Constraints on cosmic-ray efficiency in the supernova remnant RCW 86

Several young supernova remnants (SNRs) have recently been detected in the high-and very-high energy gamma-ray domains, and the nature of this emission is still hotly debated. We have studied the broadband nonthermal emission from one of these SNRs, namely RCW 86, by analyzing ∼40 months of Fermi/LAT data in the high-energy domain, and the archival X-ray data from several instruments. The derived Fermi/LAT upper limits, together with the H.E.S.S. measurements, are incompatible with a standard E−2 p hadronic emission arising from p-p interactions, and can only be accommodated by a particle spectrum harder than E−1.8 p . In such hadronic scenario, the total energy in hadrons ηCR = ECR/ESN is ∼ 0.07 d22.5/n¯¯cm−3 (with d2.5≡d/2.5kpc and n¯¯cm−3≡n¯¯/1cm−3 ), and the average magnetic field B must be larger than 50 μG in order to significantly suppress any leptonic contribution. On the other hand, the interpretation of the gamma-ray emission by inverse Compton scattering reproduces the multi-wavelength data using a reasonable value for B of 15-25 μG. In this leptonic scenario, we derive a conservative upper limit to η CR of 0.04 d22.5/n¯¯cm−3 .