First detection of a VHE gamma-ray spectral maximum from a cosmic source: HESS discovery of the Vela X nebula

The Vela supernova remnant (SNR) is a complex region containing a number of sources of non-thermal radiation. The inner section of this SNR, within 2 degrees of the pulsar PSR B0833-45 , has been observed by the HESS $\gamma$-ray atmospheric Cherenkov detector in 2004 and 2005. A strong signal is seen from an extended region to the south of the pulsar, within an integration region of radius $0.8\ensuremath{^{\circ}} $ around the position ( $\rm\alpha = 08^{h} 35^{m} 00^{s}$, $\delta = -45\ensuremath{^{\circ}} 36\arcmin$ J2000.0). The excess coincides with a region of hard X-ray emission seen by the ROSAT and ASCA satellites. The observed energy spectrum of the source between 550 GeV and 65 TeV is well fit by a power law function with photon index $\Gamma = 1.45 \pm 0.09\ensuremath{_{{\rm stat}}}\ \pm 0.2\ensuremath{_{{\rm sys}}} $ and an exponential cutoff at an energy of $13.8 \pm 2.3\ensuremath{_{{\rm stat}}}\ \pm 4.1\ensuremath{_{{\rm sys}}} $ TeV. The integral flux above 1 TeV is $(1.28 \pm 0.17\ensuremath{_{{\rm stat}}}\ \pm 0.38\ensuremath{_{{\rm sys}}} ) \times 10^{-11}\ \ensuremath{{\rm cm}^{-2}\,{\rm s}^{-1}} $. This result is the first clear measurement of a peak in the spectral energy distribution from a VHE $\gamma$-ray source, likely related to inverse Compton emission. A fit of an Inverse Compton model to the HESS spectral energy distribution gives a total energy in non-thermal electrons of ~ $2 \times 10^{45}$ erg between 5 TeV and 100 TeV, assuming a distance of 290 parsec to the pulsar. The best fit electron power law index is 2.0, with a spectral break at 67 TeV.