Limits on Enhanced Radio Wave Scattering by Supernova Remnants

We report multifrequency observations with the NRAO Very Long Baseline Array (VLBA) of the compact radio sources J0128+6306 and J0547+2721, which are viewed through the supernova remnants G127.1+0.5 and S147, respectively. Observations were made at frequencies of 1.427, 1.667, 2.271, and 4.987 GHz. The lines of sight to these sources pass through the shock wave and upstream and downstream turbulent layers of their respective supernova remnants and thus might detect cosmic-ray-generated turbulence produced during the Fermi acceleration process. For both sources, we detect interstellar scattering, characterized by a component of the angular size that scales as the square of the observing wavelength. The magnitude of the scattering is characterized by an effective scattering angular size θS0 at a frequency of 1 GHz of 13.2 ± 2.6 mas for J0128+6306 and 6.7 ± 2.2 mas for J0547+2721. These angular sizes are consistent with the incidental scattering for any line of sight out of the galaxy at similar Galactic latitudes and longitudes. There is therefore no evidence for enhanced turbulence at these supernova remnants. We establish upper limits to the supernova remnant-associated scattering measures of 8.1-14.8 m-20/3 pc for J0128+6306 and 3.0 m-20/3 pc for J0547+2721.