The possible submillimeter bump and accretion-jet in the central supermassive black hole of NGC 4993

Abstract: 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.