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Typically, integrated radio frequency continuum spectra of supernova remnants(SNRs) exhibit a power-law form due to their synchrotron emission. In numerouscases, these spectra show an exponential turnover, long assumed due to thermalfree-free absorption in the interstellar medium. We use a compilation ofGalactic radio continuum SNR spectra, with and without turnovers, to constrainthe distribution of the absorbing ionized gas. We introduce a novelparameterization of SNR spectra in terms of a characteristic frequency v_*,which depends both on the absorption turnover frequency and the power-lawslope. Normalizing to v_* and to the corresponding flux density, S_*, wedemonstrate that the stacked spectra of our sample reveal a similarity inbehavior with low scatter (r.m.s. ~15%), and a unique exponential drop-offfully consistent with the predictions of a free-free absorption process.Observed SNRs, whether exhibiting spectral turnovers or not, appear to bespatially well mixed in the Galaxy without any evident segregation betweenthem. Moreover, their Galactic distribution does not show a correlation withgeneral properties such as heliocentric distance or Galactic longitude, asmight have been expected if the absorption were due to a continuousdistribution of ionized gas. However, it naturally arises if the absorbers arediscretely distributed, as suggested by early low-frequency observations.Modeling based on HII regions tracking Galactic spiral arms successfullyreproduces the patchy absorption observed to date. While more extensivestatistical datasets should yield more precise spatial models of the absorbinggas distribution, our present conclusion regarding its inhomogeneity willremain robust.

One size fits all: Insights on Extrinsic Thermal Absorption from Similarity of Supernova Remnant Radio Continuum Spectra