X-ray Bright Galactic Nuclei Research Articles

Radio/X-Ray Luminosity Relation for X-Ray–Bright Galactic Nuclei: Implications for Weighing Supermassive Black Holes

Optically thin and geometrically thick accretion flows are known to be responsible for the observed radio/X-ray luminosity relation of X-ray–bright galactic nuclei. It has also been suggested that supermassive black hole masses can be estimated from measurements of the core radio luminosity and the X-ray luminosity by using the advection-dominated accretion flow (ADAF) model. In this study we increase the amount of data available by compiling the radio/X-ray fluxes and the mass in published literatures, and compare the observed ratio of the luminosities with predictions from various models of optically thin accretion flows. Semianalytically derived relations of the luminosities are presented in cases of the standard ADAF model and modified ADAF models, in which a truncation of inner parts of the flows and winds causing a reduction of the infalling matter are included. We show that the observed relation can indeed be used to estimate the supermassive black hole mass, provided that properties of such accretion flows are known. Having investigated sensitivities of the method on modifications of the "standard" ADAF model, we find that a general trend of model predictions from the "standard" ADAF, the truncated ADAF, and the "windy" ADAF are somewhat indistinguishable. We also find, however, that the extreme case of the windy model is inconsistent with currently available observational data unless microphysics parameters are to be substantially changed. High-resolution radio observations, however, are required to avoid the contamination of nondisk components such as a jet component, which otherwise results in an overestimation of SMBH mass.

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-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 1035LR1039 ergs s-1. We designate these as 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 (LR1039 ergs s-1), X-ray-bright (LX1043 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.