We present an analysis of archival X-ray observations of the Type IIL supernova SN 1979C .
We find that its X-ray luminosity is remarkably constant at ( 6.5 \pm 0.1 ) \times 10 ^ { 38 } erg s ^ { -1 } over a period of 12 years between 1995 and 2007 .
The high and steady luminosity is considered as possible evidence for a stellar-mass ( \sim 5–10M _ { \odot } ) black hole accreting material from either a supernova fallback disk or from a binary companion , or possibly from emission from a central pulsar wind nebula .
We find that the bright and steady X-ray light curve is not consistent with either a model for a supernova powered by magnetic braking of a rapidly rotating magnetar , or a model where the blast wave is expanding into a dense circumstellar wind .