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We present a detailed investigation of the $\gamma$-ray emission in thevicinity of the supernova remnant (SNR) W28 (G6.4$-$0.1) observed by the LargeArea Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope. We detectedsignificant $\gamma$-ray emission spatially coincident with TeV sources HESSJ1800$-$240A, B, and C, located outside the radio boundary of the SNR. Theirspectra in the 2-100 GeV band are consistent with the extrapolation of thepower-law spectra of the TeV sources. We also identified a new source of GeVemission, dubbed Source W, which lies outside the boundary of TeV sources andcoincides with radio emission from the western part of W28. All of the GeV$\gamma$-ray sources overlap with molecular clouds in the velocity range from 0to 20 km s$^{-1}$. Under the assumption that the $\gamma$-ray emission towardsHESS J1800-240A, B, and C comes from $\pi^0$ decay due to the interactionbetween the molecular clouds and cosmic rays (CRs) escaping from W28, they canbe naturally explained by a single model in which the CR diffusion coefficientis smaller than the theoretical expectation in the interstellar space. Thetotal energy of the CRs escaping from W28 is constrained through the samemodeling to be larger than $\sim$ 2 $\times$ 10$^{49}$ erg. The emission fromSource W can also be explained with the same CR escape scenario.

Detailed Investigation of the Gamma-Ray Emission in the Vicinity of SNR W28 with FERMI-LAT