Chandra and XMM-Newton X-ray observations of AWM 7 - I: Investigating
  X-ray surface brightness fluctuations
  We investigate the levels of small scale structure in surface brightness
images of the core of the X-ray bright cool-core galaxy cluster AWM 7. After
subtraction of a model of the smooth cluster emission, we find a number of
approximately radial surface brightness depressions which are not present in
simulated images and are seen in both the Chandra and XMM-Newton data. The
depressions are most strongly seen in the south of the cluster and have a
magnitude of around 4 per cent in surface brightness. We see these features in
both an energy band sensitive to the density (0.6 to 5 keV) and a band more
sensitive to the pressure (3.5 to 7.5 keV). Histograms of surface brightness in
the data, when compared to realisations of a smooth model, reveal stronger
surface brightness variations. We use the Delta-variance technique to
characterise the magnitude of the fluctuations as a function of length scale.
We find that the spectrum in the 0.6 to 5 keV band is flatter than expected for
Kolmogorov index fluctuations. If characterised by a power spectrum, on large
scales it would have an index around -1.7, rather than -3.7. The implied 3D
density fluctuations have a standard deviation of around 4 per cent. The
implied 3D pressure variations are at most 4 per cent. Most of the longer-scale
power in the density spectrum is contributed by the southern half of the
cluster, where the depressions are seen. The density variations implied by the
spectrum of the northern sector have a standard deviation of about 2 per cent.
