2008ApJ...689..231V

I have determined the expansion of the supernova remnant of SN 1604 (Kepler's supernova) based on archival Chandra ACIS-S observations made in 2000 and 2006. The measurements were done in several distinct energy bands, and were made for the remnant as a whole, and for six individual sectors. The average expansion parameter indicates that the remnant expands on average as r∝t^0.5, but there are significant differences in different parts of the remnant: the bright northwestern part expands as r∝t^0.35, whereas the rest of the remnant's expansion shows an expansion r∝t^0.6. The latter is consistent with an explosion in which the outer part of the ejecta has a negative power law slope for density (ρ∝v^–n) of n=7, or with an exponential density profile [ρ∝exp(-v/v_e)]. The expansion parameter in the southern region, in conjunction with the shock radius, indicates a rather low value (<5x10⁵⁰ erg) for the explosion energy of SN 1604 for a distance of 4 kpc. A higher explosion energy is consistent with the results if the distance is larger. The filament in the eastern part of the remnant, which is dominated by X-ray synchrotron radiation, seems to mark a region with a fast shock speed r∝t^0.7, corresponding to a shock velocity of v=4200 km/s, for a distance to SN 1604 of 4 kpc. This is consistent with the idea that X-ray synchrotron emission requires shock velocities in excess of ∼2000 km/s. The X-ray-based expansion measurements reported are consistent with results based on optical and radio measurements but disagree with previous X-ray measurements based on ROSAT and Einstein observations.