T. Deckers

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.

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 time structure of Cherenkov images generated by TeV γ-rays and by cosmic rays

Abstract The time profiles of Cherenkov images of cosmic-ray showers and of γ-rays showers are investigated, using data gathered with the HEGRA system of imaging atmospheric Cherenkov telescopes during the 1997 outbursts of Mrk 501. Photon arrival times are shown to vary across the shower images. The dominant feature is a time gradient along the major axis of the images. The gradient varies with the distance between the telescope and the shower core, and is maximal for large distances. The time profiles of cosmic-ray showers and of γ-ray showers differ in a characteristic fashion. The main features of the time profiles can be understood in terms of simple geometrical models. Use of the timing information towards improved shower reconstruction and cosmic-ray supperssion is discussed.The time profiles of Cherenkov images of cosmic-ray showers and of gamma-ray showers are investigated, using data gathered with the HEGRA system of imaging atmospheric Cherenkov telescopes during the 1997 outbursts of Mrk 501. Photon arrival times are shown to vary across the shower images. The dominant feature is a time gradient along the major axis of the images. The gradient varies with the distance between the telescope and the shower core, and is maximal for large distances. The time profiles of cosmic-ray showers and of gamma-ray showers differ in a characteristic fashion. The main features of the time profiles can be understood in terms of simple geometrical models. Use of the timing information towards improved shower reconstruction and cosmic-ray suppression is discussed.

Cosmic ray proton spectrum determined with the imaging atmospheric Cherenkov technique

The HEGRA system of 4 imaging atmospheric Cherenkov telescopes ~IACTs! has been used to determine the flux and the spectrum of cosmic ray protons over a limited energy range around 1.5 TeV. Although the IACT system is designed for the detection of g-rays with energies above 500 GeV, it has also a large detection area of .10 6 m 2 33 msr for primary protons of energies above 1 TeV and the capability to reconstruct the primary proton energy with a reasonable accuracy DE/E of 50% near this threshold. Furthermore, the principle of stereoscopic detection of air showers permits the effective suppression of air showers induced by heavier primaries already on the trigger level, and in addition on the software level by analysis of the stereoscopic images. The combination of both capabilities permits a determination of the proton spectrum almost independently of the cosmic ray chemical composition. The accuracy of our estimate of the spectral index at 1.5 TeV is limited by systematic uncertainties and is comparable to the accuracy achieved with recent balloon and space borne experiments. In this paper we describe in detail the analysis tools, namely the detailed Monte Carlo simulation, the analysis procedure and the results. We determine the local ~i.e., in the range of 1.5‐3 TeV! differential spectral index to be g p52.7260.02stat60.15syst and obtain an integral flux above 1.5 TeV of F (.1.5 TeV)53.160.6 stat61.2syst310 22 /s sr m 2 . @S0556-2821~99!04107-7#

TeV γ-ray observations of the Crab and Mkn 501 during moonshine and twilight

Abstract 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.

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.

Cherenkov light based measurement of extensive air showers around the knee with the HEGRA experiment

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.