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 yrs to 17.5 yrs .
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 \propto 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 \hskip { 1.0 pt } ^ { \prime \prime } \hskip { -7.0 pt } . \hskip { 2.0 pt } 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 .