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Gamma-Ray Emission of Supernova Remnants and the Origin of Galactic Cosmic Rays

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Abstract

The recent surveys of the Milky Way with space and ground-based gamma-ray detectors revealed hundreds of high energy (HE) and tens of very high energy (VHE) gamma-ray emitters representing several galactic source populations – supernova remnants, giant molecular clouds, star forming regions, pulsars, pulsar wind nebulae, binary systems. The major fraction of these objects remains however unidentified. In this chapter I discuss the astrophysical implications of VHE gamma-ray observations of supernova remnants (SNRs) in the context of the origin of galactic cosmic rays. These observations confirm the earlier theoretical predictions of effective acceleration of multi-TeV particles in young SNRs by strong shock waves. The interpretation of VHE gamma-ray data from several prominent representatives of young SNRs within the so-called hadronic models requires hard energy spectra of protons extending to 100 TeV, with total energy released in relativistic protons and nuclei as large as 1050 erg. Formally, this can be considered as an observational proof of the so-called SNR paradigm of the origin of galactic cosmic rays. However, the hadronic models are not free of problems related to interpretation of multi-wavelength properties of these objects. Moreover, in most of the cases the gamma-ray data can be explained by the inverse Compton scattering of electrons which are responsible also for the synchrotron X-radiation of young SNRs. These circumstances prevent us from a firm statement about the contribution of SNRs to the overall flux of galactic cosmic rays. Further observations of young SNRs, especially in the highest energy band (well above 10 TeV), can be crucial in this regard. Quite important are also the complementary observations from massive molecular clouds located within the close proximity of mid-age SNRs.

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Notes

  1. 1.

    ​Remarkably, in the survey of the galactic plane conducted with the HESS, an extended source, HESS 1731-374 with a shell-type structure has be found with no clear counterpart at other wavelengths. However, soon a new shell-type SNR was reported, both in radio and nonthermal X-rays, positionally coinciding with the gamma-ray source. If this association is correct, this would make HESS 1731-374 as the first SNR discovered in gamma-rays (Abramowsky et al. 2011).

  2. 2.

    ​Recently, the detection of 4.1 keV line emission has been reported from the youngest galactic supernova remnant G1.9+0.3 (Borkowski et al. 2010). The likely interpretation of this line is its association to 44Sc which is produced from 44Ti via electron capture.

  3. 3.

    ​Note that in pulsar wind nebulae the acceleration rate of electrons exceeds, for the given magnetic field, the corresponding rate in SNR shocks of speed v sh by a factor of (cv sh)2 ∼ 103 − 104; therefore in these objects electrons can be accelerated to PeV energies. The HESS observations of several plerions with hard gamma-ray spectra extending to tens of TeV without indication of a spectral cutoff is indeed a strong evidence that pulser wind nebulae act as extremely effective electron accelerators.

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© 2013 Springer Science+Business Media Dordrecht

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Aharonian, F. (2013). Gamma-Ray Emission of Supernova Remnants and the Origin of Galactic Cosmic Rays. In: Oswalt, T.D., Gilmore, G. (eds) Planets, Stars and Stellar Systems. Springer, Dordrecht. https://doi.org/10.1007/978-94-007-5612-0_15

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