\documentstyle[twoside,epsfig,CJK]{carticle}
%\documentclass[twoside]{carticle}
%\usepackage{epsfig}
\pagestyle{empty}
\headsep=0.6cm %%%%%%%

\setcounter{page}{303}

%\input cchead.sty
%\input Vatola.sty

\def\thefootnoot{}

\textwidth=140mm \textheight=210mm \topmargin=0pt
\oddsidemargin=0pt \evensidemargin=0pt \footskip 0.2 truecm

%%...................
%\vspace*{1\baselineskip}
\doublerulesep=0pt
%%..................
% \renewcommand{\baselinestretch}{1.4} %%cc
\parindent=7.5mm

\setlength{\parskip}{0 true pt}
    \def\hang{\hangindent\parindent}
    \def\RE{\par\hang\textindent}
    \catcode`@=11
%%%˫ҳ
    \def\@evenhead{\underline{\parbox{\textwidth}
    {\small\rm\zihao{6}\songti\thepage\hfill
    \quad\quad ACTA ASTRONOMICA SINICA \hfill
    \hspace{2mm} Vol.44\vspace{2mm}}}} %%%%%%%
%%%��ҳ
    \def\@oddhead{\underline{\parbox{\textwidth}
    {\small\rm \zihao{6}\songti suppl.\hfill J. H. Wu \& X. Z. Zhang: Is the Radio Dichotomy of AGNs Real?  %%�����ߣ���Ŀ���ƣ�
    \vspace{1.5mm}
    \hfill\thepage}}} %%%%%%%
    \normalsize\rm
\zihao{5}\songti

  %  \begin{CJK}{GB}{song}
    \begin{document}


%%ƪ��
    \thispagestyle{empty}
    \vspace*{-1.5 true cm}
    \small\rm
    \zihao{6}\songti
    \noindent
    \\%%%[-0.5 true mm]
    Vol.44~ Suppl.
    \hfill\hspace{8mm} %%% if ``October/January'' then \hspace{8/10 mm} July
    \small ACTA ASTRONOMICA SINICA
    \hfill
      Feb.,~2003 \\[-1.5 true mm] %%January
    \rule {\textwidth}{0.12 true mm}\\[-3.2 true mm]
    \rule {\textwidth}{0.12 true mm}
%%%%%%%

\input{../newcom.ctx}

\renewcommand{\baselinestretch}{1}

\setcounter{footnote}{1}

\vs\vs\vs \bc {\Large\bf
Is the Radio Dichotomy of AGNs Real?    %%�����⣩
%\\[1ex]
%$\!\!^*$
}

\vs\vs{\normalsize J. H. Wu\hs\&\hs  X. Z. Zhang}  %%���������ߣ�

\vs

(\footnotesize \nao)
\vs\vs \ec

\normalsize
\begin{quotation}

 \no{\bf A{\scriptsize\bf BSTRACT }}~~~\small
AGNs are traditionally divided into radio-loud and radio-quiet.
Optically selected AGN samples usually show bimodal distributions
in radio-loudness. However, recent radio selected AGN samples do
not have this radio dichotomy and contained a large number of
radio-intermediate objects. To avoid the apparent selection
effects in optically and radio selected AGN samples, we use an
X-ray selected sample to revisit this question and find no clear
bimodality in radio-loudness. On the other hand, when the
X-ray-loudness and radio-loudness are both been taken into
consideration, two populations of AGNs are well separated. This
result might be used in the unification scheme of AGNs.

 %\vn
 %\no{\bf Key words}~~  pulsar-mode  change-spectrum
\end{quotation}

%%%% ����
 \para{\large\bf{1}~~ Introduction}

\vs\vs AGNs are usually classified as radio-loud (hereafter RL)
and radio-quiet (hereafter RQ). Although this classification is of
some historical reason, the optically and X-ray selected samples
do show a bimodal distribution in radio-loudness, with one peak
corresponds to the RL and the other to the RQ (see Fig. 1 in Della
Ceca et al. 1994, Fig.3 in Kellermann et al. 1989, and Fig. 3 in
Visnovsky et al. 1992; Stocke et al. 1992). The distinction in
radio-loudness has been attributed to the difference in central
black hole mass or spin rate, say, the black holes in radio-loud
objects have much higher mass or much higher spin rate than the
black holes in radio-quiet objects(e.g. Loar 2000; Wilson \&
Colbert 1995).

However, it has recently been found that the ROSAT All-Sky Survey (RASS) and
the Faint Images of the Radio Sky at Twenty-centimeters (FIRST) correlated
quasars do not illustrate bimodal distribution in radio-loudness
(Brinkmann et al. 2000). What's more, the quasars discovered in the FIRST
Bright Quasar Survey (hereafter FBQS) have a smooth change in number from
the traditionally
RL to the traditionally RQ (White et al. 2000). If there is indeed no radio
dichotomy in AGNs, AGNs should be classified in another way and the
unified scheme should be reconsidered. Therefore,
whether there is a radio dichotomy in AGNs is an essential
question in AGN classification and unification.

It is well known that the optically and radio selected AGN samples suffer
from significant selection effects. However, X-ray emission has been
found to be nearly the definitive characteristic of AGNs, hence X-ray selected
AGN samples are believed to be more complete than optically and radio selected
samples. Here we use an X-ray selected sample to study the
radio-loudness of AGNs.

\para{\large\bf{2}~~The Samples}

\vs\vn{\bf{2.1~~The X-ray Sample}}

 The complete sample of X-ray
AGNs resulted from the optical follow-up observations of the RASS
sources in six areas (Appenzeller et al. 1998; Zickgraf et al.
1997). The total sky coverage is 685 $\mathrm {deg}^2$ and there
are 674 X-ray sources down to a ROSAT count rate limit 0.03
$\mathrm{cts\,s}^{-1}$. Optical identifications led to the
discovery of 166 Seyferts and 71 quasars.

\vspace*{1mm}\vn{\bf{2.2~~The Radio Sample}}

The NRAO VLA Sky Survey (NVSS, Condon et al. 1998) sample is
selected as our radio sample. The NVSS used the D configuration of
the VLA to survey the entire sky north of $-40$\,deg declination
at a frequency of 1.4\,GHz. Over 1.8 million sources were detected
to a flux density limit of about 2.5\,mJy. The NVSS reaches well
into the RQ population of AGNs.

\para{\large\bf{3}~~Cross Correlation}

\vs\vs
 The X-ray AGN sample gives only the X-ray positions. Taking
their uncertainty as 20$''$ (Zickgraf et al. 1997) and the NVSS's
positional uncertainty as 5$''$ (Condon et al. 1998), we searched
the AGNs in the NVSS catalog with a searching radius of 40$''$.
The cross correlation resulted in 32 (45\%) radio detections in 71
quasars and 34 (20\%) detections in 166 Seyferts.

\para{\large\bf{4}~~Statistics on Radio-Loudness}

\vs\vn{\bf{4.1~~Definition of Radio- and X-ray-Loudnesses}}

Sometimes the radio-loudness of AGNs is defined directly by their
radio luminosities, but the more generally used definition is the
radio-to-optical flux ratio,
$R^{\ast}=f(5GHz)/f(2500\mathrm{\AA})$ (Stocke et al. 1992).

Another equivalent parameter is the two point radio-to-optical
spectral index $\alpha_{\mathrm{ro}}$. It is defined as
\begin{equation}
\alpha_{\mathrm{ro}}=-\displaystyle\frac{\log{\displaystyle\frac{L_{2500\mathrm{\AA}}}{L_{\mathrm{5\,{\rm
GHz}}}}}}{\log{\displaystyle\frac{\nu_{2500\mathrm{\AA}}}{\nu_{\mathrm{5\,{\rm
GHz}}}}}}=\displaystyle\frac{\log R^{\ast}}{5.38}.
\end{equation}

Similarly, the two point X-ray-to-optical spectral index or X-ray-loudness
can be defined as
\begin{equation}
\alpha_{\rm
ox}=-\displaystyle\frac{\log{\displaystyle\frac{L_{2\,{\rm
keV}}}{L_{2500\,{\rm\AA}}}}}{\log{\displaystyle\frac{\nu_{2\,{\rm
keV}}}{\nu_{2500\,{\rm\AA}}}}}.
\end{equation}
It should be noted that a smaller value of $\alpha_{\mathrm{ox}}$ corresponds
to a relatively higher X-ray luminosity, or in other words, an X-ray-loud
AGN has a relatively lower $\alpha_{\mathrm{ox}}$, contrary to radio-loudness.

When calculating the fluxes and luminosities, we adopted a Hubble
constant of\linebreak
 50\,km\,s$^{-1}$\,Mpc$^{-1}$ and a
deceleration factor as $q_0=0.5$. Power laws were assumed for the
radio, optical, and X-ray spectra of quasars and Seyferts and
their radio and optical spectral indices were both adopted as
$0.5$ ($S\propto\nu^{-\alpha}$). All AGNs were assumed to have an
X-ray photon index as $\Gamma=2.3$\footnote{The X-ray photon index
is slightly different for different kind of objects and varies
with redshift. The generally accepted values are:
$\Gamma\approx2.5$ for RQ quasars, $\Gamma\approx2.1$ for RL
quasars, and $\Gamma\approx2.3$ for Seyferts. For simplicity, we
adopted 2.3 for all three kinds of objects.}
($F_{\mathrm{photon}}\propto E^{-\Gamma}$) (Brinkmann et al. 1997;
Rush et al. 1996; Yuan et al. 1998).

\begin{figure}[tbph]
\center {\includegraphics [bb=8 93 405 478,scale=0.7]{r_mb.eps}}
\center {\includegraphics [bb=17 88 402 480,scale=0.7]{alpha.eps}}
   \parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize

      Fig.\,1~~ Left: $\log R^{\ast}$
versus absolute B magnitude distribution; right: overall energy
distribution. Quasars and Seyferts are taken as a whole. The radio
detections are denoted in open circles and the upper limits in
downward arrows. The dashed line marks the generally adopted
boundary between RL and RQ.}

    \end{figure}

\vn{\bf{4.2~~The Radio and X-ray Properties of AGNs}}

 Fig. 1
demonstrates the radio-loudness versus absolute B magnitude
distribution and broad-band spectral index distribution. Quasars
and Seyferts are taken as a whole. There is no clear deficiency of
objects towards the dashed lines, the commonly adopted boundaries
between RL and RQ. Fig. 1. suggest that there is no radio
dichotomy for AGNs. This result is in agreement with recent
conclusions (Brinkmann et al. 2000; White et al. 2000). And
because our sample is X-ray selected and hence involves less
selection effects, the result is of higher confidence.

What is interesting in the right panel of Fig. 1 is that there seems to be a
gap extending from the upperleft to the lowerright (marked by the dotted
line). If it is the case, it would be that the dividing line of
$\alpha_{\mathrm{ro}}$ between `RL' and `RQ' decreases
with increasing optical-to-X-ray energy ratio. In other words, the AGNs
should be classified not only by their radio-loudness ($\alpha_{\mathrm{ro}}$),
but also by their X-ray-loudness ($\alpha_{\mathrm{ox}}$).

In fact, a similar phenomenon has been observed by Stocke et al. (1991)
when carrying out a similar statistics on the X-ray AGNs selected in the
EINSTEIN Extended Medium Sensitivity Survey (EMSS).
In their Fig. 6, both `RL' AGNs and `RQ' AGNs have an inclined
distribution in $\alpha_{\mathrm{ox}}-\alpha_{\mathrm{ro}}$ plane,
from high $\alpha_{\mathrm{ro}}$, low $\alpha_{\mathrm{ox}}$ to low
$\alpha_{\mathrm{ro}}$, high $\alpha_{\mathrm{ox}}$, similar as in Fig. 1
in our statistics. But the `RL' AGN band has a clear shift in
$\alpha_{\mathrm{ox}}$ with respect to the `RQ' AGN band, the former being
more X-ray-quiet (larger $\alpha_{\mathrm{ox}}$) than the latter.
The only difference between Stocke et al. statistics and ours is that
their sample have a lower cutoff in $\alpha_{\mathrm{ro}}$ for the `RL'
AGNs and a upper cutoff in $\alpha_{\mathrm{ro}}$ for the `RQ' AGNs.
Both cutoffs might be based on the early classification of AGNs just
after their discoveries. Our result is different from most previous
results.

\para{\large\bf{5}~~ Conclusions}

\vs\vs Based on the statistics of an X-ray selected sample, we
found no radio dichotomy in AGNs. This result is in agreement with
some recent results (Brinkmann et al. 2000; White et al. 2000),
but, of course, has higher confidence. The radio dichotomy of AGNs
might be totally of historical reason.

An interesting result of our study is that AGNs can be classified
both in radio-loudness and X-ray-loudness. This result may have
some implication in unifying the AGNs since the present unification
scheme of AGNs is based in part on the radio dichotomy of AGNs. Of
course, the NVSS, the radio survey used by us, is of limited
sensitivity, making the radio detection relatively low for the X-ray
AGNs. A deeper radio survey is desired to carry out the same statistics
as in this paper.

\bc {\bf References} \ec

\vspace{-4mm} \no \fns

 \no Appenzeller I., Thiering I., Zickgraf F.J., et al., ApJS,
1998, 117: 319\\
 \no Becker R.H., White R.L., Gregg M.D., et al., ApJS, 2001, 135: 227\\
  \no Brinkmann W.,
Laurent-Muehleisen S.A., Voges W., et al., A\&A, 2000, 356: 445\\
\no Brinkmann W., Yuan W., Siebert J., A\&A, 1997, 319: 413\\
\no Condon J.J., Cotton W.D., Greisen E.W., et al., AJ, 1998, 115:
1693\\
 \no Della Ceca R., Lamorani G., Maccacaro T., et
al., ApJ, 1994, 430: 533\\
 \no Kellermann K.I., Sramek R., Schmidt M., et al., AJ, 1989, 98: 1195\\
  \no Laor A., ApJ, 2000, 543: L111\\
   \no Rush B., Malkan M.A., Edelson R.A., ApJ, 1996, 473: 130\\
    \no Stocke J.T.,
Morris S.L., Weymann R.J., et al., ApJ, 1992, 396: 487\\
\no Visnovsky K.L., Impey C.D., Foltz C.B., et al., ApJ, 1992,
391: 560\\
 \no White R.L., Becker R.H., Gregg M.D., et
al., ApJ, 2000, 126: 133\\
 \no Wilson A.S.,Colbert E.J.M., ApJ, 1995, 438: 62\\
  \no Yuan W., Brinkmann W.,
Siebert J., et al., A\&A, 1998, 330: 108\\
 \no Zickgraf F.J., Thiering I., Krautter J., et al., A\&AS, 1997, 123:
 103\\

%\end{CJK}
\end{document}
