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 .
It is the least radio luminous and one of the least X-ray luminous SNe ever detected ( except for the unusual and very close SN 1987A ) .
The Chandra X-ray data indicate non-radiative interaction of SN ejecta with a power-law density profile ( \rho \propto r ^ { - n } with n \sim 7 ) for a pre-SN wind with a low mass-loss rate of \sim 2 \times 10 ^ { -6 } M _ { \odot } yr ^ { -1 } for a wind velocity of 10 km s ^ { -1 } , 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 \sim 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 _ { \odot } 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 \sim 10 ^ { 4 } km s ^ { -1 } , the shock front is non-radiative at the time of the observations , and the mass-loss rate is 1–2 \times 10 ^ { -4 } M _ { \odot } yr ^ { -1 } for a pre-supernova wind velocity of 10 km s ^ { -1 } .
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 , \sim 2 \times 10 ^ { -4 } M _ { \odot } yr ^ { -1 } , from the pre-supernova star .