Session 46 - Supernova Remnants.
Display session, Tuesday, January 14
Metropolitan Ballroom,

## [46.08] The X-ray Expansion of Tycho's Supernova Remnant

J. P. Hughes (Rutgers U.)

The High Resolution Imager (HRI) onboard ROSAT\/ observed Tycho's supernova remnant (SNR) for over 100,000 s in February 1995. This image is about five times deeper than any previous high resolution X-ray image of the SNR and it reveals a number of new spatial features. Most notable is the weak rim of emission seen clearly around the entire remnant, which is believed to be the blast wave propagating through the surrounding circumstellar medium. The brighter regions of the remnant, corresponding to metal-enriched ejecta, are being studied to quantify the amount and size scale of clumping. Here I report on the expansion of the remnant in the X-ray band.

This is the second observation of Tycho's SNR done by the ROSAT\/ HRI: the first occurred in July 1990 for \sim21,000 s. The time baseline between the two pointings is about 4.5 years, roughly 1% of the age of the SNR, which exploded as a supernova in AD1572. The azimuthally-averaged current expansion rate of Tycho's SNR determined from the two ROSAT\/ observations is 0.42\pm0.06^\prime\prime yr^-1 which should be compared to the mean expansion rate of 0.59^\prime\prime yr^-1. A supernova remnant in the Sedov phase of evolution should show a current expansion rate 0.4 times the time-averaged expansion rate; for Tycho we measure a ratio of expansion rates that is significantly larger \sim0.7. I conclude that this strongly indicates that Tycho's SNR has not yet entered the Sedov phase of evolution (as suggested by previous radio and optical studies), and that the dynamical evolution is still dominated by the deceleration of reverse-shocked ejecta.

Two small X-ray bright knots in the southeast are particularly interesting, since earlier work has revealed that these features are almost surely highly enriched knots of SN ejecta with very different abundance distributions. The northern one is apparently Si+S rich, while the southern one is Fe rich. I find that these knots are moving considerably more rapidly than most of the rest of the remnant, and, in fact, the current expansion rate is consistent with their time-averaged expansion rate, implying that these clumps of ejecta have not been decelerated significantly. This is the first evidence for ejecta bullets'' in the remnant of a Type Ia supernova.