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The Astrophysical Journal Letters


We report on the first detection of presupernova evolution in the X-ray regime. 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 probe the SN shell interaction with the ambient circumstellar matter (CSM). After 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 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 reaches a radius of 3 × 1017 cm from the site of the explosion, corresponding to ~104 yr in the progenitor's stellar wind history. Our analysis shows that the mass-loss rate of the progenitor has decreased constantly from = 4 × 10-4 to 4 × 10-5 M yr-1 (vw/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 (ρcsm r-1.63) than expected for a constant mass-loss rate and constant wind velocity profile (r-2). The observed evolution 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.


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