A Suzaku Observation of NGC 4593: Illuminating the Truncated Disk
  We report results from a 2007 Suzaku observation of the Seyfert 1 AGN NGC
4593. The narrow Fe K alpha emission line has a FWHM width ~4000 km/s,
indicating emission from >~ 5000 Rg. There is no evidence for a
relativistically broadened Fe K line, consistent with the presence of a
radiatively efficient outer disk which is truncated or transitions to an
interior radiatively inefficient flow. The Suzaku observation caught the source
in a low-flux state; compared to a 2002 XMM observation, the hard X-ray flux
decreased by 3.6, while the Fe K alpha line intensity and width each roughly
halved. Two model-dependent explanations for the changes in Fe line profile are
explored. In one, the Fe line width has decreased from ~10000 to ~4000 km/s
from 2002 to 2007, suggesting that the thin disk truncation/transition radius
has increased from 1000-2000 to >~5000 Rg. However, there are indications from
other compact accreting systems that such truncation radii tend to be
associated only with accretion rates relative to Eddington much lower than that
of NGC 4593. In the second (preferred) model, the line profile in the XMM
observation consists of a time-invariant narrow component plus a broad
component originating from the inner part of the truncated disk (~300 Rg) which
has responded to the drop in continuum flux. The Compton reflection component
strength R is ~1.1, consistent with the measured Fe K alpha line total EQW with
an Fe abundance 1.7 times solar. The modest soft excess has fallen by a factor
of ~20 from 2002 to 2007, ruling out emission from a region 5 lt-yr in size.
