M. Ulrich

First results on the performance of the HEGRA IACT array

Abstract First results concerning the performance characteristics of the HEGRA IACT array are given based on stereoscopic observations of the Crab Nebula with four telescopes. The system provides a γ-ray energy threshold around 0.5 TeV. The Crab signal demonstrates an angular resolution of about 0.1°. Shape cuts allow to suppress cosmic ray background by almost a factor 100, while maintaining 40% efficiency for γ-rays. The Crab signal is essentially background free. For longer observation times of order 100 h, the system in its present form provides sensitivity to point sources at a level of 3% of the Crab flux. Performance is expected to improve further with the inclusion of the fifth telescope and the implementation of advanced algorithms for shower reconstruction.

Detection of gamma rays above 1 TeV from the Crab Nebula by the second HEGRA imaging atmospheric Cherenkov telescope at La Palma

Abstract The results of observations of the Crab Nebula by the second HEGRA imaging atmospheric Cherenkov telescope during the period 1994/1995 are presented. The signal, detected at a level of 10 σ , allows one to estimate the flux and the spectral index of the γ -ray energy spectrum above 1 TeV. The analysis, based on the comparison of measurements with comprehensive Monte Carlo simulations, reveals a γ -ray flux F γ (≥ 1 TeV) ⋍ 8 × 10 −12 photons cm −2 s −1 , and indicates a rather steep integral spectrum in the energy region 1–3 TeV with a power-law index α ⋍ 1.6–1.8.

The first telescope of the HEGRA air Cherenkov imaging telescope array

Abstract In search of VHE γ ray emission from cosmic point sources a system of imaging Cherenkov telescopes is constructed at present on the Canarian island of La Palma; the first telescope has been operational since 1992. The Cherenkov light from air shower particles is collected by a 5 m2 reflector. The camera at the focus contains 37 photomultipliers which sample the images of the Cherenkov flashes. The subsequent image analysis allows the discrimination of γ ray induced events from the much more abundant charged cosmic ray induced showers. The telescope has an effective energy threshold for γ showers of about 1.5 TeV. During the first year of operation a signal from the Crab nebula was detected.

Stereoscopic imaging of air showers with the first two HEGRA Cherenkov telescopes

Abstract With systems of atmospheric Cherenkov telescopes, providing stereo images of air showers, the shower parameters, such as the direction, core location, and height of shower maximum can be determined event by event. Techniques for the reconstruction of air showers are introduced, and are demonstrated using data from the first two HEGRA Cherenkov telescopes. Applied to observations of the Crab nebula, a clear signal is observed; the angular distribution of the excess events is consistent with Monte Carlo simulations of the expected resolution.

The Cosmic Ray Tracking (CRT) detector system

Abstract The Cosmic Ray Tracking (CRT) project represents a study on the use of tracking detectors of the time projection chamber type to detect secondary cosmic ray particles in extensive air showers. In reconstructing the arrival direction of the primary cosmic ray particles, the CRT detectors take advantage of the angular correlation of secondary particles with the cosmic rays leading to these air showers. In this paper, the detector hardware including the custom-designed electronics system is described in detail. A CRT detector module provides an active area of 2.5 m 2 and allows to measure track directions with a precision of 0.4°. It consists of two circular drift chambers of 1.8 m diameter with six sense wires each, and a 10cm thick iron plate between the two chambers. Each detector has a local electronics box with a readout, trigger, and monitoring system. The detectors can distinguish penetrating muons from other types of charged secondaries. A large detector array could be used to search for γ -ray point sources at energies above several TeV and for studies of the cosmic-ray composition. Ten detectors are in operation at the site of the HEGRA air shower array.

On the accuracy of determination of fluxes and energy spectra of primary gamma rays by a single imaging atmospheric Cherenkov telescope

The strong energy dependence of the detection efficiency of gamma -rays by imaging atmospheric Cherenkov telescopes (IACT) both at the hardware and software levels, leads to significant uncertainties in the estimate of primary gamma -ray fluxes. Nevertheless, detailed Monte Carlo calculations of the principal characteristics of IACTS, in particular of collection areas and of the gamma /proton separation efficiency, allow a rather high accuracy in the evaluation of the absolute gamma -ray fluxes and in the determination of the spectral index of power-law spectra. However, the poor energy resolution of a single IACT does not permit us to distinguish features such as sharp cutoffs in primary gamma -ray spectra unambiguously. Extended sources cause additional problems in the separation and determination of the gamma -ray flux.

An improved technique for the determination of shower geometry from single and stereo IACT images

In order to better utilize the information contained in the shower images generated by imaging Cherenkov telescopes (IACTs) equipped with cameras with small pixels, images are fitted to a parametrization of image shapes gained from Monte Carlo simulations, treating the shower direction, impact point, and energy as free parameters. Monte Carlo studies were carried out for stereoscopic shower images obtained with a system of IACTs. An improvement of order 1.5 in the angular resolution and in the determination of the core position is predicted. The fitting technique can also be applied to single-telescope images. Simulations indicate that the shower direction in space can be reconstructed event-by-event with a resolution of 0.16-. Data from Crab observations with a single HEGRA telescope confirm this prediction.

Operation and performance of the Cosmic Ray Tracking (CRT) detector system

Abstract The cosmic ray tracking (CRT) detectors consist of two drift chambers of the TPC type and an iron plate as a muon filter between the two chambers. CRT detectors are designed to detect and identify secondary cosmic ray particles in extensive air showers. Each detector has a local electronics box with a readout, trigger, and monitoring system. The local computers at the detector stations are used for hit and track finding in parallel to the detector readout. Detector stations are permanently calibrated online, using the cosmic ray data. The online software, including data processing and calibration procedures, are presented in this paper. Ten detectors are in operation at the site of the HEGRA air shower array. The detector performance has been evaluated using data from these detectors and the HEGRA array as well as by Monte Carlo simulations. The spatial resolution has been found to be about 1 mm both in drift and in wire direction. The corresponding angular resolution is about 0.4° in each projection for a single track. Muons are usually identified if scattered by less than 2.5° in the iron plate, corresponding to energies above about 1 GeV. In HEGRA-triggered showers an overall muon identification efficiency of 87% is achieved, with a fraction of only 0.07% of the electrons being misidentified as muons.

Study of the temporal and spectral characteristics of TeV gamma radiation from Mkn 501 during a state of high activity by the HEGRA IACT array

During the period March-April 1997, a high flux level of TeV γ-rays was observed from Mkn 501, using the HEGRA stereoscopic system of four imaging atmospheric Cherenkov telescopes (IACTs). The almost background-free detection of γ-rays with a rate exceeding in average 100 h−1, coupled with good energy resolution of the instrument ⩽25% allowed a study of the flux variation on time scales between 5 min and days, and measurements of the differential energy spectrum of the source for selected periods. Here we briefly discuss the results of observations with emphasis on the measurements of temporal and spectral characteristics of the source at 3 subsequent periods in April when the average flux of the source showed a remarkable increase of the flux from about one to eight times the flux observed from the Crab Nebula.