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    {\small\rm\zihao{6}\songti\thepage\hfill
    \quad\quad ACTA ASTRONOMICA SINICA \hfill
    \hspace{2mm} Vol.44\vspace{2mm}}}} %%%%%%%
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    {\small\rm \zihao{6}\songti suppl.\hfill X. Liu \& J. Yang: Thomson Scattering
    Effect on Broad Line Radio Galaxy OQ208  %%（作者：题目名称）
    \vspace{1.5mm}
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\zihao{5}\songti

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    \zihao{6}\songti
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    Vol.44~ Suppl.
    \hfill\hspace{8mm} %%% if ``October/January'' then \hspace{8/10 mm} July
    \small ACTA ASTRONOMICA SINICA
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      Feb.,~2003 \\[-1.5 true mm] %%January
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\vs\vs\vs \bc {\Large\bf
Thomson Scattering Effect on Broad Line \\[2mm]Radio Galaxy OQ208   %%（文题）
%\\[1ex]
%$\!\!^*$
}

\vs\vs{\normalsize  Xiang Liu\hs\&\hs Jun Yang  }  %%（本文作者）

\vs
({\footnotesize {\it Urumqi Astronomical Observatory, NAO, CAS, Urumqi}~ 830011}) \\%%（本文作者单位）

\vs\vs \ec

\normalsize
\begin{quotation}

 \no{\bf A{\scriptsize\bf BSTRACT }}~~~\small
We propose that the Thomson scattering could have observational
effect in Compact Symmetric Objects(CSOs). We developed a
'semi-transparent screen' method for calculating the re-scattering
back photons, by which we estimated that the Thomson scattering
contributes a factor 1.7 to the lobe flux ratio in OQ208 and 2.5
to the component flux ratio in NGC4261.

 \vn
 \no{\bf Key words}~~  Thomson
scattering, CSOs, OQ208, NGC4261
\end{quotation}

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

\vs\vs
Compact symmetric objects(CSOs) have been found to be young
radio sources, probably in the early evolutionary stage of large
double-lobed radio sources. CSOs are confined in less than $1kpc$
volume, VLBI morphologies of these objects have shown the
double-lobed symmetric structure with a weak core between lobes.
The typical lobe expansion speed of CSOs is about $0.2c$ (light
speed) and the estimated kinematical ages of CSOs are less than
$10^4$ years.

The source OQ208 is a nearby radio galaxy (redshift 0.077).
Its VLBI images at low frequency show a strong
north-east lobe and a weak south-west lobe. Twin jets and a weak
core were also detected at 15 GHz images. Stanghellini et al.
(1997) suggested that the orientation of the jets is nearly $45\deg$
away from the line of sight. The 2 mini-lobes are separated by 6.6
milliarcseconds, i.e. $6.6h^{-1}pc$. The source
has been confirmed as a CSO with lobe's proper motion velocity of
$0.07h^{-1}c$ and age of $\approx$200 years (Liu et al. 2000; Stanghellini
et al. 2000). Surprisingly, the flux
densities of the 2 lobes in OQ208 have large differences, the flux
ratio of the 2 lobes is 63, 11, 10 at 1.6, 5 and 8.4 GHz
respectively (Kameno et al. 2000, Stanghellini et al. 1997, Fey et
al. 1996).

The free-free absorption (FFA) has been found in
the weak lobe of OQ208 (Kameno et al. 2000; Liu et al. 2002a), and the
fitted turnover frequency of FFA in spectrum is 4 GHz. Therefore,
the FFA could lead to the large flux ratio at 1.6 GHz. If the FFA
 is the only reason for the asymmetric flux densities in OQ208 the flux
ratio should be near unity above 4 GHz. The beaming of jets in
OQ208 can also cause the approaching lobe to have flux larger than
the receding one, this effect is well known in small viewing-angle
relativistic-beaming radio sources. However, the estimated flux
ratio from the beaming effect in OQ208 is less than 1.5 (Liu 2002b).
Considering this factor to the lobe flux ratio in OQ208 at 5 and 8.4 GHz,
there is still a factor of 6.7 and it should be considered in view
of other mechanisms.

\para{\large\bf{2}~~ Thomson scattering in the lobes of OQ208}

\vs\vs For the broad line properties and the compactness of the
radio source OQ208, it is possible that Thomson scattering could
lead to an observational effect on the lobe flux ratio in OQ208.

As the first approximation, we assume that the lobe flux ratio 6.7 as mentioned above
in OQ208 is due to Thomson scattering. As a result the mean
medium density between lobes could be obtained; then the Thomson
scattering can be calculated in detail by accounting for the
re-scattering back photons, such that a more realistic lobe flux
ratio caused by the Thomson scattering can be obtained.

For the distance between the 2 lobes in OQ208 along the line of
sight, $\Delta l = 6.6 h^{-1} pc$ (we use $h=0.75$); the flux
ratio of lobes $I_{a}/I_{r}=6.7$, and the Thomson scattering
section rate $\sigma_T=6.65\times 10^{-25} cm^{2}$, we have (see
Fig.1)

\begin{figure}[h]
\parbox[b]{6cm} {\hspace{10mm}{\includegraphics[bb=145 552 260
718,scale=0.85]{fig01.eps}}

    {\footnotesize
     Fig.\,1~~The Thomson scattering as the first
approximation}
 }
 \end{figure}
 \begin{figure}[h]

 \vspace{-71mm}
 \ \hfill \
\parbox[b]{6.5cm}
{\hspace{10mm}{\includegraphics[bb=148 460 370
714,scale=0.6]{fig02.eps}}

    {\footnotesize
    Fig.\,2~~A diagram for the calculation of the
Thomson scattering in OQ208}
 }
 \end{figure}


\begin{equation}
I_{a} = I \times \exp(-\sigma_T \overline{n}_2 l_{2})\,,
\end{equation}
\begin{equation}
I_{r} = I \times \exp[-\sigma_T (\overline{n}_1 \Delta l +
\overline{n}_2 l_{2})]\,,
\end{equation}

\begin{equation}
I_{a} / I_{r} = \exp(\sigma_T \overline{n}_1 \Delta l)\,.
\end{equation}

The $I$ is the intensity emitted by each lobe and we assume it is
same for 2 lobes, then the mean medium density is $\overline{n}_1
= 1.1 \times 10^5 cm^{-3}$.

Assuming that the medium density varies with $r$ as (Fanti 2000)
$\rho = \rho_0({r_0}/{r})^2$,  when   $ r>r_0$ , $\rho_0$ (usually
assumed as a constant) is the medium density inside $r_0$. Using
the $\overline{n}_1$ above, the production $\rho_0 r_{0}^{2}$ can
be determined. We integrate the column density from the weak lobe
along the line of sight to the point 'a' in Fig.2 where a half
flux from the weak lobe is just scattered,

\begin{equation}
\ln0.5 = -\sigma_T \int_{45}^{\theta}\rho dl\,,
\end{equation}

\no i.e.  $\theta=13.6\deg$ from the horizon line in Fig.2. A
equivalent `half transparent screen' scattering the half photons
could be estimated as

\begin{equation}
<l> = \int^{13.6}_{45}l\rho dl / \int^{13.6}_{45}\rho dl =
0.418L\,,
\end{equation}

\no $0.418L$ is at  $\theta = 30.2\deg$ from the horizon line in
Fig.2, is marked as `1'. The scattered photons will be partly
re-scattering back. As above we integrate from the screen `1' up
to the point where a half of the scattered ones will be reflected
back, it is at  $\theta = 61.6\deg$; with equation (5) the
equivalent `half transparent screen' is at $\theta =47.5\deg$, is
marked as `2', near the weak lobe.

Through the serial `half-transparent screen' before point `a', by
a complete analysis and calculation we finally obtained the sum of
photons arriving at the point `a' with respect to the ones emitted
($I$) from the weak lobe is $\approx 0.80 I$.

By the same approach we have the point `b' where a half of the
flux come from `a' is scattered, the $\theta$ at `b' is -18
degree. Again follow this way, the point `c' is the place there a
half of the flux from 'b' is scattered. Finally the sum photons
come out from `c' is $\approx 0.80 \times 0.83 \times 0.86 I=0.57
I $. The flux ratio for the approaching lobe to the receding lobe
is $\approx 1/0.59=1.7$ assuming the mean medium density is
$1.1\times 10^5 cm^{-3}$ in a few $pc$ away from the center of
OQ208, for the near broad line region of AGNs this medium density
could be reasonable.

For a more realistic example, the source NGC4261, the viewing
angle of its jets is 63 degree, the average medium density
$\approx 1 \times 10^6 cm^{-3}$, and the separation  $\approx
0.5pc$ between 2 bright components near its accretion disk (Jones
et al. 2000). We found the receding component can be dimmed by
$\approx40\%$ through Thomson scattering, this can account for a
part of the weakness of the receding jet in NGC4261, although
free-free absorption was suggested for the weakness by Jones et
al.(2000).

In summary, the beaming effect cannot fully account for the large
flux ratio of lobes in near broad line region of CSOs, we propose
that the Thomson scattering could cause observational effect in
these compact sources. We have developed a `semi-transparent
screen' method for calculating the re-scattering back photons; we
estimated that the Thomson scattering contributes a factor 1.7 to
the lobe flux ratio in OQ208, and 2.5 to the component flux ratio
in NGC4261.

\vs\vn{\bf ACKNOWLEDGEMENTS}~~
 L.X. thanks Prof. Jiang Dongrong and Dr. Cao Xinwu for valuable
discussions, and Dr. Wang Weihua for help on the paper. This work is
supported by the National Natural Science Foundation of China (19970314).

\bc {\bf References} \ec

\vspace{-4mm} \no \fns

 \nref Fanti C.,  in the Proceedings of the 5th European VLBI Network
Symposium, In: Conway J.E., et al. eds., published by Onsala Space
Observatory, Chalmers Technical University, 2000 \nref Fey A.L.,
Clegg A.W., Fomalont E.B.,  ApJS, 1996, 105, 299 \nref Kameno S.,
Horiuchi S., Shen Z.-Q., et al., PASJ, 2000, 52, 209 \nref Jones
D.L., Wehrle, A.E., Meier, D.L., et al., ApJ, 2000,534, 165 \nref
Liu Xiang, Stanghellini C., Dallacasa D., et al.,  Chin. Phys.
Lett., 2000, 17(4), 307 \nref Liu Xiang, Stanghellini C.,
Dallacasa D., et al., A\&A, 2002a, 385, 768 \nref  Liu Xiang, PhD
thesis, 2002b \nref Stanghellini C., Bondi M., Dallacasa D., et
al., A\&A, 1997, 318, 376 \nref Stanghellini C., Dallacasa D.,
Bondi M., et al.,  in the Proceedings of the 5th European VLBI
Network Symposium, In: Conway J.E., ed., published by Onsala Space
Observatory, Chalmers Technical University, 2000

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