  We report the detection of a narrow Fe-K emission line in the Seyfert 1
galaxy NGC 5548 with the Chandra High-Energy Transmission Gratings. In the
galaxy frame we measure a center energy of 6.40 \pm 0.03 keV, a FWHM of 4515
(+3525,-2645) km/s, and an equivalent width of 133 (+62,-54) eV. The line
energy is consistent with an origin in cold, neutral matter, but ionization
states up to ~Fe XVIII are not ruled out. Assuming Keplerian motion, the
velocity width is consistent with the line being produced in the outer
optical/UV broad-line region (BLR) at about a light-month from the central
X-ray source. We cannot rule out some contribution to the Fe-K line from a
putative, parsec-scale obscuring torus. The continuum intensity during the
Chandra observation was a factor ~2 less than typical historical levels. If the
X-ray continuum was > than a factor 2 higher in the recent past before the
Chandra observation and the narrow Fe-K line intensity had not yet responded,
then the predicted line equivalent width and intensity for an origin in the BLR
is within the 90% measurement errors. Retroactive spectral fitting of archival
ASCA data shows evidence that the narrow Fe-K has a variable intensity. It will
be important to measure the non-disk component of the Fe-K line in other AGN to
obtain a more complete understanding of the total Fe-K line emission.
Measurement of the narrow-line parameters provides powerful diagnostics of
conditions beyond the accretion disk and is necessary to properly deconvolve
the broad, relativistic Fe-K line.
