The efficient low-mass Seyfert MCG-05-23-016

Aims. The Seyfert 1.9 galaxy MCG-05-23-016 has been shown to exhibit a complex X-ray spectrum. This source has moderate X-ray luminosity, hosts a comparably low-mass black hole, but accretes at a high Eddington rate, and allows us to study a super massive black hole in an early stage. Methods. Three observations of the INTEGRAL satellite simultaneous with pointed Swift/XRT observations performed from December 2006 to June 2007 are used in combination with public data from the INTEGRAL archive to study the variability of the hard X-ray components and to generate a high-quality spectrum from 1 to 150 keV. Results. The AGN shows little variability in the hard X-ray spectrum, with some indication of a variation in the high-energy cut-of energy ranging from 50 keV to >> 100 keV, with an electron plasma temperature in the 10-90 keV range. The reflection component is not evident and, if present, the reflected fraction can be constrained to R < 0.3 for the combined data set. Comparison to previous observations shows that the reflection component has to be variable. No variability in the UV and optical range is observed on a time scale of 1.5 years. Conclusions. The hard X-ray spectrum of MCG-05-23-016 appears to be stable with the luminosity and underlying power law varying moderately and the optical/UV flux staying constant. The reflection component and the iron K alpha line seem to have decreased between December 2005 and the observations presented here. The spectral energy distribution appears to be similar to that of Galactic black hole systems, e. g. XTE 1118+ 480 in the low state. The AGN exhibits a remarkably high Eddington ratio of L-bol/L-Edd greater than or similar to 0.8 (or L-bol/L-Edd greater than or similar to 0.1, if we consider a higher mass of the central engine) and, at the same time, a low cut-off energy around 70 keV. Objects like MCG-05-23-016 might indicate the early stages of super massive black holes, in which a strong accretion flow feeds the central engine.