NEUTRAL PION EMISSION FROM ACCELERATED PROTONS IN THE SUPERNOVA REMNANT W44

Journal Article · Thu Dec 15 04:00:00 EST 2011 · Astrophysical Journal Letters

DOI:https://doi.org/10.1088/2041-8205/742/2/L30· OSTI ID:22047322

Giuliani, A; Caraveo, P; Chen, A; Contessi, T ^[1]; Cardillo, M; Tavani, M; Costa, E; Monte, E Del; Donnarumma, I; Evangelista, Y; Feroci, M ^[2]; Fukui, Y; Yoshiike, S; Torii, K ^[3]; Dubner, G; Castelletti, G ^[4]; Barbiellini, G ^[5]; Bulgarelli, A; Gianotti, F ^[6]; Cattaneo, P W ^[7] ; others, and

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1. INAF-IASF Milano, via E. Bassini 15, 20133 Milano (Italy)
2. INAF/IASF-Roma,via Del Fosso del Cavaliere 100, 00133 Roma (Italy)
3. Department of Astrophysics, Nagoya University, Chikusa-ku, Nagoya 464-8602 (Japan)
4. Instituto de Astronomia y Fisica del Espacio (IAFE, CONICET-UBA), 1428 Buenos Aires (Argentina)
5. Dipartimento di Fisica and INFN, Via Valerio 2, I-34127 Trieste (Italy)
6. INAF/IASF-Bologna, Via Gobetti 101, I-40129 Bologna (Italy)
7. INFN-Pavia, Via Bassi 6, I-27100 Pavia (Italy)

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 ({approx}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{sub c} = 6 {+-} 1 GeV. Direct evidence for pion emission is then established in an SNR for the first time.

• OSTI ID: 22047322
• Journal Information: Astrophysical Journal Letters, Journal Name: Astrophysical Journal Letters Journal Issue: 2 Vol. 742; ISSN 2041-8205
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ACCELERATION
ASTROPHYSICS
COSMIC PROTONS
DENSITY
ENERGY SPECTRA
GAMMA RADIATION
GAMMA SPECTRA
GEV RANGE
INDEXES
LEPTONS
MEV RANGE
PARTICLE DECAY
PIONS NEUTRAL
PLASMA
RADIOWAVE RADIATION
SUPERNOVA REMNANTS