Yuan, Markoff, Falcke, Biermann 2002: NGC 4258: Jet-Dominated LLAGN
NGC 4258: a jet-dominated low-luminosity AGN?
Feng Yuan, Sera Markoff, Heino Falcke, & Peter L. Biermann
Max-Planck-Institut für Radioastronomie, Auf den Hügel 69, D-53121 Bonn, Germany
Low-luminosity AGNs (LLAGNs) are a very important class of sources
since they occupy a significant fraction of local galaxies. Their
spectra differ significantly from the canonical luminous AGNs, most
notably by the absence of the ``big blue bump''. In the present
paper, taking a typical LLAGN--NGC 4258--as an example, we investigate
the origin of their spectral emission. The observational data of
NGC 4258 is extremely abundant, including water maser emission,
putting very strict constraints to its theoretical models. The
infrared (IR) spectrum is well described by a steep power-law form
Fnu ~ nu-1.4, and may extend to the optical/UV
band. Up until now there is no model which can explain such a steep
spectrum, and we here propose a coupled jet plus accretion disk model
for NGC 4258. The accretion disk is composed of an inner ADAF (or
radiatively inefficient accretion flow) and an outer standard thin
disk. A shock occurs when the accretion flow is ejected out of the
ADAF to form the jet near the black hole, accelerating the electrons
into a power-law energy distribution. The synchrotron and
self-Comptonized emission from these electrons greatly dominates over
the underlying accretion disk and can well explain the spectrum
ranging from IR to X-ray bands. The further propagation of the
shocked gas in the jet can explain the flat radio spectrum of NGC
4258. Several predictions of our model are presented for testing
against future observations, and we briefly discuss the application of
the model to other LLAGNs.
Paper: Available in PostScript and (AA)LaTex. Please send an email request to email@example.com
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