  NGC~5548 is an X-ray bright Seyfert 1 active galaxy. It exhibits a variety of
spectroscopic features in the soft X-ray band, including in particular the
absorption by the AGN outflows of a broad range of ionization states, with
column densities up to 1E27 /m^2, and having speeds up to several thousand
kilometers per second. The known emission features are in broad agreement with
photoionized X-ray narrow and broad emission line models. We report on an X-ray
spectroscopic study using 1.1 Ms XMM-Newton and 0.9 Ms Chandra grating
observations of NGC 5548 spanning two decades. The aim is to search and
characterize any potential spectroscopic features in addition to the known
primary spectral components that are already modeled in high precision. We
detect a weak unidentified excess emission feature at 18.4 Angstrom (18.1
Angstrom in the restframe). The feature is seen at >5 sigma statistical
significance taking into account the look elsewhere effect. No known
instrumental issues, atomic transitions, and astrophysical effects can explain
this excess. The observed intensity of the possible feature seems to
anti-correlate in time with the hardness ratio of the source. However, the
variability might not be intrinsic, it might be caused by the time-variable
obscuration by the outflows. An intriguing possibility is the line emission
from charge exchange between a partially ionized outflow and a neutral layer in
the same outflow, or in the close environment. Other possibilities, such as
emission from a highly-ionized component with high outflowing speed, cannot be
fully ruled out.
