X‐Ray, Optical, and Radio Observations of the Type II Supernovae 1999em and 1998S

Observations of the Type II-P (plateau) Supernova (SN) 1999em and Type IIn (narrow emission line) SN 1998S have enabled estimation of the profile of the SN ejecta, the structure of the circumstellar medium (CSM) established by the pre-SN stellar wind, and the nature of the shock interaction. SN 1999em is the first Type II-P detected at both X-ray and radio wavelengths. The Chandra X-ray data indicate non-radiative interaction of SN ejecta with a power-law density profile (rho \propto r^{-n} with n ~ 7) with a pre-SN wind with a low mass-loss rate of ~2 \times 10^{-6} M_sun/yr for a wind velocity of 10 km/sec, in agreement with radio mass-loss rate estimates. The Chandra data show an unexpected, temporary rise in the 0.4--2.0 keV X-ray flux at ~100 days after explosion. SN 1998S, at an age of >3 years, is still bright in X-rays and is increasing in flux density at cm radio wavelengths. Spectral fits to the Chandra data show that many heavy elements (Ne, Al, Si, S, Ar, and Fe) are overabundant with respect to solar values. We compare the observed elemental abundances and abundance ratios to theoretical calculations and find that our data are consistent with a progenitor mass of approximately 15-20 M_sun if the heavy element ejecta are radially mixed out to a high velocity. If the X-ray emission is from the reverse shock wave region, the supernova density profile must be moderately flat at a velocity ~10^4 km/sec, the shock front is non-radiative at the time of the observations, and the mass-loss rate is 1-2 \times 10^{-4} M_sun/yr for a pre-supernova wind velocity of 10 km/sec. This result is also supported by modeling of the radio emission which implies that SN 1998S is surrounded by a clumpy or filamentary CSM established by a high mass-loss rate, ~2 \times 10^{-4} M_sun/yr, from the pre-supernova star.