Scaling variability from stellar to supermassive black holes

We investigate the correspondence between the variability seen in the stellar and supermassive black holes. Galactic black hole (GBH) power density spectra (PDS) are generally complex, and dependent on spectral state. In the low/hard state the high-frequency rollover in the PDS moves in a way which is not simply related to luminosity. Hence this feature can only be used as an approximate indicator rather than as an accurate tracer of black hole mass in active galactic nuclei (AGNs). The X-ray spectrum in the high/soft state is dominated by the disc in the GBH, which is rather stable. We show that the PDS of the Comptonized tail in GBHs can be much more variable, and that it is this which should be compared to AGNs due to their much lower disc temperature. This bandpass effect removes a problem in interpreting the (often highly variable) narrow-line Seyfert 1 (NLS1) galaxies as the counterparts of the high mass accretion rate GBHs. Additionally, we speculate that some NLS1s (e.g. Akn 564) are counterparts of the very high state. The Comptonized tail in this state is also highly variable, but with PDS which can be roughly described as band-limited noise. This shape is similar to that seen in the low/hard state, so merely seeing such band-limited noise in the power spectrum of an AGN does not necessarily imply low luminosity. We also stress that Cygnus X-1, often used for comparison with AGNs, is not a typical black hole system due to its persistent nature. In particular, the shape of its power spectrum in the high/soft state is markedly different from that of other (transient) GBH systems in this state. The fact that the NLS1s NGC 4051 and MCG -6-30-15 do appear to show a power spectrum similar to that of the high/soft state of Cyg X-1 may lend observational support to theoretical speculation that the hydrogen ionization disc instability does not operate in AGNs.