Journal
ASTROPHYSICAL JOURNAL LETTERS
Volume 742, Issue 2, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/2041-8205/742/2/L30
Keywords
acceleration of particles; cosmic rays; gamma rays: general; ISM: supernova remnants
Categories
Funding
- Italian Space Agency (ASI) with Italian Institute of Astrophysics (INAF)
- Italian Institute of Nuclear Physics (INFN)
- ASI [I/042/10/0]
- CONICET
- ANPCyT
- UBACYT
- Grants-in-Aid for Scientific Research [23740149, 21253003] Funding Source: KAKEN
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We present the AGILE gamma-ray observations in the energy range 50 MeV-10 GeV of the supernova remnant (SNR) W44, one of the most interesting systems for studying cosmic-ray production. W44 is an intermediate-age SNR (similar to 20,000 years) and its ejecta expand in a dense medium as shown by a prominent radio shell, nearby molecular clouds, and bright [S II] emitting regions. We extend our gamma-ray analysis to energies substantially lower than previous measurements which could not conclusively establish the nature of the radiation. We find that gamma-ray emission matches remarkably well both the position and shape of the inner SNR shocked plasma. Furthermore, the gamma-ray spectrum shows a prominent peak near 1 GeV with a clear decrement at energies below a few hundreds of MeV as expected from neutral pion decay. Here we demonstrate that (1) hadron-dominated models are consistent with all W44 multiwavelength constraints derived from radio, optical, X-ray, and gamma-ray observations; (2) ad hoc lepton-dominated models fail to explain simultaneously the well-constrained gamma-ray and radio spectra, and require a circumstellar density much larger than the value derived from observations; and (3) the hadron energy spectrum is well described by a power law (with index s = 3.0 +/- 0.1) and a low-energy cut-off at E-c = 6 +/- 1 GeV. Direct evidence for pion emission is then established in an SNR for the first time.
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