed AGN are the now-established TeV-emitting BL Lac type objects H 1426+428 and 1ES 1959+650. Furthermore the BL Lac object 1ES 2344+514 and the radio galaxy M 87 show evidence for a signal on a4 σ level. The observation of 1ES 2344+514 together with the Whipple results firmly establishes this AGN as a TeV source. Several objects (PKS 2155-304, BL Lacertae, 3C 066A) that have been claimed as TeV γ-ray emitters by other groups are included in this data sample but could not be confirmed using data analysed here. The upper limits from several AGN included in this analysis are compared with predictions in the frame-work of SSC models.

Detection of Cherenkov light from air showers with Geigermode-APDs

Abstract We have detected Cherenkov light from air showers with Geigermode-APDs (G-APDs). G-APDs are novel semiconductor photon detectors, which offer several advantages compared to conventional photomultiplier tubes in the field of ground-based very high energy (VHE) γ -ray astronomy. In a field test with the MAGIC telescope we have tested the efficiency of a G-APD/light catcher setup to detect coinciding Cherenkov light from air showers. From our test we estimate a detection efficiency of our G-APD/light catcher setup to be ≥ 60 % higher than the efficiency of a MAGIC camera pixel. The dark count rates of the tested G-APD at ambient temperature are 10 times lower than the rates of detected photons from the night sky light background (NSB). This relaxes the need for cooling the G-APD to the level where the noise rate is sufficiently low. According to this recent test, G-APDs promise a major progress in ground-based VHE γ-ray astronomy.

Phase-resolved Crab pulsar measurements from 25 to 400 GeV with the MAGIC telescopes

We report on observations of the Crab pulsar with the MAGIC telescopes. Our data were taken in both monoscopic (> 25 GeV) and stereoscopic (> 50 GeV) observation modes. Two peaks were detected with both modes and phase-resolved energy spectra were calculated. By comparing with Fermi-LAT measurements, we find that the energy spectrum of the Crab pulsar does not follow a power law with an exponential cutoff, but has an additional hard component, extending up to at least 400 GeV. This suggests that the emission above 25 GeV is not dominated by curvature radiation, as suggested in the standard scenarios of the OG and SG models.

VHE gamma-ray measurements of the Crab nebula and pulsar by MAGIC

In the last two years 2010 and 2011 the Crab nebula and its pulsar have surprised the gamma-ray astrophysics community at least twice: the former with GeV flares detected by AGILE and Fermi-LAT, the latter with very high energy (VHE, E > 100 GeV) pulsed emission extending up to 400 GeV detected by MAGIC and VERITAS. These exciting results show that there is still much left to understand about this object. We report here the results of the MAGIC stereoscopic observations of the Crab nebula and pulsar carried on between October 2009 and April 2011. For the nebula, the analysis of these data yields a differential energy spectrum with an unprecedented precision spanning from 50 GeV to 45 TeV, a precise determination of its inverse Compton (IC) peak, and the measurement of the VHE emission during the GeV flare of April 2011. Implications on the modeling of the Crab nebula, especially regarding the strength of the magnetic field, are discussed. For the pulsar, a phase-resolved energy spectrum and a lightcurve i...