RCW 86 is a young supernova remnant ( SNR ) showing a shell-type structure at several wavelengths and is thought to be an efficient cosmic-ray ( CR ) accelerator .
Earlier Fermi Large Area Telescope results reported the detection of \gamma -ray emission coincident with the position of RCW 86 but its origin ( leptonic or hadronic ) remained unclear due to the poor statistics .
Thanks to 6.5 years of data acquired by the Fermi -LAT and the new event reconstruction Pass 8 , we report the significant detection of spatially extended emission coming from RCW 86 .
The spectrum is described by a power-law function with a very hard photon index ( \Gamma = 1.42 \pm 0.1 _ { stat } \pm 0.06 _ { syst } ) in the 0.1–500 GeV range and an energy flux above 100 MeV of ( 2.91 \pm 0.8 _ { stat } \pm 0.12 _ { syst } ) \times 10 ^ { -11 } erg cm ^ { -2 } s ^ { -1 } .
Gathering all the available multiwavelength ( MWL ) data , we perform a broadband modeling of the nonthermal emission of RCW 86 to constrain parameters of the nearby medium and bring new hints about the origin of the \gamma -ray emission .
For the whole SNR , the modeling favors a leptonic scenario in the framework of a two-zone model with an average magnetic field of 10.2 \pm 0.7 \mu G and a limit on the maximum energy injected into protons of 2 \times 10 ^ { 49 } erg for a density of 1 cm ^ { -3 } .
In addition , parameter values are derived for the North-East ( NE ) and South-West ( SW ) regions of RCW 86 , providing the first indication of a higher magnetic field in the SW region .