DISCOVERY OF GAMMA-RAY EMISSION FROM THE SHELL-TYPE SUPERNOVA REMNANT RCW 86 WITH HESS

The shell-type supernova remnant (SNR) RCW 86, possibly associated with the historical supernova SN 185, with its relatively large size (about 40' in diameter) and the presence of nonthermal X-rays is a promising target for gamma-ray observations. The high sensitivity, good angular resolution of a few arcminutes and the large field of view of the High Energy Stereoscopic System (HESS) make it ideally suited for the study of gamma-ray morphology of such extended sources. HESS observations have indeed led to the discovery of the SNR RCW 86 in very high energy (VHE; E > 100 GeV) gamma-rays. With 31 hr of observation time, the source is detected with a statistical significance of 8.5 sigma and is significantly more extended than the HESS point-spread function. Morphological studies have been performed and show that the gamma-ray flux does not correlate perfectly with the X-ray emission. The flux from the remnant is similar to 10% of the flux from the Crab nebula, with a similar photon index of about 2.5. Possible origins of the VHE gamma-ray emission, via either Inverse Compton scattering by electrons or the decay of neutral pions produced by proton interactions, are discussed on the basis of spectral features obtained both in the X-ray and gamma-ray regimes.