AGILE detection of GeV gamma-ray emission from the SNR W28

TL;DR

This paper reports AGILE/GRID gamma-ray observations of the supernova remnant W28, supporting a hadronic origin of gamma-ray emission through interactions with molecular clouds, and compares these findings with TeV and molecular line data.

Contribution

First detailed AGILE gamma-ray observations of W28 that confirm hadronic processes as the source of gamma-ray emission in this SNR.

Findings

AGILE detected gamma-ray flux of (14 +- 5) x 10^-8 ph cm^-2 s^-1 for E > 400 MeV.

Gamma-ray emission correlates with TeV observations and molecular cloud locations.

A hadronic interaction and diffusion model explains the spectral and morphological features.

Abstract

Supernova remnants (SNRs) are believed to be the main sources of Galactic cosmic rays. Molecular clouds associated with SNRs can produce gamma-ray emission through the interaction of accelerated particles with the concentrated gas. The middle aged SNR W28, for its associated system of dense molecular clouds, provides an excellent opportunity to test this hypothesis. We present the AGILE/GRID observations of SNR W28, and compare them with observations at other wavelengths (TeV and 12CO J=1-->0 molecular line emission). The gamma-ray flux detected by AGILE from the dominant source associated with W28 is (14 +- 5) 10^-8 ph cm^-2 s^-1 for E > 400 MeV. This source is positionally well correlated with the TeV emission observed by the HESS telescope. The local variations of the GeV to TeV flux ratio suggest 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 AGILE in the MeV-GeV energy range and by 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.