Context. A sample of 14 young open star clusters has been observed in the TeV energy regime with the stereoscopic system of the HEGRA (High Energy Gamma-Ray Astronomy) Cherenkov telescopes from 1997 to 2002, resulting in more than 300 h of observation time.Aims. Young open star clusters may contribute to the acceleration of cosmic rays. The detection of gamma-rays (from decaying pi(0)s produced in hadronic interactions) from these objects could be evidence for such a contribution. The results of our observations are compared to available gamma-ray data and to a simple hadronic model in the framework of shock front acceleration of cosmic rays in the stellar winds of the cluster members to test the potential of the presently available data on young open star clusters to constrain this type of model.Methods. The stereoscopic system of HEGRA Cherenkov telescopes makes use of the atmospheric imaging technique. Air showers initiated by primary Gamma-Rays are recorded as elliptical images in the telescope cameras. The images from the different telescopes are then superimposed to reconstruct the parameters of the primary particle. This technique ( stereoscopy) was pioneered by the HEGRA experiment.Results. No significant excess has been found in the analysed data set of young open star clusters. The derived upper limit on the TeV gamma-ray flux from Berkeley 87 and the available EGRET data from the same direction do not allow us to fully constrain the simple hadronic model used here. The comparison of the upper limits derived for all 14 objects with the flux detected from TeV J2032+4130 (under the assumption of an association of the TeV-signal with the compact stellar association CygOB2) suggests that gamma-ray emission from young open star clusters as an object class cannot be ruled out.

We present results of a search for TeV gamma ray emission from the microquasar SS-433 and the surrounding region covering a similar to 8 degrees x 8 degrees field of view. Analysis of data taken with the HEGRA stereoscopic system of imaging atmospheric. Cerenkov imaging telescopes reveals no evidence of steady or variable emission from any position. Observation times of over 100 h have been achieved in central parts of the field of view. We set 99% confidence level upper limits to a number of a-priori-chosen objects of interest, including SS-443 and its interaction regions, 32 pulsars, 3 supernova remnants and the GeV source GeV J1907+0537. Our upper limit of 3.2% Crab flux ( for energies E > 0.8 TeV) for the eastern-lobe region e3 of SS-433 permits, after comparison with X-ray fluxes, a lower limit of B >= 19 mu G on the post-shocked magnetic field in this region. An ensemble upper limit at 0.3% Crab flux ( E > 0.7 TeV) from a subset ( 11) of the 32 pulsars implies a maximum of 4.5% of the spin-down pulsar power is available for TeV gamma ray production. For one of the SNR in our FoV, 3C 396, recent Chandra observations suggest that a central pulsar-driven wind nebula may be the source of X-ray emission. Our upper limit implies that a maximum of 0.1% of the spin-down power from the central source of 3C 396 would be available for TeV.- rays.

The unidentified TeV source in Cygnus is now confirmed by follow-up observations from 2002 with the HEGRA stereoscopic system of Cherenkov Telescopes. Using all data ( 1999 to 2002) we confirm this new source as steady in flux over the four years of data taking, extended with radius 6.2' (+/-1.2(stat)' +/- 0.9(sys)') and exhibiting a hard spectrum with photon index -1.9. It is located in the direction of the dense OB stellar association, Cygnus OB2. Its integral flux above energies E > 1 TeV amounts to similar to5% of the Crab assuming a Gaussian profile for the intrinsic source morphology. There is no obvious counterpart at radio, optical nor X-ray energies, leaving TeV J2032+4130 presently unidentified. Observational parameters of this source are updated here and some astrophysical discussion is provided. Also included are upper limits for a number of other interesting sources in the FoV, including the famous microquasar Cygnus X-3.

A sample of 54 selected Active Galactic Nuclei (AGN) has been observed with the HEGRA stereoscopic system of Cherenkov Telescopes between 1996 and 2002 in the TeV energy regime. The observations were motivated by the positive results obtained for Mkn 421 and Mkn 501. The distances of the selected objects vary over a large range of redshifts between z = 0.004 and z = 0.7. Among the observed AGN are the meanwhile established TeV-emitting BL Lac type objects H1426+428 and 1ES1959+650. Furthermore the BL Lac object 1ES2344+514 and the radio galaxy M87 show evidence for a signal on a 4 sigma level. The observation of 1ES2344+514 together with the Whipple results firmly establishes this AGN as a TeV source. Several objects (PKS2155-304, BL Lacertae, 3C066A) that have been claimed as TeV gamma-ray emitters by other groups are included in this data sample but could not be confirmed using data analysed here. The upper limits of several AGN included in this analysis are compared with predictions in the frame-work of SSC models.

The array of 5 imaging atmospheric Cerenkov telescopes (IACTs) deployed at La Palma (Canary Islands), and operated by the HEGRA (High Energy Gamma Ray Astronomy) collaboration, was used for observations of the Monoceros Loop SNR region for a total of about 120 hrs and 20 hrs in ON-source and OFF-source mode, respectively. The giant molecular cloud Rosette Nebula appears in the sky region, close to the south-east part of the SNR rim. Using the HEGRA system of IACTs of rather large field of view (4.3 degree in diameter), we have mapped the extended sky region of 3degreesx3degrees associated with the Monoceros SNR/Rosette Nebula, which is centered towards the hard spectrum X-ray point source SAX J0635+533. The EGRET unidentified source of diffuse gamma-ray emission (3EG J0634+0521) observed in the energy range between 100 MeV-10 GeV, was effectively in the field of view of our present observations. Also, the GeV source GeV J0633+0645 was within the available field of view. The performance of the IACTs array reveals an energy threshold of 500 GeV and an angular resolution of 0.1degrees for gamma-rays. In what follows, we present the result of the data analysis and its physical interpretation.

The Crab supernova remnant has been observed regularly with the stereoscopic system of five imaging air Cerenkov telescopes that was part of the High Energy Gamma Ray Astronomy (HEGRA) experiment. In total, close to 400 hr of useful data have been collected from 1997 to 2002. The differential energy spectrum of the combined data set can be approximated by a power law-type energy spectrum: dPhi/dE = Phi(0)(E/TeV)(Gamma), Phi(0) = (2.83 +/- 0.04(stat) +/- 0.6(sys))10(-11) photons cm(-2) s(-1) TeV-1, and Gamma = -2.62 +/- 0.02(stat) +/- 0.05(sys). The spectrum extends up to energies of 80 TeV and is well matched by model calculations in the framework of inverse Compton scattering of various seed photons in the nebula, including for the first time a recently detected compact emission region at millimeter wavelengths. The observed indications for a gradual steepening of the energy spectrum in data is expected in the inverse Compton emission model. The average magnetic field in the emitting volume is determined to be 161.6 +/- 0.8(stat) +/- 18(sys) muG. The presence of protons in the nebula is not required to explain the observed flux, and upper limits on the injected power of protons are calculated to be as low as 20% of the total spin-down luminosity for bulk Lorentz factors of the wind in the range of 10(4)-10(6). The position and size of the emission region have been studied over a wide range of energies. The position is shifted by 1300 to the west of the pulsar, with a systematic uncertainty of 25". No significant shift in the position with energy is observed. The size of the emission region is constrained to be less than 20 at energies between 1 and 10 TeV. Above 30 TeV the size is constrained to be less than 3'. No indication of pulsed emission has been found, and upper limits in differential bins of energy have been calculated reaching typically 1%-3% of the unpulsed component.

Using the HEGRA system of imaging atmospheric Cherenkov telescopes, the Andromeda galaxy (M31) was surveyed for TeV gamma ray emission. Given the large field of view of the HEGRA telescopes, three pointings were sufficient to cover all of M31, including also M32 and NGC 205. No indications for point sources of TeV gamma rays were found. Upper limits are given at a level of a few percent of the Crab flux. A specific search for monoenergetic gamma-ray lines from annihilation of supersymmetric dark matter particles accumulating near the center of M31 resulted in flux limits in the 10(-13) cm(-2) s(-1) range, well above the predicted MSSM flux levels except for models with pronounced dark-matter spikes or strongly enhanced annihilation rates.

Between the beginning of 1997 and fall 2002, the HEGRA collaboration was operating a stereoscopic system of 4 (later 5) Imaging Atmospheric Cherenkov Telescopes (IACT) on the Canary Island La Palma. We review the calibration schemes which were developed for the system, and report on the performance of the detector over this period. The system had an energy threshold of 500 GeV under optimum detector conditions at zenith. With the calibration schemes described here, a systematic accuracy of 15 percent on the absolute energy scale has been achieved. The continuous sensitivity monitoring provides a relative accuracy of a few percent, and shows that the threshold did not exceed 600 GeV throughout the whole period of operation. The readout electronics and the imaging quality of the dishes were well monitored and stable. The absolute pointing had an accuracy of better than 25 �� .

The HEGRA system of imaging air Cherenkov telescopes has been used to observe the BL Lac object H1426+428 (z= 0:129) for 217.5 hours in 2002. In this data set alone, the source is detected at a confidence level of 5:3, confirming this object as a TeV source. The overall flux level during the observations in 2002 is found to be a factor of 2.5 lower than during the previous observations by HEGRA in 1999 and 2000. A new spectral analysis has been carried out, improving the signal-to-noise ratio at the expense of a slightly increased systematic uncertainty and reducing the relative energy resolution to E=E 12% over a wide range of energies. The new method has also been applied to the previously published data set taken in 1999 and 2000, confirming the earlier claim of a flattening of the energy spectrum between 1 and 5 TeV. The data set taken in 2002 shows again a signal at energies above 1 TeV. We combine the energy spectra as determined by the CAT and VERITAS groups with our reanalyzed result of the 1999 and 2000 data set and apply a correction to account for eects of absorption of high energy photons on extragalactic background light in the optical to mid infrared band. The shape of the inferred source spectrum is mostly sensitive to the characteristics of the extragalactic background light between wavelengths of 1 and 15 m.

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.

In the first months of 2001 the AGN Mkn 421 showed highly variable, strong TeV activity at flux levels frequently exceeding 1 Crab.Here we present the light curve and energy spectrum of Mkn 421 as measured with the HEGRA stand alone telescope CT1.Around 30% of the data were taken under moonlight conditions.The spectrum shows a significant exponential energy cutoff at around 3.4 TeV.The results from the dark night-and the moon data are in excellent agreement with each other.A significant spectral shape variation depending on the flux level has been found.The TeV light curve is also found to be highly correlated with the X-ray light curve of the RXTE / ASM satellite, showing no significant time lag larger than 0.2 d.The derived correlation coefficient of 0.83 corresponds to a 5.2 σ significance.

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 ± 26.8 events above 730 GeV corresponds to an integral flux of 3.3% of the Crab flux or Nγ(E > 730 GeV) = (0.96 ± 0.23) × 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.

The nearby BL Lacertae (BL Lac) object Markarian 421 (Mkn 421) at a red shift z = 0.031 was observed to undergo strong TeV gamma-ray outbursts in the observational periods from December 1999 until May 2001. The time averaged flux level F(E > 1 TeV) in the 1999/2000 season was (1.43 +/- 0.04) x 10(-11) ph cm(-2) s(-1), whereas in the 2000/2001 season the average integral flux increased to (4.19 +/- 0.04) x 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)(stat)(+0.9 0.8)(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 +/- 0.09(stat) for 1999/2000 to 2.19 +/- 0.02(sta)t in 2000/2001. The energy spectra derived for different average flux levels ranging from 0.5 to 10 x 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 DeltaE = 2.6 +/- 0.6(stat) +/- 0.6(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.

Deep observation (similar to113 hrs) of the Cygnus region at TeV energies using the HEGRA stereoscopic system of air Cerenkov telescopes has serendipitously revealed a signal positionally inside the core of the OB association Cygnus OB2, at the edge of the 95% error circle of the EGRET source 3EG J2033+4118, and similar to0.5degrees north of Cyg X-3. The source centre of gravity is RA alpha(J2000): 20(hr)32(m)07(s) +/- 9.2(stat)(s) +/- 2.2(sys)(s), Dec delta(J2000): +41degrees30'30" +/- 2.0(stat)' +/- 0.4(sys)'. The source is steady, has a post-trial significance of +4.6sigma, indication for extension with radius 5.6' at the similar to3sigma level, and has a differential power-law flux with hard photon index of -1.9 +/- 0.3(stat) +/- 0.3(sys). The integral flux above 1 TeV amounts similar to3% that of the Crab. No counterpart for the TeV source at other wavelengths is presently identified, and its extension would disfavour an exclusive pulsar or AGN origin. If associated with Cygnus OB2, this dense concentration of young, massive stars provides an environment conducive to multi-TeV particle acceleration and likely subsequent interaction with a nearby gas cloud. Alternatively, one could envisage gamma-ray production via a jet-driven termination shock.

The detection of TeV gamma-rays from the blazar H1426+428 at an integral flux level of (4 +/- 2(stat) +/- 1(syst)) x 10(-12) erg cm-(2) s(-1) above 1 TeV with the HEGRA imaging atmospheric Cherenkov telescope system is reported. H1426+428 is located at a redshift of z = 0.129, which makes it the most distant source detected in TeV gamma-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 proportional to 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.

A search for potential point sources of very high energy gamma rays has been carried out on the data taken simultaneously by the HEGRA AIROBICC and Scintillator arrays from August 1994 to March 2000. The list of sought sources includes supernova remnants, pulsars, AGNs and binary systems. The energy threshold is around 15 TeV. For the Crab Nebula, a modest excess of 2.5 standard deviations above the cosmic ray background has been observed. Flux upper limits (at 90% c.l.) of around 1.3 times the flux of the Crab Nebula are obtained, in average, for the candidate sources. A different search procedure has been used for an all-sky search which yields absolute flux upper limits between 4 and 9 crabs depending on declination, in the band from delta = 0 to delta = 60degrees.