We present detailed analysis of the two gamma-ray sources , 1FGL J1801.3 - 2322c and 1FGL J1800.5 - 2359c , that have been found toward the supernova remnant ( SNR ) W28 with the Large Area Telescope ( LAT ) on board the Fermi Gamma-ray Space Telescope .
1FGL J1801.3 - 2322c is found to be an extended source within the boundary of SNR W28 , and to extensively overlap with the TeV gamma-ray source HESS J1801 - 233 , which is associated with a dense molecular cloud interacting with the supernova remnant .
The gamma-ray spectrum measured with LAT from 0.2–100 GeV can be described by a broken power-law function with a break of \sim 1 GeV , and photon indices of 2.09 \pm 0.08 ( stat ) \pm 0.28 ( sys ) below the break and 2.74 \pm 0.06 ( stat ) \pm 0.09 ( sys ) above the break .
Given the clear association between HESS J1801 - 233 and the shocked molecular cloud and a smoothly connected spectrum in the GeV–TeV band , we consider the origin of the gamma-ray emission in both GeV and TeV ranges to be the interaction between particles accelerated in the SNR and the molecular cloud .
The decay of neutral pions produced in interactions between accelerated hadrons and dense molecular gas provide a reasonable explanation for the broadband gamma-ray spectrum .
1FGL J1800.5 - 2359c located outside the southern boundary of SNR W28 , can not be resolved .
An upper limit on the size of the gamma-ray emission was estimated to be \sim 16 ^ { \prime } using events above \sim 2 GeV under the assumption of a circular shape with uniform surface brightness .
It appears to coincide with the TeV source HESS J1800 - 240B , which is considered to be associated with a dense molecular cloud that contains the ultra compact H II region W28A2 ( G5.89 - 0.39 ) .
We found no significant gamma-ray emission in the LAT energy band at the positions of TeV sources HESS J1800 - 230A and HESS J1800 - 230C .
The LAT data for HESS J1800 - 230A combined with the TeV data points indicate a spectral break between 10 GeV and 100 GeV .