Spectral variability and reverberation time delays in the Suzaku X-ray
  spectrum of NGC 4051
  Long-exposure Suzaku X-ray observations of the active galaxy NGC 4051 from
2005 and 2008 are analysed, in an attempt to reach a self-consistent
understanding of the spectral variability on long timescales and at high time
resolution. Principal components analysis and a maximum likelihood method of
power spectrum analysis are used. In common with other type I AGN, the spectral
variability is dominated by a varying-normalisation power-law component
together with a quasi-steady, hard-spectrum offset component that contains Fe K
atomic features. NGC 4051 displays a strong excess over a power-law at energies
above 20 keV, some fraction of which also appears to vary with the power-law
continuum. The fast-timescale power spectrum has a shape consistent with
previous determinations, but significant differences are found between the low
and high flux states of the source, demonstrating that the power spectrum is
non-stationary. Frequency-dependent time lags between the hard and soft bands
of up to 970+/-225s are measured. The existence of the observed lags excludes
the possibility that the hard spectral component originates as reflection from
the inner accretion disk. We instead show that the frequency- and
energy-dependent time lags may be explained as reverberation, caused by
reflection from a thick shell of material with maximum lags of about 10,000s.
If the reflecting material surrounds the AGN, it extends to a distance about
1.5x10^14 cm, 600 gravitational radii, from the illuminating source and the
global covering factor is C(g) >~ 0.4, confirming suggestions that type I AGN
have high covering factors of absorbing and reflecting material. Given the
spectral and timing similarities with other type I AGN, we infer that this
structure is common in the type I population. [abridged]
