Context : Several young supernova remnants ( SNRs ) have recently been detected in the high-energy ( HE ; 0.1 < E < 100 GeV ) and very-high-energy ( VHE ; E > 100 GeV ) gamma-ray domains .
As exemplified by RX J1713.7 - 3946 , the nature of this emission has been hotly debated , and direct evidence for the efficient acceleration of cosmic-ray protons at the SNR shocks still remains elusive .

Aims : We study the broadband gamma-ray emission from one of these young SNRs , namely RCW 86 , for which several observational lines of evidence indirectly point towards the presence of accelerated hadrons .
We then attempt to detect any putative hadronic signal from this SNR in the available gamma-ray data , in order to assess the level of acceleration efficiency .

Methods : We analyzed more than 40 months of data acquired by the Large Area Telescope ( LAT ) on-board the Fermi Gamma-Ray Space Telescope in the HE domain , and gathered all of the relevant multi-wavelength ( from radio to VHE gamma-rays ) information about the broadband nonthermal emission from RCW 86 .
For this purpose , we re-analyzed the archival X-ray data from the ASCA /Gas Imaging Spectrometer ( GIS ) , the XMM-Newton /EPIC-MOS , and the RXTE /Proportional Counter Array ( PCA ) .

Results : Beyond the expected Galactic diffuse background , no significant gamma-ray emission in the direction of RCW 86 is detected in any of the 0.1–1 , 1–10 and 10–100 GeV Fermi -LAT maps .
The derived HE upper limits , together with the H.E.S.S .
measurements in the VHE domain , are incompatible with a standard E _ { p } ^ { -2 } hadronic emission arising from proton-proton interactions , and can only be accommodated by a spectral index \Gamma \leq 1.8 , i.e .
a value in-between the standard ( test-particle ) index and the asymptotic limit of theoretical particle spectra in the case of strongly modified shocks .
In such a hadronic scenario , the total energy in accelerated particles is at the level of \eta _ { CR } = E _ { { CR } } /E _ { { SN } } \sim 0.07 d ^ { 2 } _ { { 2.5 kpc } } / \bar { n } _ { { cm - 3 } } ( with the distance d _ { 2.5 kpc } \equiv d/2.5 kpc and the effective density \bar { n } _ { { cm - 3 } } \equiv \bar { n } /1 cm ^ { -3 } ) , and the average magnetic field must be stronger than 50 \mu G in order to significantly suppress any leptonic contribution .
On the other hand , the interpretation of the gamma-ray emission by inverse Compton scattering of high energy electrons reproduces the multi-wavelength data using a reasonable value for the average magnetic field of 15–25 \mu G. In this leptonic scenario , we derive a conservative upper limit to \eta _ { CR } of 0.04 d ^ { 2 } _ { { 2.5 kpc } } / \bar { n } _ { { cm - 3 } } .
We discuss these results in the light of existing estimates of the magnetic field strength , the effective density and the acceleration efficiency in RCW 86 .

Conclusions :