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} PerformanceVerification observations of the nearby Seyfert 1 galaxy MCG-6-30-15.Confirming the results of Fabian et al. (1994), we find definite evidence forthe {\sc O\,vii} and {\sc O\,viii} K-shell absorption edges of the warmabsorber and a doubling of the warm absorber column density within the 3 weeksseparating the two observations. No intra-day {\it flux-correlated} variabilityof 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. Possibleinterpretations are discussed but a contradiction remains: the constancy of theionization state of the warm absorber argues that it lies at large distancesfrom the central source whereas the short term change in column density arguesfor small distances. Fluorescent iron emission is examined. As found by Fabianet 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 findvery rapid primary X-ray variability. Assuming relativistic beaming to beunimportant, the derived efficiency is comparable to the maximum obtainablefrom accretion onto a Schwarzschild black hole. Correlated variability outsideof the energy range of {\it ASCA} might exceed this maximum, thus requiringefficient accretion onto a Kerr hole.