2001ApJ...561L.107I

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 3x10¹⁷ 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 M{dot}=4x10^–4 to 4x10^–5 M_☉ 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 (ρ_csm∝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.