X-ray emission from NGC 4321 (M 100): detection of supernova 1979C

In a 42.8 ks ROSAT HRI X-ray observation of the spiral galaxy NGC 4321 (M 100) X-ray emission from the supernova 1979C is discovered, sixteen years after its outburst, with an (0.1-2.4 keV) X-ray luminosity of L_x = 1.0 x 10(39) erg s(-1) . No X-ray emission is observed from the three other historical supernovae in NGC 4321 (SN 1901B, SN 1914A and SN 1959E). In addition to SN 1979C, seven X-ray point sources are detected inside the D₂₅ ellipse of the galaxy, with luminosities ranging from 4.2 x 10(38) to 6.5 x 10(39) erg s(-1) . Apart from two bright sources in the nuclear region of NGC 4321, none of the other point-like X-ray sources show any time variability over the observation period. An unresolved diffuse emission component fills the entire optical extent of the galaxy. The total luminosity of the diffuse component is 3.5 x 10(40) erg s(-1) . Point sources contribute 1.4 x 10(40) erg s(-1) to the total luminosity of 5.5 x 10(40) erg s(-1) . Three archival Einstein HRI observations of NGC 4321 were merged into a single 41.3 ks observation. Six point-like X-ray sources are detected inside the D₂₅ ellipse of NGC 4321 with Einstein (0.1-4.5 keV) luminosities in the range 1.1 - 5.1 x 10(39) erg s(-1) . Three of the sources coincide with the positions of ROSAT sources (the two bulge sources and a southern interarm source). Comparison of the ROSAT and Einstein luminosities show that the sources are variable. Einstein upper limits are evaluated at the positions of all other ROSAT sources and historical supernovae in NGC 4321.