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\begin{document}

\begin{frontmatter}

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\title{Jet-disk coupling model for low luminosity AGNs}

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%\author{Feng Yuan\corauthref{cor1}}
%\author{Sera Markoff\corauthref{cor2}}
%\author{Heino Falcke, Peter Biermann\corauthref{cor3}}
%author{Peter Biermann\corauthref{cor3}}
%\corauth[cor1]{Harvard-Smithsonian Center for Astrophysics, 
%60 Garden Street, MS 51,
%Cambridge, MA 02138, USA}
%\ead{fyuan@cfa.harvard.edu}
%\corauth[cor2]{Center for Space Research, MIT, Cambridge, MA, USA}
%\author{Heino Falcke, Peter Biermann}
%\corauth[cor3]{Max-Planck Institut f\"{u}r Radioastronomie,
%          Auf dem H\"{u}gel 69, D-53121 Bonn, Germany}

\author{Feng Yuan}
\address{Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA}
\ead{fyuan@cfa.harvard.edu}
\author{Sera Markoff}
\address{Center for Space Research, MIT, Cambridge, MA, USA}
\author{Heino Falcke, Peter L. Biermann}
\address{Max-Planck Institut f\"{u}r Radioastronomie, Bonn, Germany}

\begin{abstract}

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''
Up until now there is no model which can explain the spectra of LLAGNs.
Taking NGC~4258 as an example, we propose a coupled jet-disk
model for some LLAGNs.
\end{abstract}

\begin{keyword}
% keywords here, in the form: keyword \sep keyword

accretion, accretion disks---galaxies: active -- galaxies: nuclei
% PACS codes here, in the form: \PACS code \sep code

\end{keyword}

\end{frontmatter}

% main text
\section{Introduction: observational Results of LLAGNs}
\label{}


LLAGNs are very common in the local universe,
           e.g., 43\% of all northern galaxies brighter
           than $B_T=12.5$ mag are active in the form of emission-line
           nuclei classified as Seyferts or LINERs 
	(Ho 2002).

On the other hand, observations indicate that LLAGNs
seem to be quite different with luminous AGNs.
Main observational results for LLAGNs are (e.g., Ho 1999; Terashima et al.
2002; Ho 2002),

  \begin{itemize}
    \item The bolometric luminosity is low relative to their Eddington 
luminosity;
    \item The Fe K$\alpha$ line is extremely weak or absent;
    \item Spectral energy distribution 
    \begin{itemize}
     \item optical/UV slope is quite steep, with the averaged power-law
     index being 1.5 ($f_\nu \propto \nu^{-1.5}$), while in luminous AGNs it is
0.5-1.0;
     \item there is no big blue bump
     \item X-ray spectral index is very variable among LLAGNs, $\alpha=0.6-1.2$
     \item the spectrum peaks at mid-infrared
     \item the radio spectrum is flat or inverted
    \end{itemize}
   \end{itemize}

\section{Can ADAFs Interpret the Spectra of LLAGNs?}
% \label{}

\begin{figure}
\psfig{file=p_fyuan_1.ps,width=11.5cm,angle=270}
\caption{The schematic diagram of jet-disk coupling model for NGC4258.
At a certain radius $r_j$, some fraction of accretion flow is transferred
into the jet. Assuming $r_j$ is smaller than the sonic radius of ADAF,
a standing shock should occur at $r_j$ due to the bending. This ``bending 
shock''
will accelerate the post-shock
electrons into a power law energy distribution.}
\end{figure}

The above observational results suggest the existence of radiatively
inefficient accretion flows, such as an ADAFs, in LLAGNs.
However, the existence of ADAF does not mean the spectra of LLAGNs can 
be fitted by ADAF since the emission from other components 
such as jets may dominate 
over ADAFs as in Blazars. 

Several work has been done to fit the spectra of
LLAGNs using ADAF or ADAF-SSD (SSD: Shakura \& Sunayev disk)  
models, including Sgr A* (Narayan et al. 1998), 
NGC~4258 (Gammie, Narayan \& Blandford 1999),
M~81 and NGC~4579 (Quataert et al. 1999). 
While the model can successfully explain the most important
features of low-luminosity and absence of big blue bump of those sources,
the detailed spectral fitting is not very satisfactory in some aspects.
For all the sources, the model underpredicts the
detected the low-frequency radio flux by more than one order of magnitude.
For NGC~4258, the infrared spectrum is produced 
by the outer thin disk, therefore a very hard infrared 
spectrum with the classical
form of $f_{\nu}\propto \nu^{1/3}$ is predicted, which is in
serious conflict with the observational result of $f_{\nu}\propto \nu^{-1.4}$
(Chary et al. 2000). For M~81 and NGC~4579, the predicted optical/UV spectrum 
is too steep compared to observation (Quataert et al. 1999).

\section{Jet-disk coupling model for LLAGNs}

NGC~4258 is an ideal object to investigate the physics of LLAGNs
due to the precise determination of the mass of the central
black hole, its distance to us, and its abundant spectral data ranging
from radio to X-ray especially at infrared waveband. 
Its spectrum seems to be very typical for LLAGNs.
We propose a jet-disk coupling model for NGC~4258 (Yuan et al. 2002). 

\begin{figure}
\psfig{file=p_fyuan_2.ps,width=8.cm,angle=270}
\caption{The jet-disk model for NGC~4258. 
The infrared and X-ray spectra
are produced by the synchrotron and SSC emission of the power-law electrons
accelerated in the bending shock at the jet base,
the radio spectrum
by the electrons accelerated by internal shocks in the outer part of the jet.}
\end{figure}

Fig. 1 shows the schematic diagram of our jet-disk coupling
model. 
After obtaining the global solution of ADAF,
we can calculate
the post-shock equilibrium temperature of the post-shock plasma from shock
jump conditions,
$$T^{eq}_2 \simeq \frac{(\Gamma+1)}{2(\Gamma+1)^2}
\frac{m_p}{k}v_1^2(r_0)=\frac{1}{5} \frac{m_p}{k}v_1^2(r_0). $$ 
The electrons will be accelerated into a power-law form after the shock.
A fraction of shock energy, $\epsilon_e$ and $\epsilon_B$,
 will be distributed into
the electrons and magnetic field,
$$\epsilon_e=\frac{p-1}{p-2}\frac{\gamma_{\rm
min}n_e m_ec^2}{e}, \hspace{1cm} 
\epsilon_B=\frac{B^2}{8\pi e}.$$ Here $\gamma_{\rm min}$ is the 
minimum Lolentz factor of power-law electrons. The electrons number density 
is determined by the mass loss rate in jet, 
$ \dot{M}_{\rm jet}=4\pi r_0^2 n_e m_p v_2.  $
Then we can calculate the emission from the jet
and underlying disk, 
with $\dot{M}_{\rm disk}=10\dot{M}_{\rm jet}$. 
Fig. 2 shows the fitting results. 
We see that 
the emission from the disk is much weaker
than from the jet. This is because most of the gravitational
energy is stored in the ADAF rather than radiated away; 
and at the bending shock, a large fraction of the stored energy,
$\epsilon_e \approx 0.2-0.3$,
will be distributed in the electrons and radiated away immediately.
Other observational features of LLAGNs, and also Sgr A*,
can be interpreted by this model as well (Yuan et al. 2002; Yuan, Markoff, 
\& Falcke 2001).

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\begin{thebibliography}{}

% \bibitem[Names(Year)]{label} or \bibitem[Names(Year)Long names]{label}.
% (\harvarditem{Name}{Year}{label} is also supported.)
% Text of bibliographic item


\bibitem[]{} Chary, R., et al. High-Resolution Infrared 
Imaging of the Compact Nuclear
Source in NGC 4258. 2000, ApJ, 531, 756

\bibitem[]{} Gammie, C., Narayan, R., \& Blandford, R. What Is the Accretion Rate in NGC 4258? 1999, ApJ, 516, 177

\bibitem[]{} Ho, L.~C. The Spectral Energy Distributions of 
Low-Luminosity Active
Galactic Nuclei. 1999, ApJ, 516, 672

\bibitem[]{} Ho, L.~C., Nonstandard Central Engines in Nearby Galaxies,
Issues in Unification of Active Galactic Nuclei, 2002,
ASP Conference Proceedings, Vol. 258.
Edited by R. Maiolino, A. Marconi, and N. Nagar, p165

\bibitem[]{} Narayan, R. et al. Advection-dominated 
accretion model of Sagittarius A*:
evidence for a black hole at the Galactic center, 
1998, ApJ, 492, 554

\bibitem[]{} Quataert, E. et al.  
Possible Evidence for Truncated Thin Disks in the
Low-Luminosity Active Galactic Nuclei M81 and NGC 4579, 1999, ApJ, 525, L89

\bibitem[]{} Terashima, Y., et al. 
X-Ray Properties of LINERs and Low-Luminosity Seyfert
Galaxies Observed with ASCA. I. Observations and Results. 2002, ApJS, 139, 1

\bibitem[]{} Yuan, F., Markoff, S., \& Falcke, H., A Jet-ADAF model for Sgr A*.
2002,  A\&A, 383, 854

\bibitem[]{} Yuan, F., Markoff, S., Falcke, H., \& Biermann, P. 
NGC 4258: A jet-dominated low-luminosity AGN? 2002, A\&A,
391, 139

\end{thebibliography}

\end{document}