We report on the first detection of a pre-supernova ( SN ) evolution in the X-ray regime .
The results are based on ROSAT observations of SN 1993J ranging from six days to five years after the outburst .
The X-ray observations are used to probe the SN shell interaction with the ambient circumstellar matter ( CSM ) .
After exploring various scenarios that might explain the observed X-ray lightcurve with a t ^ { -0.27 } rate of decline , we present a coherent picture in terms of the interaction of the SN shock front with the CSM deposited by the progenitor ’ s stellar wind .
During the observed period , the SN shell has reached a radius of 3 \times 10 ^ { 17 } cm from the site of the explosion , corresponding to \sim 10 ^ { 4 } years in the progenitors stellar wind history .
Our analysis shows that the mass-loss rate of the progenitor has decreased constantly from \dot { M } = 4 \times 10 ^ { -4 } to 4 \times 10 ^ { -5 } ~ { } { M } _ { \sun } ~ { } { yr } ^ { -1 } ( v _ { w } / 10 ~ { } { km~ { } s } ^ { % -1 } ) during the late stage of the evolution .
Assuming a spherically symmetric expansion , the circumstellar matter density profile is found to be significantly flatter ( \rho _ { csm } \propto r ^ { -1.63 } ) than expected for a constant mass-loss rate and constant wind velocity profile ( r ^ { -2 } ) .
The observed evolution either reflects a decrease in the mass-loss rate , an increase in the wind speed or a combination of both , indicating that the progenitor likely was making a transition from the red to the blue supergiant phase during the late stage of its evolution .