  A long-standing question is whether active galactic nuclei (AGN) vary like
Galactic black hole systems when appropriately scaled up by mass (refs 1-3). If
so, we can then determine how AGN should behave on cosmological timescales by
studying the brighter and much faster varying Galactic systems. As X-ray
emission is produced very close to the black holes, it provides one of the best
diagnostics of their behaviour. A characteristic timescale, which potentially
could tell us about the mass of the black hole, is found in the X-ray
variations from both AGN and Galactic black holes (refs 1-6), but whether it is
physically meaningful to compare the two has been questioned (ref 7). Here we
report that, after correcting for variations in the accretion rate, the
timescales can be physically linked, revealing that the accretion process is
exactly the same for small and large black holes. Strong support for this
linkage comes, perhaps surprisingly, from the permitted optical emission lines
in AGN whose widths (in both broad-line AGN and narrow-emission-line Seyfert 1
galaxies) correlate strongly with the characteristic X-ray timescale, exactly
as expected from the AGN black hole masses and accretion rates. So AGN really
are just scaled-up Galactic black holes.
