The Crab Nebula and Pulsar between 500 GeV and 80 TeV: Observations with the HEGRA stereoscopic air Cherenkov telescopes

Abstract

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: dφ/dE = φ0(E/TeV) Γ, φ0 = (2.83 ± 0.04stat ± 0.6sys)10-11 photons cm-2 s -1 TeV-1, and Γ = -2.62 ± 0.02 stat ± 0.05sys. 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 ± 0.8stat ± 18sys μG. 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 104-106. 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.