Two ROSAT high resolution images separated by nearly five years have been used to determine the expansion of the X-ray remnant of Tycho ’ s supernova ( SN1572 ) .
The current expansion rate averaged over the entire remnant is 0.124 \pm 0.011 \% yr ^ { -1 } , which , when combined with the known age of the remnant , determines the mean expansion parameter m , defined as R \propto t ^ { m } , to be 0.54 \pm 0.05 .
There are significant radial and azimuthal variations of the X-ray expansion rate .
The radial expansion in particular shows highly significant evidence for the more rapid expansion of the forward blast wave as compared to the reverse-shocked ejecta , an effect that has not been seen previously .
The expansion parameter varies from m = 0.71 \pm 0.06 at the outermost edge of Tycho ’ s supernova remnant ( SNR ) to a value of m = 0.34 \pm 0.10 on the inside edge of the bright rim of emission .
These values are consistent with the rates expected for a remnant with constant density ejecta evolving into a uniform interstellar medium during the ejecta-dominated phase of evolution .
Based on the size , age , and X-ray expansion rates , I obtain values for the explosion energy and ambient density of E \approx 4 - 5 \times 10 ^ { 50 } ergs and n _ { 0 } \approx 0.35 - 0.45 cm ^ { -3 } .
As is also the case for Cas A and Kepler ’ s SNR , the X-ray expansion rate of Tycho ’ s SNR appears to be significantly higher than the radio expansion rate .
In the case of Tycho ’ s SNR , however , the difference between radio and X-ray expansion rates is clearly associated with the motion of the forward shock .