On the Existence of “Radio Thermally Active” Galactic Supernova Remnants

Abstract

In this paper, we investigate the possibility of significant production of thermal bremsstrahlung radiation at radio continuum frequencies that could be linked to some Galactic supernova remnants (SNRs). The main targets for this investigation are SNRs expanding in high-density environments. There are several indicators of radio thermal bremsstrahlung radiation from SNRs, such as a flattening at higher frequencies and thermal absorption at lower frequencies intrinsic to an SNR. In this work, we discuss the radio continuum properties of three SNRs that are the best candidates for testing our hypothesis of significant thermal emission. In the case of SNRs IC 443 and 3C 391, thermal absorption has been previously detected. For IC 443, the contribution of thermal emission at 1 GHz, from our model fit is 3%-57%. It is similar to the estimate obtained from the thermal absorption properties (10%-40% at 1 GHz). In the case of the 3C 391 the conclusions are not so clear. The results from our model fit (thermal emission contribution of 10%-25% at 1 GHz) and results obtained from the low-frequency absorption (thermal contribution of 0.15%-7% at 1 GHz) do not overlap. For the SNR 3C 396 we suggest that if previously detected thermal absorption could be intrinsic to the SNR then the thermal emission (<47% at 1 GHz from our model fit) could be significant enough to shape the radio continuum spectrum at high frequencies. Polarization observations for these SNRs can constrain the strength of a thermal component. Reliable observations at low frequencies (<100 MHz) are needed as well as more data at high radio frequencies (>1 GHz), in order to make stronger conclusions about the existence of "radio thermally active" SNRs.

A New Look at the Integrated Radio/Microwave Continuum Spectrum of Galactic Supernova Remnant Ic 443

“…No indications of significant thermal bremsstrahlung emission from the SNR have been found by Planck Collaboration Int. XXXI (2016) in contrast to the results of Onić et al (2012).…”

“…Generally, verification of several theoretical models (e.g., non-linear particle acceleration in young SNRs, significant thermal bremsstrahlung emission from the SNRs expanding in the dense environment, models of dust emission linked to the SNRs, etc) rely, particularly on a good knowledge of the high-frequency part of the radio as well as the microwave continuum of SNRs (Reynolds & Ellison 1992;Scaife et al 2007;Onić et al 2012;Onić & Urošević 2015;Onić 2015;Génova-Santos et al 2016). The ground based radio-observations of SNRs at frequencies higher than around 10 GHz generally suffer from the transparency issues due to the existence of Earth's atmosphere.…”

On the radio spectra of supernova remnants

“…Urošević and Pannuti 2005;Urošević et al 2007;Onić et al 2012). Especially at the higher radio frequencies, the thermal bremsstrahlung component becomes significant (S ν ∝ ν −0.1 ), and when combined with the nominal synchrotron emission detected from SNRs, the spectrum appears flatter than expected when extrapolating from lower radio frequencies where synchrotron component is dominant (S ν ∝ ν −α ).…”

Planckintermediate results

“…The 10.7 GHz and 30 GHz flux densities are consistent with a single power law from 1-30 GHz, with the single outlier at 8 GHz being little more than 1σ away. Both the model of Onić et al (2012) and the single power law predict the flux density at 30 GHz perfectly, but they over predict the higherfrequency emission at 40 and143 GHz by factors of more than 3 and 2, respectively. Therefore, the shape of the IC 443 SED requires a dip in emissivity in the microwave, rather than an excess due to free-free emission.…”

“…The utility of the microwave flux densities is illustrated by comparing the observations of IC 443 to extrapolations based only on lower-frequency data. In one recent paper, there is a claim for an additional emission mechanism at 10 GHz and above, with thermal free-free emission contributing at a level comparable to or higher than synchrotron radiation (Onić et al 2012). The highest-frequency data point considered in that paper was at 8 GHz.…”