2004ApJ...607..855K

We present a new method of determining the systemic velocity of Galactic supernova remnants (SNRs) based on H I absorption of their linearly polarized radio continuum emission. Conventional H I observations of total power emission are limited by H I emission and self-absorption along the line of sight, but since H I emission is unpolarized, the only limits on measurements of absorption of the polarized emission are noise and velocity resolution. This leads to lower uncertainties and makes it possible to obtain absorption profiles for virtually all Galactic SNRs with very precise H I column densities. To demonstrate the new technique, we have obtained H I absorption profiles from Tycho's supernova remnant (G120.1+1.4). Absorption profiles of the polarized emission are very similar to those of the total power emission. Optical depths from the polarization profiles are slightly larger because of small-scale emission features. We also observed polarization absorption profiles of the Boomerang pulsar wind nebula (part of G106.3+2.7) and the plerionic SNR DA 495 (G65.7+1.2), remnants that are so faint that absorption profiles cannot be obtained in total power.