Observations with the HEGRA stereoscopic system

The HEGRA system of imaging air Cherenkov telescopes has successfully pioneered the stereoscopic observation technique of extensive air showers. The observational method is briefly described and important results of recent observations of sources of photons with TeV($=10^{12}$ eV)-energies are summarized: The first detection of a TeV $\gamma$-ray signal from the shell-type supernova remnant Cassiopeia A and preliminary results obtained from the observation of strong variability of the extragalactic source Mkn 421 during observations carried out from February to May 2000.Comment: 4 pages, 3 figures, to appear in the Proceedings of the XXIst Rencontres de Moriond "Very High Energy Phenomena in the Universe", Les Arcs, France, Jan. 20-27 (2001

Cross Calibration of Imaging Air Cherenkov Telescopes with Fermi

An updated model for the synchrotron and inverse Compton emission from a population of high energy electrons of the Crab Nebula is used to reproduce the measured spectral energy distribution from radio to high energy gamma-rays. By comparing the predicted inverse Compton component with recent Fermi measurements of the nebula's emission, it is possible to determine the average magnetic field in the nebula and to derive the underlying electron energy distribution. The model calculation can then be used to cross calibrate the Fermi observations with ground based air shower measurements. The resulting energy calibration factors are derived and can be used for combining broad energy measurements taken with Fermi in conjunction with ground based measurements.Comment: 2009 Fermi Symposium, eConf Proceedings C091122, 5 pages, 5 figures, 3 table

Multi-wavelength observations of the TeV Blazars Mkn~421, 1ES1959+650, and H1426+428 with the HEGRA Cherenkov telescopes and the RXTE X-ray satellite

Recent results obtained with the HEGRA system of imaging Cherenkov telescopes on the TeV emission of the Blazars Mkn~421 ($z=0.030$), 1ES1959+650 ($z=0.047$), and H1426+428 ($z=0.129$) are reported. For Mkn~421, a close connection of the average flux level and spectral shape has been observed during the periods of increased activity in the years 2000 and 2001. Simultaneously taken data with the RXTE X-ray satellite reveal a complex light curve at X-ray and TeV energies. After a deep exposure of 94 hrs, the object 1ES1959+650 was detected at the significance level of 5.4$\sigma$ with a soft energy spectrum following a power-law with a photon-index of $3.3 \pm 0.7$. During recent observations in May 2002, the source has shown increased activity with indications for a flattening of the energy spectrum. The high energy peaked Blazar H1426+428 has recently been identified as a source of TeV photons. Since the source is fairly distant (z=0.129), absorption of TeV photons due to pair-production on the optical and near infrared extragalactic light becomes important and should leave a signature in the observed TeV energy spectrum. Notably, the TeV energy spectrum determined with the HEGRA system of Cherenkov telescopes agrees with the expectation of a strongly absorbed source spectrum

Large zenith angle observations of Mkn 421 with H.E.S.S

Mkn 421 was observed during 2004 with the fully operational High Energy Stereoscopic System (H.E.S.S.) in Namibia. The observations were carried out at zenith angles of 60°-67°, which result in an average energy threshold of 1.5 TeV and a collection area reaching 2 km² at 10 TeV. The light-curve of integrated fluxes above 2 TeV shows changes of the diurnal ux up to a factor of 4.3. For nights of high flux, intra-night variability is detected with a decay time of less than 1 hour. The time averaged energy spectrum is curved and is well described by a power-law with a photon index 2.1±0.1 and an exponential cutoff at 3.1 (+0.5 - 0.4)TeV and an average integral flux above 2 TeV of 3 Crab ux units. Significant variations of the spectral shape are detected with a spectral hardening as the ux increases. Contemporaneous multi-wavelength observations at lower energies (X-rays and gamma-rays above ± 300 GeV) indicate smaller relative variability amplitudes than seen above 2 TeV during the high flux state