X-ray detection of presupernova evolution for the SN 1993J progenitor

We report on the first detection of presupernova evolution in the X-ray reg ime. The results are based on ROSAT observations of SN 1993J ranging from 6 days to 5 years after the outburst. The X-ray observations are used to pro be the SN shell interaction with the ambient circumstellar matter (CSM). Af ter exploring various scenarios that might explain the observed X-ray light curve with a t(-0.27) rate of decline, we present a coherent picture in te rms 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 reaches a radius of 3 x 10(17) cm from the site of the explosion, corresponding to similar to 10(4) yr in the progenitor's stellar wind history. Our analysis shows that the mass-loss rate of the progenitor has decreased constantly f rom (M) over dot = 4 x 10(-4) to 4 x 10(-5) M. yr(-1) (v(w)/10 km s(-1)) du ring the late stage of the evolution. Assuming a spherically symmetric expa nsion, the circumstellar matter density profile is found to be significantl y flatter (rho (csm) proportional to r(-1.63)) than expected for a constant mass-loss rate and constant wind velocity profile (r(-2)). The observed ev olution 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 la te stage of its evolution.