SIMBAD references

W28 is one of the archetypal supernova remnants (SNRs) interacting with molecular clouds (MCs). HESS observation found four TeV sources which are coincident with the MCs around W28, but Fermi LAT detected no prominent GeV counterparts for two of them. An accumulative diffusion model is established in this Letter, and the energetic protons colliding with the nearby MCs are considered to be an accumulation of the diffusive protons escaping from the shock front throughout the history of the SNR expansion. We have fitted the γ-ray spectra of the four sources and naturally explained the GeV spectral break of the north-eastern source (source N) and the non-significant GeV emission of southern sources A and C. The distances of sources A and C from the SNR centre are found to be much larger than those of sources N and B, which may be the basic reason for the faint GeV γ-rays of the two former sources.