We present the first high-resolution X-ray study of emission line variability
with superorbital phase in the neutron star binary LMC X-4. Our analysis
provides new evidence from X-ray spectroscopy confirming accretion disk
precession as the origin of the superorbital period. The spectra, obtained with
the Chandra High-Energy Transmission Grating Spectrometer (HETGS) and the
XMM-Newton Reflection Grating Spectrometer (RGS), contain a number of emission
features, including lines from hydrogen-like and helium-like species of N, O,
Ne, and Fe, a narrow O VII RRC, and fluorescent emission from cold Fe. We use
the narrow RRC and the He-alpha triplets to constrain the temperature and
density of the (photoionized) gas. By comparing spectra from different
superorbital phases, we attempt to isolate the contributions to line emission
from the accretion disk and the stellar wind. There is also evidence for highly
ionized iron redshifted and blueshifted by ~25,000 km/s. We argue that this
emission originates in the inner accretion disk, and show that the emission
line properties in LMC X-4 are natural consequences of accretion disk
precession.Comment: 12 pages, 8 figures, uses emulateap