Neutral pion emission from accelerated protons in the supernova remnant W44

A. Giuliani, M. Cardillo, M. Tavani, Y. Fukui, S. Yoshiike, K. Torii, G. Dubner, G. Castelletti, G. Barbiellini, A. Bulgarelli, P. Caraveo, E. Costa, P.W. Cattaneo, A. Chen, T. Contessi, E. Del Monte, I. Donnarumma, Y. Evangelista, M. Feroci, F. GianottiF. Lazzarotto, F. Lucarelli, F. Longo, M. Marisaldi, S. Mereghetti, L. Pacciani, A. Pellizzoni, G. Piano, P. Picozza, C. Pittori, G. Pucella, M. Rapisarda, A. Rappoldi, S. Sabatini, P. Soffitta, E. Striani, M. Trifoglio, A. Trois, S. Vercellone, F. Verrecchia, V. Vittorini, S. Colafrancesco, P. Giommi, G. Bignami

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Abstract

We present the AGILE gamma-ray observations in the energy range 50MeV-10GeV of the supernova remnant (SNR) W44, one of the most interesting systems for studying cosmic-ray production. W44 is an intermediate-age SNR (∼20, 000 years) and its ejecta expand in a dense medium as shown by a prominent radio shell, nearby molecular clouds, and bright [SII] 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 1GeV 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 Ec= 6 ± 1GeV. Direct evidence for pion emission is then established in an SNR for the first time. © 2011. The American Astronomical Society. All rights reserved..
Original languageEnglish
Article numberL30
Pages (from-to)-
JournalAstrophysical Journal Letters
Volume742
Issue number2
DOIs
Publication statusPublished - 1 Dec 2011
Externally publishedYes

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All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Giuliani, A., Cardillo, M., Tavani, M., Fukui, Y., Yoshiike, S., Torii, K., ... Bignami, G. (2011). Neutral pion emission from accelerated protons in the supernova remnant W44. Astrophysical Journal Letters, 742(2), -. [L30]. https://doi.org/10.1088/2041-8205/742/2/L30