ASCA PV observations of the Seyfert 1 galaxy MCG-6-30-15 : rapid variability of the warm absorber

We present a detailed re-analysis of the two {\it ASCA} Performance Verification observations of the nearby Seyfert 1 galaxy MCG-6-30-15. Confirming the results of Fabian et al. (1994), we find definite evidence for the {\sc O\,vii} and {\sc O\,viii} K-shell absorption edges of the warm absorber and a doubling of the warm absorber column density within the 3 weeks separating the two observations. No intra-day {\it flux-correlated} variability of the warm absorber is found. However, we report the discovery of an `event' in which the warm absorber parameters temporarily change for \sim10\,000\thinspace s before returning to their original values. Possible interpretations are discussed but a contradiction remains: the constancy of the ionization state of the warm absorber argues that it lies at large distances from the central source whereas the short term change in column density argues for small distances. Fluorescent iron emission is examined. As found by Fabian et al. (1994), the iron line is broad and strong (equivalent width \sim300\thinspace eV). The line profile is also suggestive of it being skewed. Such a line would be expected from a relativistic accretion disk. We also find very rapid primary X-ray variability. Assuming relativistic beaming to be unimportant, the derived efficiency is comparable to the maximum obtainable from accretion onto a Schwarzschild black hole. Correlated variability outside of the energy range of {\it ASCA} might exceed this maximum, thus requiring efficient accretion onto a Kerr hole.