Chandra and ASCA X-ray observations of the type II-L supernova SN1979C in NGC 4321

We report on the X-ray observation of the radio-selected supernova SN 1979C carried out with ASCA in 1997 December and serendipitously available from a Chandra Guaranteed Time Observation in 1999 November. The supernova, of t ype SN II-linear (SN II-L), was first observed in the optical and occurred in the weakly barred, almost face-on spiral galaxy NGC 4321 (M100). The gal axy, a member of the Virgo S Cluster, is at a distance of 17.1 Mpc and cont ains at least three other supernovae discovered in this century. The useful exposure time was similar to 25 ks for the Solid-State Imaging Spectromete r (SIS), similar to 28 ks for the Gas Scintillation Imaging Spectrometer, a nd similar to2.5 ks for the Chandra Advanced CCD Imaging Spectrometer. No p oint source was detected at the radio position of SN 1979C in a 3' diameter , half-power response circle in the ASCA data. The background- and galaxy-s ubtracted supernova (SN) signal had a 3 sigma upper limit to the flux of 6. 3 x 10(-14) ergs cm(-2) s(-1) in the full ASCA SIS band (0.4-10.0 keV) and a 3 sigma upper limit of less than (3-4) x 10(-14) erg cm(-2) s(-1) in the 2-10 keV band. In the Chandra data, a source at the position of SN 1979C is marginally detected at energies below 2 keV at a flux consistent with the ROSAT HRI detection in 1995. At energies above 2 keV, no source is detected with an upper limit of similar to3 x 10(-14) ergs cm(-2) s(-1). These meas urements give the first ever X-ray flux limit of a Type II-L SN above 2 keV , which is an important diagnostic of the outgoing shock wave plowing throu gh the circumstellar medium.