  Broad Fe K emission lines have been widely observed in the X-ray spectra of
black hole systems, and in neutron star systems as well. The intrinsically
narrow Fe K fluorescent line is generally believed to be part of the reflection
spectrum originating in an illuminated accretion disk, and broadened by strong
relativistic effects. However, the nature of the lines in neutron star LMXBs
has been under debate. We therefore obtained the longest, high-resolution X-ray
spectrum of a neutron star LMXB to date with a 300 ks Chandra HETGS observation
of Serpens X-1. The observation was taken under the "continuous clocking" mode
and thus free of photon pile-up effects. We carry out a systematic analysis and
find that the blurred reflection model fits the Fe line of Serpens X-1
significantly better than a broad Gaussian component does, implying that the
relativistic reflection scenario is much preferred. Chandra HETGS also provides
highest spectral resolution view of the Fe K region and we find no strong
evidence for additional narrow lines.
