The variable iron K emission line in MCG—6-30-15

We report on the variability of the iron K emission line in the Seyfert 1 galaxy MCG—6-30-15 during a four-day ASCA observation. The line consists of a narrow core at an energy of about 6.4 keV, and a broad red wing extending to below 5 keV, which are interpreted as line emission arising from the inner parts of an accretion disc. The narrow core correlates well with the continuum flux whereas the broad wing weakly anticorrelates. When the source is brightest, the line is dominated by the narrow core, whilst, during a deep minimum, the narrow core is very weak and a huge red tail appears. However, at other times when the continuum shows rather rapid changes, the broad wing is more variable than the narrow core, and shows evidence for correlated changes contrary to its long-time-scale behaviour. The peculiar line profile during the deep minimum spectrum suggests that the line-emitting region is very close to a central spinning (Kerr) black hole where enormous gravitational effects operate.