Letter to the Editor

AGILE detection of GeV $\sf$ γ-ray emission from the SNR W28

¹ INAF/IASF - Milano, via E. Bassini 15, 20133 Milano, Italy e-mail: giuliani@iasf-milano.inaf.it
² INAF/IASF - Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy
³ Dip. di Fisica, Univ. “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
⁴ INAF/IASF - Bologna, via Gobetti 101, 40129 Bologna, Italy
⁵ INFN - Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
⁶ Department of Astrophysics, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
⁷ Max - Planck - Institut für Kernphysik, 69117 Heidelberg, Germany
⁸ Dublin Institute for Advanced Sudies, Dublin 2, Ireland
⁹ AS - ASDC, via G. Galilei, 00044 Frascati (Roma), Italy
¹⁰ Dip. di Fisica and INFN, via Valerio 2, 34127 Trieste, Italy
¹¹ INFN - Pavia, via Bassi 6, 27100 Pavia, Italy
¹² CIFS - Torino, viale Settimio Severo 3, 10133, Torino, Italy
¹³ ENEA - Bologna, via dei Martiri di Monte Sole 4, 40129 Bologna, Italy
¹⁴ INFN - Roma “La Sapienza”, Piazzale A. Moro 2, 00185 Roma, Italy
¹⁵ INAF - Osservatorio Astronomico di Cagliari, localita' Poggio dei Pini, strada 54, 09012 Capoterra, Italy
¹⁶ Dip. di Fisica, Univ. dell'Insubria, via Valleggio 11, 22100 Como, Italy
¹⁷ ENEA - Roma, via E. Fermi 45, 00044 Frascati (Roma), Italy
¹⁸ INAF/IASF - Palermo, via La Malfa 153, 90146 Palermo, Italy
¹⁹ ASI, viale Liegi 26 , 00198 Roma, Italy

Received: 13 February 2010
Accepted: 5 May 2010

Abstract

Aims. 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.

Methods. We present the AGILE/GRID observations of SNR W28, and compare them with observations at other wavelengths (TeV and ¹²CO (J=10) molecular line emission).

Results. 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 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.