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

Kavli Affiliate: Luis Ho

| First 5 Authors: Zhaolong Yu, Feng Yuan, Luis Ho, ,

| Summary:

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.

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