Fermi Gamma-ray Space Telescope

Detailed Investigation of the Gamma-Ray Emission in the Vicinity of SNR W28 with Fermi Large Area Telescope

Yoshitaka Hanabata
on behalf of the Fermi-LAT collaboration

Abstract:

We present a detailed investigation of the region around the middle-aged supernova remnant (SNR) W28 with Fermi Large Area Telescope. W28 is surrounded by dense gas, and the gamma-ray emission from the SNR can be explained by the decay of neutral pions produced by the interaction between the cosmic rays (CRs) accelerated in the SNR and adjacent molecular clouds. Furthermore, three TeV gamma-ray sources outside the boundary of W28, overlapping with clouds, have been detected by H.E.S.S. These results suggest that CRs have escaped from the SNR and the gamma rays come from the clouds illuminated by the CRs. Therefore, W28 is one of the best sites for study of the CR diffusion process from SNRs. To tightly constrain the diffusion coefficient of the escaped CRs, we investigated the spatial correlation of the GeV emission with the H.E.S.S. sources and found good correspondences of the morphologies. With the wide-band gamma-ray spectra, these emissions can be naturally explained by a single model in which the CR diffusion coefficient around W28 is smaller than the theoretical expectation in the interstellar space. The total energy of the CRs escaping from W28 is constrained through the modeling to be larger than 2x10E49 erg, in agreement with the conjecture that SNRs are the main accelerator of the Galactic CRs.