Leaked GeV CRs from a broken shell: Explaining 9 years Fermi-LAT data of SNR W28
Supernova remnant (SNR)\,W28 is well known for its classic hadronic scenario, in which the TeV cosmic rays (CRs) released at the early stage of this mid-aged SNR are illuminating nearby molecular clouds (MCs). Overwhelming evidences have shown that the northeast of the SNR (W28-North) has already encountered with the MC clumps. Through this broken shell -- W28-North, we believe the CRs with energy down to $<$1\,GeV to be able to be injected into nearby MCs. To further testify this hadronic scenario, we first analyse the 9 years Fermi-LAT data in/around W28 with energy down to 0.3\,GeV. Our Fermi-LAT analysis display a 10-200 GeV skymap which spatially matches well with the known TeV sources -- HESS\,J1801-233 (W28-North), HESS\,J1800-240\,A,\,B\,\&\,C (240\,A\,B\,\&\,C). At low energy band, we has discovered a 0.5-1\,GeV blob located to the south of 240\,B\,\&\,C, and a low flux of 0.3-1\,GeV at 240\,A. A hadronic model is build to explain our analysis results and previous multi-wavelength observations of W28. Our model consists of three CR sources: The run-away CRs escaped from a strong shock; The leaked GeV CRs from the broken shell -- W28-North; And the local CR sea. Through modelling the SNR evolution, CR acceleration\,\&\,releasing, we have explained the GeV-TeV emission in/around SNR\,W28 (except for 240\,A) in one model. Both the damping of the magnetic waves by the neutrals and the decreased acceleration efficiency are taken into account in our model due to the mid-age of SNR W28.