The extraordinarily rapid expansion of the X-ray remnant of Kepler's supernova (SN 1604)

Four individual high-resolution X-ray images from ROSAT and the Einstein Ob servatory have been used to measure the expansion rate of the remnant of Ke pler's supernova (SN 1604). Highly significant measurements of the expansio n have been made for time baselines varying from 5.5 to 17.5 yr. All measur ements are consistent with a current expansion rate averaged over the entir e remnant of 0.239(-0.015-0.010)(+0.015+0.017)% yr(-1),which, when combined with the known age of the remnant, determines the expansion parameter m, d efined as R proportional to t(m), to be 0.93(-0.06-0.04)(+0.06+0.07). The e rror bars on these results include both statistical (first pair of errors) and systematic (second pair) uncertainty. According to this result, the X-r ay remnant is expanding at a rate that is remarkably close to free expansio n 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 p resented based on two ROSAT images that were registered to an accuracy bett er than 0."5. Significant radial and azimuthal variations that appear to ar ise 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 accepte d dynamical models for the evolution of Kepler's SNR.