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\title{The UV/radio correlation of quasars \protect\\
 and its implications for unified schemes.}
%\subtitle{Basic Instructions}

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\author{Heino Falcke}
\institute{University of Maryland, Department of Astronomy, College
Park, MD 20742-2421, USA (hfalcke\atsign{}astro.umd.edu)}
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\runningtitle{The UV/radio correlation of quasars}
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\section{Introduction}


\footnotetext{Talk presented
at the IAU Symp. 175, ``Extragalactic Radio Sources'', Bologna 1994}What causes the differences in the central engines of AGN? Are they
intrinsically different or are the differences entirely due to
environment and orientation? We have postulated that there is
basically only one engine, consisting of a black hole and a closely
coupled (even symbiotic) jet/disk system with very similar parameters
in all AGN \cite{FB95}.  This has lead to interesting results
concerning the modelling of the UV/radio correlations \cite{FMB95} of
quasars and the difference between FR\,I and FR\,II radio
galaxies \cite{FGB95}.

\section{The UV/radio correlation for quasars}
If there is a closely coupled jet/disk system in AGN we would expect
to find a strong correlation between accretion disk luminosity (UV
bump) and radio core luminosity in quasars; and indeed correlations
between optical (e.g. O[III]  \cite{MRS93}) and radio emission have
been demonstrated. We tried to improve upon those correlations by
estimating the UV-bump luminosity of quasars directly from
optical/UV/x-ray observations of quasars  \cite{SM89} {\it and}
indirectly from emission line luminosities  \cite{BG92}. We combined
all estimates for each source into a single estimate for the disk
luminosity $L_{\rm disk}$ and plotted this vs.~their VLA radio core
fluxes at 5 GHz  \cite{FMB95}.  The UV/radio distribution can
then be compared to a simple emission model which takes mass and
energy conservation in a coupled jet/disk system into account
 \cite{FB95}: \parindent0cm

-- We find a strong correlation between UV/bump and radio core
luminosity in radio weak quasars and a similar trend in radio loud
quasars. The former correlation implies that radio emission and
UV-bump in {\it radio weak} quasars have a similar origin (central engine
rather than starburst).

-- Flat-spectrum compact quasars have brighter radio cores than steep
spectrum quasars at the same $L_{\rm disk}$ consistent with the former
being relativistically boosted. Width of the distribution and location
of the flat-spectrum cores are well modelled by randomly oriented
relativistic jets with bulk Lorentzfactor $\gamma\sim6$ at $L_{\rm
disk}\sim10^{46}$ erg/sec.

-- The cores of radio loud quasars (with bright extended emission) are
brighter than those of radio weak quasars at the same $L_{\rm
disk}$. Hence, the radio-loud/radio-weak dichotomy is already
established on the pc scale.

-- The total jet power of radio loud quasars must be comparable to
their accretion disk luminosity to explain the bright synchrotron
emission from their radio cores. This indicates that the jet is
produced in the innermost region of the disk where most of the energy
is available. Radio loud quasars are very efficient and require that
relativistic electrons are in equipartition with the magnetic field
(having a minimum Lorentzfactor of $\gamma_{\rm e}\sim100$). Radio
weak quasars can be modelled by basically the same jet if the
available electrons are accelerated into a powerlaw starting at
$\gamma_{\rm e}\sim1$.

-- There is a distinct population of a few flat-spectrum quasars which
have {\it total} radio luminosities intermediate between radio loud
and radio weak quasars (flat-spectrum intermediate quasars =
FIQ). Despite being dominated by a variable, flat-spectrum radio core
-- which usually is indicative of relativistically boosted jets --
their cores are equally bright or even weaker than the cores of
lobe-dominated quasars at the same $L_{\rm disk}$. If the FIQ are
boosted quasars their parent population can not be the radio loud
quasars as they lack the steep-spectrum lobe emission and their cores
are to weak. The only possible parent population therefore are radio
weak quasars. Number and offset of the FIQ in the PG sample would then
both indicate bulk Lorentz factors of $\gamma\sim3-5$ in {\it radio weak}
quasars!


-- In the optically and the radio selected sample there is a void of
radio loud {\it quasars} below a critical $L_{\rm disk}$. We identify
this with the FR\,I/FR\,II break and suggest that this might be
caused by a torus with power-dependent opening angle \cite{FGB95}. At
low powers the torus closes, obscures the central engine for all
aspect angles (no quasar signatures) and starts to disrupt the radio
jet (causing the FR\,I morphology). The power-dependent torus will
change the length/number statistics for quasars and radio galaxies and
the jet torus interaction can modify the jet on the pc scale.


\begin{thebibliography}{xxxxx}  
\bibitem[]{BG92}Boroson, T.A., Green, R.F. 1992, {\it ApJS} {\bf 80}, 109
\bibitem[]{FB95}{Falcke, H., Biermann, P. L. 1995, {\it A\&A} {\bf 293}, 665}
\bibitem[]{FGB95}{Falcke, H., Gopal-Krishna, Biermann, P.L. 1995b, {\it A\&A} {\bf 298}, 395}
\bibitem[]{FMB95}{Falcke, H., Malkan, M., Biermann, P.L. 1995, {\it A\&A}
{\bf 298}, 375}
\bibitem[]{MRS93}{Miller, P., Rawlings, S., Saunders, R. 1993, {\it MNRAS} {\bf
263}, 425}
\bibitem[]{SM89}Sun, W.H., Malkan, M.A. 1989, {\it ApJ} {\bf 346}, 68 
\end{thebibliography}
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