Title:X-ray observation of radio supernova SN1979C by ASCA

Abstract: We report on the X-ray observation of the radio selected supernova SN1979C carried out with ASCA in December 1997. The supernova of type II$_{L}$ was first observed in the optical and occurred in the weakly barred, almost face on spiral galaxy NGC 4321 (M100) which is at a distance of 17.1 Mpc, and contains at least three other supernovae discovered in this century. No point source was detected at the radio position of SN1979C in a 3' diameter half power response circle in a 27.3 ks SIS exposure. The background and galaxy subtracted SN signal had a 3$\sigma$ upper limit to the count rate of 1.2$\times 10^{-3}$ cps in the full ASCA SIS band (0.4-10.0 keV). These measurements give the first ever x-ray flux limit of a Type II$_{L}$ SN in the higher energy band ($\geq$ 2 keV) which is an important diagnostic of the {\it outgoing} shock wave ploughing through the circumstellar medium.