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

It has been suggested that advection-dominated accretion flows (ADAFs) are responsible for the X-ray activity in nearby galactic nuclei. These X-ray-b right galactic nuclei (XBGNs) are a heterogeneous group that include LINERs , low- to moderate-luminosity Seyfert galaxies, and narrow-line X-ray galax ies with 2-10 keV X-ray luminosities in the range similar to 10(39) to simi lar to 10(43) ergs s(-1). In the absence of a radio jet, the core 15 GHz ra dio luminosity of an ADAF is relatively low and roughly proportional to the mass of the central black hole. The predicted radio luminosity depends pri marily on the black hole mass and for XBGNs typically falls in the range 10 (35) less than or similar to LR less than or similar to 10(39) ergs s(-1). We designate these as "radio-quiet" XBGNs. However, some level of jet activ ity seems to be present in most sources and the radio emission can be consi derably 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 ho le spins in both cases. Even in the presence of a radio jet, high-resolutio n, high-frequency radio imaging of nearby XBGNs could identify compact, inv erted-spectrum ADAF radio sources. The unique radio/X-ray luminosity relati on is confirmed in a few cases in which black hole masses are known and cou ld be used as a tool to estimate unknown black hole masses. For radio-dim ( L-R less than or similar to 10(39) ergs s(-1)), X-ray-bright (L-X greater t han or similar to 10(43) ergs s(-1)) sources, which are primarily Seyfert g alaxies, the X-ray emission mechanism is not accounted for by pure ADAFs an d radio activities are likely to be similar to those of the radio-quiet AGN s.