  We use four Chandra gratings spectra of the neutron star low-mass X-ray
binary 4U 1820-30 to better understand the nature of certain X-ray absorption
lines in X-ray binaries, including the NeII, NeIII, NeIX, OVII, and OVIII
lines. The equivalent widths of the lines are generally consistent between the
observations, as expected if these lines originate in the hot interstellar
medium. No evidence was found that the lines were blueshifted, again supporting
the interstellar medium origin, though this may be due to poor statistics.
There is apparent variability in the OVIII Ly-alpha line equivalent width
providing some evidence that at least some of the OVIII absorption arises
within the system. However, the significance is marginal (2.4-sigma), and the
lack of variation in the other lines casts some doubt on the reality of the
variability. From calculating the equivalent hydrogen column densities for a
range of Doppler parameters, we find they are consistent with the interstellar
origin of the lines. Additionally we fit the spectra with photoionization
models for locally absorbing material, and find that they can reproduce the
spectrum well, but only when there is an extremely low filling factor. We
conclude that both the ISM and local absorption remain possible for the origin
of the lines, but that more sensitive observations are needed to search for
low-level variability.
