  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, long assumed due to thermal
free-free absorption in the interstellar medium. We use a compilation of
Galactic radio continuum SNR spectra, with and without turnovers, to constrain
the distribution of the absorbing ionized gas. We introduce a novel
parameterization of SNR spectra in terms of a characteristic frequency v_*,
which depends both on the absorption turnover frequency and the power-law
slope. Normalizing 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 (r.m.s. ~15%), and a unique exponential drop-off
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 ionized gas. However, it naturally arises if the absorbers are
discretely distributed, as suggested by early low-frequency observations.
Modeling based on HII 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.
