For the first time an excess of photons above an energy threshold of 730 GeV from the giant radio galaxy M 87 has been measured at a significance level above 4 σ. The data have been taken during the ...years 1998 and 1999 with the HEGRA stereoscopic system of 5 imaging atmospheric Cherenkov telescopes. The excess of $107.4\,\pm\,26.8$ events above 730 GeV corresponds to an integral flux of 3.3% of the Crab flux or $N_\gamma(E > 730\,\mbox{GeV}) = (0.96 \pm 0.23) \times 10^{-12}$ phot cm-2 s-1. M 87 is located at the center of the Virgo cluster of galaxies at a relatively small redshift of $z = 0.00436$ and is a promising candidate among the class of giant radio galaxies for the emission of TeV γ-radiation. The detection of TeV γ-rays from M 87 – if confirmed – would establish a new class of extragalactic source in this energy regime since all other AGN detected to date at TeV energies are BL Lac type objects.
TeV γ-rays from the BL Lac object 1ES 1959+650 have been measured during the years 2000 and 2001 with a significance of 5.2 σ at a value of 5.3% of the Crab flux and in May 2002 during strong ...outbursts with >$23\,\sigma$ at a flux level of up to 2.2 Crab, making 1ES 1959+650 the TeV Blazar with the third best event statistics. The deep observation of 197.4 h has been performed with the HEGRA stereoscopic system of 5 imaging atmospheric Cherenkov telescopes (IACT system). 1ES 1959+650 is located at a redshift of $z = 0.047$, providing an intermediate distance between the nearby Blazars Mkn 421 and Mkn 501, and the much more distant object H1426+428. This makes 1ES 1959+650 an important member of the class of TeV Blazars in view of the absorption of TeV photons by the diffuse extragalactic background radiation (DEBRA). The differential energy spectrum of 1ES 1959+650 during the flares can be fitted by a power law with a spectral index of $2.83 \pm 0.14_{\mbox{\tiny stat}} \pm 0.08_{\mbox{\tiny sys}}$ or by a power law with an exponential cut-off at $(4.2^{+0.8}_{-0.6~{\mbox{\tiny stat}}} \pm 0.9_{\mbox{\tiny sys}})$ TeV and a spectral index of $1.83 \pm 0.15_{\mbox{\tiny stat}} \pm 0.08_{\mbox{\tiny sys}}$. The low state differential energy spectrum obtained with lower statistics can be described by a pure power law with a spectral index of $3.18 \pm 0.17_{\mbox{\tiny stat}} \pm 0.08_{\mbox{\tiny sys}}$.
The detection of TeV γ-rays from the blazar H 1426+428 at an integral flux level of $(4 \pm 2_{\mathrm{stat}} \pm 1_{\mathrm{syst}}) \times 10^{-12}~\mathrm{erg}~\mathrm{cm}^{-2}~\mathrm{s}^{-1}$ ...above 1 TeV with the HEGRA imaging atmospheric Cherenkov telescope system is reported. H 1426+428 is located at a redshift of $z =$ 0.129, which makes it the most distant source detected in TeV γ-rays so far. The TeV radiation is expected to be strongly absorbed by the diffuse extragalactic background radiation (DEBRA). The observed energy spectrum of TeV photons is in good agreement with an intrinsic power law spectrum of the source $\propto E^{-1.9}$ corrected for DEBRA absorption. Statistical errors as well as uncertainties about the intrinsic source spectrum, however, do not permit strong statements about the density of the DEBRA infrared photon field.
The level of the night-sky background light at La Palma and Namibia was determined, with emphasis on the wavelength region and solid angle coverage relevant for the operation of imaging atmospheric ...Cherenkov telescopes. The dependence of the night-sky background light both on celestial coordinates (alt,az) and on galactic coordinates (
b,
l) was measured, with an angular resolution of about 1°. Average light levels near the zenith are similar in both locations −2.2×10
12–
2.6×10
12
photons
sr
−1
s
−1
m
−2
for
300
nm<λ<650
nm
. With increasing zenith angle the level of background light increases at La Palma, whereas a constant level is measured in Namibia. Near the center of the Milky Way, background light levels are increased by a factor up to 4 and more. Also the level of light backscattered from the ground has been studied.
Between early 1997 and late 2002, the HEGRA collaboration operated a stereoscopic system of four (later five) imaging atmospheric Cherenkov telescopes. In this paper we present the calibration ...schemes which were developed for the system, and report on the performance of the detector over the years. In general, the telescope system was very well understood, regarding both the absolute calibration and the slight changes in performance over the years. The system had an energy threshold of 500 GeV for observations at zenith and under optimum detector conditions. With the corresponding calibration schemes, a systematic accuracy of 15% on the absolute energy scale has been achieved. The continuous sensitivity monitoring provided a relative accuracy of a few percent, and showed that the threshold did not exceed 600 GeV throughout the entire operation time. The readout electronics and the imaging quality of the dishes were well monitored and stable. The absolute pointing had an accuracy of at least 25
″; this number was guaranteed throughout the whole lifetime of the experiment.
Data taken with the HEGRA system of imaging atmospheric Cherenkov telescopes during the 1997 flares of Markarian 501 (Mkn 501) are reanalyzed using an algorithm providing improved energy resolution. ...A resolution of 10% to 12% is obtained by accounting for the variation of the Cherenkov light yield with the height of the shower maximum in the atmosphere. The improved energy resolution is particularly relevant for the study of the high-energy cutoff in the spectrum, which might be caused by interactions with the intergalactic infrared background radiation. The reanalysis presented here confirms the results obtained in the previous analysis, but hints a steeper slope of the spectrum in the region around 20 TeV.
PDF
