On the Origin of Ultraviolet Emission and the Accretion Model of Low-luminosity AGNs

TL;DR

This paper demonstrates that ADAF models can successfully explain the ultraviolet, X-ray, and radio emissions of low-luminosity AGNs, challenging the idea that all AGNs share the same accretion processes.

Contribution

It provides detailed ADAF model calculations that fit observed spectra of LLAGNs, supporting ADAF over thin disk models for low-luminosity AGNs.

Findings

ADAF models fit the spectral energy distributions of LLAGNs.

UV emission can be explained by ADAF, jet synchrotron, and truncated disk emissions.

Theoretical reasons favor ADAF models over thin disk scenarios.

Abstract

Low-luminosity active galactic nuclei (LLAGNs) are generally believed to be powered by an inner radiatively inefficient, advection-dominated accretion flow (ADAF), an outer truncated thin disk, and a jet. Maoz (2007) recently challenged this picture based on the observation that the strength of ultraviolet emission relative to the X-ray and radio bands does not depart from empirical trends defined by more luminous sources. He advocates that AGNs across all luminosities have essentially the same accretion and radiative processes, which in luminous sources are described by a standard optically thick, geometrically thin disk. We calculate ADAF models and demonstrate that they can successfully fit the observed spectral energy distributions of the LLAGNs in Maoz's sample. Our model naturally accommodates the radio and X-ray emission, and the ultraviolet flux is well explained by a combination of the first-order Compton scattering in the ADAF, synchrotron emission in the jet, and black body emission in the truncated thin disk. It is premature to dismiss the ADAF model for LLAGNs. The UV data can be fit equally well using a standard thin disk, but an additional corona and jet would be required to account for the X-ray and radio emission. We argue that there are strong theoretical reasons to prefer the ADAF model over the thin disk scenario. We discuss testable predictions that can potentially discriminate between the two accretion models.