  X-ray charge-coupled devices (CCDs) are the workhorse detectors of modern
X-ray astronomy. Typically covering the 0.3-10.0 keV energy range, CCDs are
able to detect photoelectric absorption edges and K shell lines from most
abundant metals. New CCDs also offer resolutions of 30-50 (E/dE), which is
sufficient to detect lines in hot plasmas and to resolve many lines shaped by
dynamical processes in accretion flows. The spectral capabilities of X-ray CCDs
have been particularly important in detecting relativistic emission lines from
the inner disks around accreting neutron stars and black holes. One drawback of
X-ray CCDs is that spectra can be distorted by photon "pile-up", wherein two or
more photons may be registered as a single event during one frame time. We have
conducted a large number of simulations using a statistical model of photon
pile-up to assess its impacts on relativistic disk line and continuum spectra
from stellar-mass black holes and neutron stars. The simulations cover the
range of current X-ray CCD spectrometers and operational modes typically used
to observe neutron stars and black holes in X-ray binaries. Our results suggest
that severe photon pile-up acts to falsely narrow emission lines, leading to
falsely large disk radii and falsely low spin values. In contrast, our
simulations suggest that disk continua affected by severe pile-up are measured
to have falsely low flux values, leading to falsely small radii and falsely
high spin values. The results of these simulations and existing data appear to
suggest that relativistic disk spectroscopy is generally robust against pile-up
when this effect is modest.
