Neutral Pion Emission from Accelerated Protons in the Supernova Remnant W44

The Astrophysical Journal Letters (Impact Factor: 5.34). 11/2011; 742(2):L30. DOI: 10.1088/2041-8205/742/2/L30
Source: arXiv


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 (~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 Ec = 6 ± 1 GeV. Direct evidence for pion emission is then established in an SNR for the first time.

Download full-text


Available from: Piergiorgio Picozza
  • Source
    • "(Courtesy of Dr William Hanlon.) that high Mach number shocks stretch and amplify magnetic field as well as accelerate cosmic ray electrons with energy up to ∼ 100 TeV [16]. While, it was hard to doubt that this was accompanied by proton acceleration , direct evidence has only been presented recently by Fermi and AGILE observations which exhibit the predicted " pion bump " in the γ-ray observations in middleaged supernova remnants expanding into dense molecular gas [17] [18] (Figure 4). No less dramatic have been the observations by Atmospheric Cerenkov Telescopes (ACTs) of TeV γ-rays which show evidence for efficient acceleration beyond ∼ 100 TeV (e.g., [19] [20]). "
    [Show abstract] [Hide abstract]
    ABSTRACT: Physicists have pondered the origin of cosmic rays for over a hundred years. However the last few years have seen an upsurge in the observation, progress in the theory and a genuine increase in the importance attached to the topic due to its intimate connection to the indirect detection of evidence for dark matter. The intent of this talk is to set the stage for the meeting by reviewing some of the basic features of the entire cosmic ray spectrum from GeV to ZeV energy and some of the models that have been developed. The connection will also be made to recent developments in understanding general astrophysical particle acceleration in pulsar wind nebulae, relativistic jets and gamma ray bursts. The prospects for future discoveries, which may elucidate the origin of cosmic rays, are bright.
    Preview · Article · Sep 2014 · Nuclear Physics B - Proceedings Supplements
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Recently, the gamma‐ray telescopes AGILE and Fermi observed several middle‐aged supernova remnants (SNRs) interacting with molecular clouds. A plausible emission mechanism of the gamma‐rays is the decay of neutral pions produced by cosmic ray (CR) nuclei (hadronic processes). However, observations do not rule out contributions from bremsstrahlung emission due to CR electrons. TeV gamma‐ray telescopes also observed many SNRs and discovered many unidentified sources. It is still unclear whether the TeV gamma‐ray emission is produced via leptonic processes or hadronic processes. In this Letter, we propose that annihilation emission of secondary positrons produced by CR nuclei is a diagnostic tool of the hadronic processes. We investigate MeV emissions from secondary positrons and electrons produced by CR protons in molecular clouds. The annihilation emission of the secondary positrons from SNRs can be robustly estimated from the observed gamma‐ray flux. The expected flux of the annihilation line from SNRs observed by AGILE and Fermi is sufficient for the future Advanced Compton Telescope to detect. Moreover, synchrotron emission from secondary positrons and electrons and bremsstrahlung emission from CR protons can be also observed by the future X‐ray telescope NuSTAR and Astro‐H.
    Full-text · Article · Mar 2011 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Recent observations reveal that spectral breaks at ∼GeV are commonly present in Galactic γ-ray supernova remnants (SNRs) interacting with molecular clouds and that most of them have a spectral (E2dF/dE) ‘platform’ extending from the break to lower energies. In Paper I, we developed an accumulative diffusion model by considering an accumulation of the diffusive protons escaping from the shock front throughout the history of the SNR expansion. In this paper, we improve the model by incorporating the finite volume of molecular clouds, demonstrate the model dependence on particle diffusion parameters and cloud size, and apply it to nine interacting SNRs (W28, W41, W44, W49B, W51C, Cygnus Loop, IC443, CTB 37A and G349.7+0.2). This refined model naturally explains the GeV spectral breaks and, in particular, the ‘platforms’, together with available TeV data. We find that the index of the diffusion coefficient δ is in the range 0.5–0.7, similar to the Galactic averaged value, and the diffusion coefficient for cosmic rays around the SNRs is essentially two orders of magnitude lower than the Galactic average (χ∼ 0.01), which is a good indication for the suppression of cosmic-ray diffusion near SNRs.
    Full-text · Article · Aug 2011 · Monthly Notices of the Royal Astronomical Society
Show more