I have determined the expansion of the supernova remnant of SN1604 ( 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 \propto t ^ { 0.5 } , but there are significant differences in different parts of the remnant : the bright northwestern part expands as r \propto t ^ { 0.35 } , whereas the rest of the remnant ’ s expansion shows an expansion r \propto 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 ( \rho \propto v ^ { - n } ) of n = 7 , or with an exponential density profile ( \rho \propto \exp ( - v / v _ { e } ) ) .
The expansion parameter in the southern region , in conjunction with the shock radius , indicate a rather low value ( < 5 \times 10 ^ { 50 } erg ) for the explosion energy of SN1604 for a distance of 4 kpc .
An 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 \propto t ^ { 0.7 } , corresponding to a shock velocity of v = 4200 km s ^ { -1 } , for a distance to SN1604 of 4 kpc .
This is consistent with the idea that X-ray synchrotron emission requires shock velocities in excess of \sim 2000 km s ^ { -1 } .

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 .