Fermi Large Area Telescope Detection of Supernova Remnant RCW 86

Using $5.4$ year Fermi-LAT data, we report the detection of GeV $\gamma$-ray emission from the shell-type supernova remnant RCW 86 (G315.4-2.3) with a significance of $\sim5.1\sigma$. The data slightly favors an extended emission of this supernova remnant. The spectral index of RCW 86 is found to be very hard, $\Gamma\sim1.4$, in the $0.4$ to $300$ GeV range. A one zone leptonic model can well fit the multi-wavelength data from radio to very high energy $\gamma$-rays. The very hard GeV $\gamma$-ray spectrum and the inferred low gas density seem to disfavor the hadronic origin of the $\gamma$-rays. The $\gamma$-ray behavior of RCW 86 is very similar to several other TeV shell-type supernova remnants, e.g., RX J1713.7-3946, RX J0852.0-4622, SN 1006 and HESS J1731-347.