2000ApJ...545L..53H

Two ROSAT high-resolution images separated by nearly 5 years have been used to determine the expansion of the X-ray remnant of Tycho's supernova (SN 1572). The current expansion rate averaged over the entire remnant is 0.124%±0.011%.yr^–1, which, when combined with the known age of the remnant, determines the mean expansion parameter m, defined as R∝t^m, to be 0.54±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±0.06 at the outermost edge of Tycho's supernova remnant (SNR) to a value of m=0.34±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~4-5x10⁵⁰ ergs and n₀~0.3-0.6 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.