The spectrum of TeV gamma rays from the Crab Nebula

The spectrum of gamma rays from the Crab Nebula has been measured in the en ergy range 500 GeV-8 TeV at the Whipple Observatory by the atmospheric Cere nkov technique. Two methods of analysis that were used to derive spectra, i n order to reduce the chance of calibration errors, gave good agreement, as did analysis of observations made with changed equipment several years apa rt. It is concluded that stable and reliable energy spectra can now be made in the TeV range. The spectrum can be represented in this energy range by the power-law fit, J = (3.20 +/- 0.17 +/- 0.6) x 10(-7) x (E/1 TeV)(-2.49+/ -0.06+/-0.04) m(-2) s(-1) TeV-1, or by the following form, which extends mu ch better to the GeV domain: J = (3.25 +/- 0.14 +/- 0.6) x 10(-7)E(-2.44+/- 0.06+/-0.04-0.151 log10 E) m(-2) s(-1) TeV-1 (E in TeV). The integral flux above 1 TeV is (2.1 +/- 0.2 +/- 0.3) x 10(-7) m(-2) s(-1). Using the comple te spectrum of the Crab Nebula, the spectrum of relativistic electrons is d educed, and the spectrum of the inverse Compton emission that they would ge nerate is in good agreement with the observed gamma-ray flux from 1 GeV to many TeV, if the magnetic field in the region where these scattered photons originate (essentially the X-ray-emitting region, around 0.4 pc from the p ulsar) is similar to 16 nT (160 mu G), in reasonable agreement with the hel d deduced by Aharonian & Atoyan. If the same field strength were present th roughout the nebula, there would be no clear need for an additional radiati on source in the GeV domain such as has recently been suggested; the result s give an indication that the magnetic held is well below the often-assumed equipartition strength (35-60 nT). Further accurate gamma-ray spectral mea surements over the range from 1 GeV to tens of TeV have the potential to pr obe the growth in the magnetic held in the inner region of the nebula.