  NGC 4993, as a host galaxy of the electromagnetic counterpart of the first
gravitational-wave detection of a binary neutron-star merger, was observed by
many powerful telescopes from radio to $\gamma$-ray waveband. The weak nuclear
activities of NGC 4993 suggest that it is a low-luminosity active galactic
nuclear (LLAGN). We build the multi-waveband spectral energy distributions
(SEDs) of NGC 4993 from literatures. We find that the radio spectrum at $\sim
100-300$ GHz is much steeper than that of low-frequency waveband (e.g., 6-100
GHz), where this break was also found in the supermassive black holes in our
galaxy center (Sgr A*), and in some other nearby AGNs. The radio emission above
and below this break may has different physical origins, which provide an
opportunity to probe the accretion and jet properties. We model the
multi-waveband SEDs of NGC 4993 with an advection-dominated accretion flow
(ADAF)-jet model. We find that the high-frequency steep radio emission at
millimeter waveband is consistent with the prediction of the ADAF, while the
low-frequency flat radio spectrum is better fitted by the jet. Further more,
the X-ray emission can be also explained by the ADAF model simultaneously. From
the model fits, we estimate important parameters of the central engine (e.g.,
accretion rate near the horizon of the black hole and mass-loss rate in jet)
for NGC 4993. This result strengthen that the millimiter, submillimer and deep
X-ray observations are crucial to understand the weak or quiescent activities
in supermassive black-hole systems. Further simultaneous millimeter and X-ray
monitoring of this kind of LLAGNs will help us to better understand the
physical origin of multiwaveband emission.
