The nearby BL Lacertae (BL Lac) object Markarian 421 (Mkn 421) at a red shift $z=0.031$ was observed to undergo strong TeV γ-ray outbursts in the observational periods from December 1999 until May ...2001. The time averaged flux level $F(E>1{\rm \,TeV})$ in the 1999/2000 season was $(1.43\pm0.04)$ $\times$ 10-11 ph cm-2 s-1, whereas in the 2000/2001 season the average integral flux increased to $(4.19\pm0.04)$ $\times$ 10-11 ph cm-2 s-1. Both energy spectra are curved and well fit by a power law with an exponential cut-off energy at $3.6(+0.4-0.3)_{\rm stat}(+0.9-0.8)_{\rm sys}$ TeV. The respective energy spectra averaged over each of the two time periods indicate a spectral hardening for the 2000/2001 spectrum. The photon index changes from $2.39\pm0.09_{\rm stat}$ for 1999/2000 to $2.19\pm0.02_{\rm stat}$ in 2000/2001. The energy spectra derived for different average flux levels ranging from 0.5 to 10 $\times$ 10-11 ph cm-2 s-1 follow a clear correlation of photon index and flux level. Generally, the energy spectra are harder for high flux levels. From January to April 2001 Mkn 421 showed rapid variability (doubling time as short as 20 min), accompanied with a spectral hardening with increasing flux level within individual nights. For two successive nights (MJD 51989-51991, March 21-23, 2001), this correlation of spectral hardness and change in flux has been observed within a few hours. The cut-off energy for the Mkn 421 TeV spectrum remains within the errors constant for the different flux levels and differs by $\Delta E=2.6\pm0.6_{\rm stat}\pm0.6_{\rm sys}$ TeV from the value determined for Mkn 501. This indicates that the observed exponential cut-off in the energy spectrum of Mkn 421 is not solely caused by absorption of multi-TeV photons by pair-production processes with photons of the extragalactic near/mid infrared background radiation.
TeV γ-ray signals from the Crab Nebula and Mkn 501 were detected with the HEGRA CT1 imaging Cerenkov telescope during periods when the moon was shining and during twilight. This was accomplished by ...lowering the high voltage supply of the photomultipliers in fixed steps up to 13%. No other adjustments were made and no filters were used. Laser runs could not establish any nonlinearity in the gain of the individual pixels, and the trigger rate was uniform over the whole camera. The energy threshold was increased by up to a factor of two, depending on the amount of HV reduction. In a series of observations lasting 11.7 hours, a signal with a 3.4σ significance was detected from the Crab. During the 1997 multiple flare episode of Mkn 501 a 26σ combined excess was recorded during 134 hours of observations under various moonshine/twilight conditions. The results show that this technique can easily be adapted to increase the exposure of a source, which is important for sources showing rapid time variability such as AGNs or GRBs. Observations can be made up to ∼ 20° angular separation from the moon and until the moon is 85% illuminated (ten to eleven days before and after new moon), as well as during 20 to 40 minutes during twilight, before the commencement of astronomical darkness.
Using the HEGRA system of imaging atmospheric Cherenkov telescopes, a region of the Galactic plane ($-10^\circ < b < 5^\circ$, $38^\circ < l < 43^\circ$) was surveyed for TeV gamma-ray emission, both ...from point sources and of diffuse nature. The region covered includes 15 known pulsars, 6 known supernova remnants (SNR) and one unidentified EGRET source. No evidence for emission from point sources was detected; upper limits are typically below 0.1 Crab units for the flux above 1 TeV. For the diffuse gamma-ray flux from the Galactic plane, an upper limit of $6.1\times10^{-15}$ ph cm-2 s-1 sr-1 MeV-1 was derived under the assumption that the spatial distribution measured by the EGRET instrument extends to the TeV regime. This upper flux limit is a factor of about 1.5 larger than the flux expected from the ensemble of gamma-ray unresolved Galactic cosmic ray sources.
The H.E.S.S. observatory was recently extended with a fifth telescope located at the center of the array - H.E.S.S. II. With a reflector roughly six times the area of the smaller telescopes and four ...times more pixels per sky area, this new telescope can resolve images of particle showers in the atmosphere in unprecedented detail and explore the gamma-ray sky in the poorly studied regime around a few tens of Giga electron-volt. H.E.S.S. II has been equipped with a high-performance drive system that can deliver the high torque necessary to accelerate and slew the 600 tonnes telescope while keeping a good tracking accuracy. A modular design with a high degree of redundancy has been employed to achieve stability of operation and to ensure that the telescope can be moved to a safe position within a short period of time. Each axis is driven by four 28 kW servo motors which are pair-wise torque-biased and synchronized through a state of the art Programmable Logic Controller (PLC). With this system, a fast repositioning and a minimal settling time has been achieved - crucial when studying transient sources such as gamma-ray bursts which are a prime target for this telescope. This contribution will report on the successful commissioning of the H.E.S.S. II drive system in the first half of 2012 at the H.E.S.S. site in Namibia. The technical implementation and the performance of the drive system will be presented.
Data of the wide angle atmospheric Cherenkov light detector array AIROBICC and the scintillator matrix of the HEGRA air shower detector complex are combined to determine the energy spectrum and ...coarse composition of charged cosmic rays in the energy interval from 300 TeV to 10 PeV.
The GERDA collaboration is performing a search for neutrinoless double beta decay of ^{76}Ge with the eponymous detector. The experiment has been installed and commissioned at the Laboratori ...Nazionali del Gran Sasso and has started operation in November 2011. The design, construction and first operational results are described, along with detailed information from the R&D phase.
Astropart.Phys.20:129-143,2003 Mirror facets of the H.E.S.S. imaging atmospheric Cherenkov telescopes are
aligned using stars imaged onto the closed lid of the PMT camera, viewed by a
CCD camera. The ...alignment procedure works reliably and includes the automatic
analysis of CCD images and control of the facet alignment actuators. On-axis,
80% of the reflected light is contained in a circle of less than 1 mrad
diameter. The spot widens with increasing angle to the telescope axis. In
accordance with simulations, the spot size has roughly doubled at an angle of
1.4 degr. from the axis. The expected variation of spot size with elevation due
to deformations of the support structure is visible, but is completely
non-critical over the usual working range. Overall, the optical quality of the
telescope exceeds the specifications.
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