Chandra and ASCA X-Ray Observations of the Type II-L Supernova SN 1979C 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 type 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 galaxy, a member of the Virgo S Cluster, is at a distance of 17.1 Mpc and contains at least three other supernovae discovered in this century. The useful exposure time was ∼25 ks for the Solid-State Imaging Spectrometer (SIS), ∼28 ks for the Gas Scintillation Imaging Spectrometer, and ∼2.5 ks for the Chandra Advanced CCD Imaging Spectrometer. No point 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-subtracted supernova (SN) signal had a 3 σ upper limit to the flux of 6.3 × 10-14 ergs cm-2 s-1 in the full ASCA SIS band (0.4–10.0 keV) and a 3 σ upper limit of less than (3–4) × 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 ∼3 × 10-14 ergs cm-2 s-1. These measurements 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 through the circumstellar medium.