AGILE detection of GeV $\sf$<i>γ</i>-ray emission from the SNR W28

Abstract

<i>Aims. <i/>Supernova remnants (SNRs) are believed to be the main sources of Galactic cosmic rays. Molecular clouds associated with SNRs can produce gamma-ray emission by means of the interaction of accelerated particles with the concentrated gas. The middle-aged SNR W28, because of its associated system of dense molecular clouds, provides an excellent opportunity to test this hypothesis.<i>Methods. <i/>We present the AGILE/GRID observations of SNR W28, and compare them with observations at other wavelengths (TeV and <sup>12<sup/>CO <i>(J=1<i/><i>0)<i/> molecular line emission). <i>Results. <i/>The gamma-ray flux detected by AGILE from the dominant source associated with W28 is (14 <i>±<i/> 5) × 10<sup>-8<sup/> ph cm<sup>-2<sup/> s<sup>-1<sup/> for <i>E > 400<i/> MeV. This source is positionally well correlated with the TeV emission observed by the HESS telescope. The local variations in the GeV to TeV flux ratio imply that there is a difference between the CR spectra of the north-west and south molecular cloud complexes. A model based on a hadronic-induced interaction and diffusion with two molecular clouds at different distances from the W28 shell can explain both the morphological and spectral features observed by both AGILE in the MeV-GeV energy range and the HESS telescope in the TeV energy range. The combined set of AGILE and H.E.S.S. data strongly support a hadronic model for the gamma-ray production in W28.