On the supernova remnants with flat radio spectra

Abstract

A considerable fraction of Galactic supernova remnants (SNRs) characterize flat spectral indices ($\alpha<0.5$). There are several explanations of the flat radio spectra of SNRs in the present literature. The most of models involve a significant contribution of the second-order Fermi mechanism but some of them also discuss high compressions (>4), contribution of secondary electrons left over from the decay of charged pions, as well as the possibility of thermal contamination. In the case of expansion in high density environment, intrinsic thermal bremsstrahlung could theoretically shape the radio spectrum of an SNR and also account for observable curved -- "concave up" radio spectra of some Galactic SNRs. This model could also shed a light on the question of flat spectral indices determined in some Galactic SNRs. On the other hand, present knowledge of the radio continuum spectra (integrated flux densities at different frequencies) of SNRs prevent definite conclusions about the significance of proposed models so the question on flat spectral indices still remains open. New observations, especially at high radio continuum frequencies, are expected to solve these questions in the near future. Finally, as there is a significant connection between the majority of Galactic SNRs with flat integrated radio spectrum and their detection in $\gamma$-rays as well as detection of radiative recombination continua in their X-ray spectra, the analysis of high energy properties of these SNRs is very important.Comment: Accepted for publication in Astrophysics and Space Science. arXiv admin note: text overlap with arXiv:1207.2057 by other author

Statistical Analysis of Supernova Remnants in the Large Magellanic Cloud

“…We note that the total contribution of SNRs to the non-thermal flux density of the 6 We note here that radio flux from some SNRs might also have a significant thermal component (Urošević 2014;Onić 2013) but to some degree it is compensated by using the lower limit for the fraction of the thermal radiation of the LMC. LMC is much higher (∼ 6%) than that found by Gordon et al (1998) of M 33 (≈ 2% − 3%).…”

On the radio spectra of supernova remnants

“…7 Plasma β = p/p mag , where p is the gas pressure, and p mag is the magnetic pressure. For a detailed review on SNRs with flat radio spectra, see Onić (2013b) and references therein.…”

“…On the other hand, the high frequency thermal emission naturally leads to low-frequency thermal absorption. Due to this we should expect that this concave-up form of spectrum turnovers into the concave-down form at the low-frequency end of the radio domain (for details see Onić et al 2012;Onić 2013aOnić , 2013b.…”

Investigating Galactic Supernova Remnant Candidates Using LOFAR

“…Low-frequency (350MHz) Galactic plane observations are useful for investigating SNRs and SNR candidates, particularly for differentiating between SNR candidates and H II regions, due to the typically steeper radio spectral indices of SNRs (α≈−0.5; Onić 2013) as compared to H II regions (α0); where S ν ∝ν α for S ν integrated flux density in Jy and ν frequency in Hz (Onić 2013). This means that SNRs are brighter at lower frequencies, while H II regions are brighter at higher frequencies.…”