Abstract
The theoretical fundamentals of formation of the supernova remnant (SNR) continuum radio spectra are presented in this review. Mainly based on the Fermi 1 theory—also known as diffuse shock acceleration (DSA)—the different shapes (linear or curved in log-log scale) of SNR radio spectra are predicted for both young and evolved SNRs. On the other hand, some particular forms of spectra of older SNRs can be predicted by including the additional processes such as Fermi 2 acceleration mechanism or thermal bremsstrahlung radiation. Also, all of these theoretically predicted forms of radio spectra are compared with real spectra obtained from observations. Finally this review can represent some kind of “atlas” with initial patterns for the different kinds of SNR radio spectra—it should be helpful for radio astronomers in their interpretation of the observed radio spectra.
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Notes
The magnetic perturbations (from which a high-energy charge particle can be reflected), are connected with turbulent fluid motion in the downstream region. Fast particles are prevented from streaming away upstream of a shock front by the scattering of Alfvén waves which they themselves generate and essentially represent magnetic perturbations in the upstream region.
The relation between the characteristic acceleration times to energy E for DSA and Fermi 2 mechanisms is given by \(t_{\mathrm{acc}}(\mathrm{DSA})\propto M_{\mathrm{A}2}^{-2}t_{\mathrm{acc}} (\mathrm{Fermi}~2)\) (Reynolds 2008), where MA2=u2/vA is the Alfvén Mach number of the downstream flow; u2 is the downstream fluid velocity and vA is the Alfvén speed, defined by vA=B/(μ0 ϱ)1/2, where B is the magnetic field strength, μ0 is the magnetic permeability of the vacuum and ϱ is the mass density.
SNR spectral indices are mainly between 0.2 and 0.8.
The magnetic field amplification is driven by the non-linear effects, when pressure of CRs, in addition to the gas pressure, represents a significant part of the total pressure (Bell 2004). A detailed description of the non-linear DSA will be presented later in this review.
The idealized vertical profile of the shock discontinuity is degenerated into the weak subshock discontinuity plus the wide transition region, which together represent a modified shock.
Plasma β=p/pmag, where p is the gas pressure, and pmag is the magnetic pressure.
The lifetime of an SNR, as defined by McKee and Ostriker (1977), can be up to one million years—this timescale is comparable to the typical timescale of synchrotron loss for electrons which radiate at the highest radio frequencies.
Crab nebula and SNR 3C58 represent particular type of SNR, the so-called pulsar wind nebulae, which are not target of this review. Contrary to shell-like, composite and mixed-morphology SNRs which are target of this review, the pulsar wind nebulae typically have flat spectral indices between 0.0 and 0.3 (e.g. Reynolds et al. 2012).
Foster et al. (2006) concluded that OA184 is rather a Galactic HII region then an SNR.
It results in the concave-up form of overall radio spectrum.
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Acknowledgements
I would like to thank the referee for excellent comments and suggestions. Also, I would like to thank N. Duric, T. Pannuti, D. Onić, B. Arbutina, and M. Pavlović for help in many aspects: exciting discussions, careful reviewing and editing of typescript, providing some data and references, etc. Their comments provided that the final version of this review has appeared in significantly better form. Additionally I acknowledge the financial support of the Ministry of Education, Science, and Technological Development of the Republic of Serbia through the project No. 176005 “Emission Nebulae: Structure and Evolution”.
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Urošević, D. On the radio spectra of supernova remnants. Astrophys Space Sci 354, 541–552 (2014). https://doi.org/10.1007/s10509-014-2095-4
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DOI: https://doi.org/10.1007/s10509-014-2095-4