The Expansion of the X-ray Remnant of Tycho's Supernova (SN1572)
  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 $0.124\pm0.011$ % per yr, which, when combined with the known age of the remnant, determines the mean expansion parameter $m$, defined as $R\propto t^m$, to be $0.54\pm0.05$. 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 $m=0.71\pm0.06$ at the outermost edge of Tycho's supernova remnant (SNR) to a value of $m=0.34\pm0.10$ 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 $E\approx 4-5\times 10^{50} \rm ergs$ and $n_0 \approx 0.35-0.45 \rm cm^{-3}$. 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. 
John P. Hughes (Rutgers University)