1999ApJ...527..298H

Four individual high-resolution X-ray images from ROSAT and the Einstein Observatory have been used to measure the expansion rate of the remnant of Kepler's supernova (SN 1604). Highly significant measurements of the expansion have been made for time baselines varying from 5.5 to 17.5 yr. All measurements are consistent with a current expansion rate averaged over the entire remnant of 0.239^+0.015+0.017_{–0.015–0.010}% yr^–1, which, when combined with the known age of the remnant, determines the expansion parameter m, defined as R∝t^m, to be 0.93^+0.06+0.07_{–0.06–0.04}. The error bars on these results include both statistical (first pair of errors) and systematic (second pair) uncertainty. According to this result, the X-ray remnant is expanding at a rate that is remarkably close to free expansion and nearly twice as fast as the mean expansion rate of the radio remnant. The expansion rates as a function of radius and azimuthal angle are also presented based on two ROSAT images that were registered to an accuracy better than 0".5. Significant radial and azimuthal variations that appear to arise from the motion of individual X-ray knots are seen. The high expansion rate of the X-ray remnant appears to be inconsistent with currently accepted dynamical models for the evolution of Kepler's SNR.