One size fits all: Insights into extrinsic thermal absorption based on the similarity of supernova remnant radio-continuum spectra

Typically, integrated radio frequency continuum spectra of supernova remnants (SNRs) exhibit a power-law form due to their synchrotron emission. In numerous cases, these spectra show an exponential turnover, which has long been assumed to be due to thermal free-free absorption in the interstellar medium. We used a compilation of Galactic radio continuum SNR spectra, with and without turnovers, to constrain the distribution of the absorbing ionised gas. We introduce a novel parameterisation of SNR spectra in terms of a characteristic frequency, ν * which depends both on the absorption turnover frequency and the power-law slope. Normalising to v * and to the corresponding flux density, S * we demonstrate that the stacked spectra of our sample reveal a similarity in behavior with low scatter (root mean square, rms, of ~15%), and a unique exponential drop-off that is fully consistent with the predictions of a free-free absorption process. Observed SNRs, whether exhibiting spectral turnovers or not, appear to be spatially well-mixed in the Galaxy without any evident segregation between them. Moreover, their Galactic distribution does not show a correlation with general properties such as heliocentric distance or Galactic longitude, as might have been expected if the absorption were due to a continuous distribution of ionised gas. However, it naturally arises if the absorbers are discretely distributed, as suggested by early low-frequency observations. Modelling based on H II regions tracking Galactic spiral arms successfully reproduces the patchy absorption observed to date. While more extensive statistical datasets should yield more precise spatial models of the absorbing gas distribution, our present conclusion regarding its inhomogeneity will remain robust.