On radio and X-ray emission mechanisms in nearby, X-ray-bright galactic nuclei

Abstract

It has been suggested that advection-dominated accretion flows (ADAFs) are responsible for the X-ray activity in nearby galactic nuclei. These X-ray-bright galactic nuclei (XBGNs) are a heterogeneous group that include LINERs, low-to moderate-luminosity Seyfert galaxies, and narrow-line X-ray galaxies with 2-10 keV X-ray luminosities in the range ∼1039 to ∼1043 ergs s-1. In the absence of a radio jet, the core 15 GHz radio luminosity of an ADAF is relatively low and roughly proportional to the mass of the central black hole. The predicted radio luminosity depends primarily on the black hole mass and for XBGNs typically falls in the range 1035 ≲ LR ≲ 1039 ergs s-1. We designate these as "radio-quiet" XBGNs. However, some level of jet activity seems to be present in most sources and the radio emission can be considerably larger than that from the ADAF core. We discuss connections between radio-bright XBGNs and radio-loud, powerful active galactic nuclei (AGNs) and suggest that the radio activities are directly correlated with black hole spins in both cases. Even in the presence of a radio jet, high-resolution, high-frequency radio imaging of nearby XBGNs could identify compact, inverted-spectrum ADAF radio sources. The unique radio/X-ray luminosity relation is confirmed in a few cases in which black hole masses are known and could be used as a tool to estimate unknown black hole masses. For radio-dim (LR ≲ 1039 ergs s-1), X-ray-bright (LX ≳ 1043 ergs s-1) sources, which are primarily Seyfert galaxies, the X-ray emission mechanism is not accounted for by pure ADAFs and radio activities are likely to be similar to those of the radio-quiet AGNs.