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: d Phi /dE = Phi sub(0) (E/TeV) Gamma , Phi sub(0) = (2.83 plus or minus 0.04 sub(stat) plus or minus 0.6 sub(sys)) 10 super(-11) photons cm super(-2) s super(-1) TeV super(-1), and Gamma = -2.62 plus or minus 0.02 sub(stat) plus or minus 0.05 sub(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 plus or minus 0.8 sub(stat) plus or minus 18 sub(sys) mu G. 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 super(4)-10 super(6). The position and size of the emission region have been studied over a wide range of energies. The position is shifted by 13" 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 2' 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.
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′ (±$1.2^\prime_{\rm stat}$ ± $0.9^\prime_{\rm sys}$) 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 ~5% 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.
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 $3.18 \pm 0.17_{\mbox{\tiny stat}} \pm 0.08_{\mbox{\tiny sys}}$.
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 now-established TeV-emitting BL Lac type objects H 1426+428 and 1ES 1959+650. Furthermore the BL Lac object 1ES 2344+514 and the radio galaxy M 87 show evidence for a signal on a 4 σ level. The observation of 1ES 2344+514 together with the Whipple results firmly establishes this AGN as a TeV source. Several objects (PKS 2155-304, BL Lacertae, 3C 066A) that have been claimed as TeV ${\mathrm \gamma}$-ray emitters by other groups are included in this data sample but could not be confirmed using data analysed here. The upper limits from several AGN included in this analysis are compared with predictions in the frame-work of SSC models.
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.
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 γ-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 γ-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 Cyg OB2) suggests that γ-ray emission from young open star clusters as an object class cannot be ruled out.
We present results of a search for TeV γ-ray emission from the microquasar SS-433 and the surrounding region covering a ~$8^\circ$ $\times$ $8^\circ$ field of view. Analysis of data taken with the ...HEGRA stereoscopic system of imaging atmospheric Čerenkov 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 $B\geq19~\mu$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 γ-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 γ-rays.
PDF
