A. Heusler

The Crab Nebula and Pulsar between 500 GeV and 80 TeV: Observations with the HEGRA Stereoscopic Air Cerenkov Telescopes

The Crab supernova remnant has been observed regularly with the stereoscopic system of 5 imaging air Cherenkov telescopes that was part of the High Energy Gamma Ray Astronomy (HEGRA) experiment. In total, close to 400 hours of useful data have been collected from 1997 until 2002. 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 mm-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

First results on the performance of the HEGRA IACT array

(161.6\pm0.8mathrm{stat}\pm18_\mathrm{sys}) \mu

Is the Giant Radio Galaxy M 87 a TeV Gamma-Ray Emitter?

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 being as low as 20 % of the total spin down luminosity for bulk Lorentz factors of the wind in the range of

An unidentified TeV source in the vicinity of Cygnus OB2

10^4-10^6

The Energy Spectrum of TeV Gamma Rays from the Crab Nebula as Measured by the HEGRA System of Imaging Air Cerenkov Telescopes

.The position and size of the emission region have been studied over a wide range of energies. The position is shifted by 13\arcsec to the west of the pulsar with a systematic uncertainty of 25\arcsec. No significant shift in the position with energy is observed. The size of the emission region is constrained to be less than 2\arcmin at energies between 1 and 10 TeV. Above 30 TeV the size is constrained to be less than 3\arcmin.No indications for pulsed emission has been found and upper limits in differential bins of energy have been calculated reaching typically 1-3 % of the unpulsed component.

Variations of the TeV energy spectrum at different flux levels of Mkn 421 observed with the HEGRA system of Cherenkov telescopes

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.

TeV gamma rays from the blazar H 1426+428 and the diffuse extragalactic background radiation

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.

SIMULTANEOUS X-RAY AND TeV GAMMA-RAY OBSERVATION OF THE TeV BLAZAR MARKARIAN 421 DURING 2000 FEBRUARY AND MAY

Deep observation (113 hrs) of the Cygnus region at TeV energies using the HEGRA stereoscopic system of air ˇ 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, and0:5 north of Cyg X-3. The source centre of gravity is RAJ2000 :2 0 hr 32 m 07 s 9:2 s 2:2 s ,D ecJ2000:+4130 0 30 00 2:0 0 0:4 0. The source is steady, has a post-trial significance of+4.6, indication for extension with radius 5:6 0 at the3 level, and has a dierential power-law flux with hard photon index of 1:9 0:3stat 0:3sys. The integral flux above 1 TeV amounts3% 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-ray production via a jet-driven termination shock.

Correlated Intense X-Ray and TeV Activity of Markarian 501 in 1998 June

The Crab Nebula has been observed by the HEGRA (High-Energy Gamma-Ray Astronomy) stereoscopic system of imaging air Cerenkov telescopes (IACTs) for a total of ~200 hr during two observational campaigns: from 1997 September to 1998 March and from 1998 August to 1999 April. The recent detailed studies of system performance give an energy threshold and an energy resolution for γ-rays of 500 GeV and ~18%, respectively. The Crab energy spectrum was measured with the HEGRA IACT system in a very broad energy range up to 20 TeV, using observations at zenith angles up to 65°. The Crab data can be fitted in the energy range from 1 to 20 TeV by a simple power law, which yields dJγ/dE = (2.79 ± 0.02 ± 0.5) × 10-7(E/1 TeV)-2.59±0.03±0.05 photons m-2 s-1 TeV-1. The Crab Nebula energy spectrum, as measured with the HEGRA IACT system, agrees within 15% in the absolute scale and within 0.1 units in the power-law index with the latest measurements by the Whipple, CANGAROO, and CAT groups, consistent within the statistical and systematic errors quoted by the experiments. The pure power-law spectrum of TeV γ-rays from the Crab Nebula constrains the physics parameters of the nebula environment as well as the models of photon emission.

Detection of TeV gamma-rays from the BL Lac 1ES 1959+650 in its low states and during a major outburst in 2002

F. Aharonian, A. Akhperjanian, M. Beilicke, K. Bernlohr, H. Borst, H. Bojahr, O. Bolz, T. Coarasa, J. Contreras, J. Cortina, L. Costamante, S. Denninghoff, V. Fonseca, M. Girma, N. Gotting, G. Heinzelmann, G. Hermann, A. Heusler, W. Hofmann, D. Horns, I. Jung, R. Kankanyan, M. Kestel, J. Kettler, A. Kohnle, A. Konopelko, H. Kornmeyer, D. Kranich, H. Krawczynski, H. Lampeitl, M. Lopez, E. Lorenz, F. Lucarelli, O. Mang, H. Meyer, R. Mirzoyan, M. Milite, A. Moralejo, E. Ona, M. Panter, A. Plyasheshnikov, G. Puhlhofer, G. Rauterberg, R. Reyes, W. Rhode, J. Ripken, G. Rowell, V. Sahakian, M. Samorski, M. Schilling, M. Siems, D. Sobzynska, W. Stamm, M. Tluczykont, H.J. Volk, C. A. Wiedner, W. Wittek and R. A. Remillard