We report on the X-ray observation of the radio selected supernova SN1979C 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 \sim 25 ks for the Solid-State Imaging Spectrometer ( SIS ) , \sim 28 ks for the Gas Scintillation Imaging Spectrometer ( GIS ) , and \sim 2.5 ks for Chandra ’ s Advanced CCD Imaging Spectrometer ( ACIS ) .
No point source was detected at the radio position of SN1979C in a 3 ’ diameter half power response circle in the ASCA data .
The background and galaxy subtracted SN signal had a 3 \sigma upper limit to the flux of 6.3 \times 10 ^ { -14 } ergs s ^ { -1 } cm ^ { -2 } in the full ASCA SIS band ( 0.4-10.0 keV ) and a 3 \sigma upper limit of < 3-4 \times 10 ^ { -14 } erg s ^ { -1 } cm ^ { -2 } in the 2-10 keV band .
In the Chandra data , a source at the position of SN1979C 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 \sim 3 \times 10 ^ { -14 } ergs s ^ { -1 } cm ^ { -2 } .
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 ploughing through the circumstellar medium .