2007ApJ...659.1257R

We derive the preshock density and scale length along the line of sight for the collisionless shock from a deep HST image that resolves the Hα filament in SN 1006 and updated model calculations. The very deep ACS high-resolution image of the Balmer line filament in the northwest quadrant shows that 0.25/cm3≤n₀≤0.4/cm3 and that the scale along the line of sight is about 2x10¹⁸ cm, while bright features within the filament correspond to ripples with radii of curvature less than 1/10 that size. The derived densities are within the broad range of earlier density estimates, and they agree well with the ionization timescale derived from the Chandra X-ray spectrum of a region just behind the optical filament. This provides a test for widely used models of the X-ray emission from SNR shocks. The scale and amplitude of the ripples are consistent with expectations for a shock propagating through interstellar gas with ∼20% density fluctuations on parsec scales as expected from studies of interstellar turbulence. One bulge in the filament corresponds to a knot of ejecta overtaking the blast wave, however. The interaction results from the rapid deceleration of the blast wave as it encounters an interstellar cloud.