CIRCUMSTELLAR INTERACTION IN SN 1993J

摘要

The radio and X-ray observations of SN 1993J during the first year can be consistently explained as a result of interaction of the expanding ejecta with a circumstellar medium. The density of the circumstellar gas can be deduced from the free-free absorption of the radio emission and from the X-ray luminosity. During the first 2 weeks, both sets of observations indicate a mass loss-rate of ～4 x 10~(-5) solar mass yr~(-1) for a wind velocity of 10 km s~(-1). The subsequent radio and X-ray observations indicate a density gradient ∝ r~(-s), with 1.5 approx < s approx< 1.7, as opposed to the r~(-2) gradient expected for a steady, spherically symmetric wind. This may be caused either by a variation of the mass-loss rate from the progenitor system or by a non-spherically symmetric geometry. To explain the properties of the X-ray emission, a steep density gradient in the ejecta is needed. During the first months, most of the observed X-ray emission originates from the circumstellar shock, which is adiabatic, while the reverse shock is radiative. To avoid excessive Comptonization in the X-ray range collisionless heating must be ineffective. The soft X-rays observed at 220 days probably originate from the reverse shock. The ionization and temperature structures of the circumstellar gas are calculated; we find that the temperature is in excess of 10~5 K and the medium is nearly completely ionized by the shock radiation after the formation of the shocks. Pre-acceleration of the circumstellar gas by the radiation from the outbreak can explain the observed high velocity for the circumstellar N Ⅴ and Hα lines. The high luminosity of the lines indicates that the circumstellar medium close to the supernova progenitor had a complex structure.