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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 D. R. Jiang \& X. Y. Hong: VLBI Observation
    of Radio Jets in AGNs  %%�����ߣ���Ŀ���ƣ�
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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
VLBI Observation of Radio Jets in AGNs    %%�����⣩
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%$\!\!^*$
}

\vs\vs{\normalsize D.R. Jiang\hs\&\hs X.Y. Hong}  %%���������ߣ�

\vs (\footnotesize \sho)\\
Email: djiang@center.shao.ac.cn
\vs\vs \ec

\normalsize
\begin{quotation}

 \no{\bf A{\scriptsize\bf BSTRACT }}~~~\small
Very Long Baseline Interferemetry (VLBI) with its high angular
resolution plays an important role in the study of radio jets in
AGNs. We present the preliminary results of the ongoing works in
our group in recent years, including observations of EGRET AGNs,
intermediate BL Lac objects and broad absorption line quasars.

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

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

\vs\vs Very Long Baseline Interferometry (VLBI) has made many
important contributions to the study of the radio jets in Active
Galactic Nuclei (AGNs). The superluminal motions detected in the
VLBI jets of the radio-loud AGNs suggest there are relativistic
bulk motions of the emitting particles. The alignment between the
pc scale jet and kpc scale jet suggests the energy transportation
from the central engine to the radio emission region at large
scale. The VLBI observation at 43\,GHz of nearby active galaxy M87
shows that the jet is formed and collimated on the very small
scale (Junor, Biretta, \& Livio 1999). The VLBI observations of
the $H_{2}O$ megamaser in the nucleus of NGC4258 provided the
strong evidence of the supermassive black hole in the center
(Miyoshi et al. 1995, Greenhill et al. 1995). The VLBI
observations of NGC4162 revealed there is a gap between the
counter-jet and VLBI core which is mostly due to the absorption of
the accretion disk (Jones et al. 2000). The relativistic beam
model can explain most of the VLBI observational results, and the
morphology of the radio jet  in the AGNs depends strongly on the
observing direction. VLBI still plays the important role in the
investigation of radio jets in AGNs. This paper presents the
preliminary results of some VLBI investigations of the AGNs in our
group in the recent years.

\para{\large\bf{2}~~ VLBI Observations of the EGRET AGNs}

\vs\vs The Energetic Gamma-Ray Experiment Telescope(EGRET), on the
Compton Gamma Ray Observatory (CGRO), has detected the
$\gamma$-ray emission at energies above 100\,MeV from the 66 AGNs
with high-confidence identification (Hartman et al. 1999). All the
EGRET AGNs are radio-loud, and most of them belong to the class
Blazars in which a high fraction of the superluminal motions (SLM)
were detected. On the basis of 22\,GHz and 43\,GHz VLBA multiepoch
observations carried out for 42 of the 66 EGRET AGNs , Jorstad et
al. (2001) concluded that the superluminal motions in $\gamma-ray
$ loud quasars are higher than those in $\gamma$-ray quiet
population. To study the connection between the radio structure
and $\gamma$-ray emission and to enlarge the sample, we applied
the VLA, Very Long Baseline Array (VLBA) and European VLBI Network
(EVN) observations for 15 EGRET sources which previously have no
good pc and/or kpc scale images, and 13 sources are quasars and 2
are BL Lac Objects. This is an ongoing project.

 The preliminary results show a variaty of morphology at VLBI scale:
 7 sources are core dominated as expected for strongly beamed sources,
 5 sources have a long and well defined jet, one source
 contains a lot of extended emission, 2 sources are symmetric/double.
 The extended emissions in 4 sources are detected at VLA scale.
 The VLA C-configuration 8.5\,GHz observation (the lowest resolution)
 reveals that the 3 sources, 1604+159, 2022-077
 and 1229-021, have the two-sided asymmetric structure,
 and the
 jets become one-sided at higher angular resolutions.
 Fig.1 shows the images of 1604+159 at different angular
 resolutions.
 Such structure would not be expected for presumably highly-beamed
 sources of the $\gamma$-ray loud AGNs.
 It is essential to search the proper motion in the inner
 jet to estimate the beaming levels in these
 sources. If these sources are not highly beamed in radio
 wavelength, one possible explanation is that the beaming
 direction in the $\gamma$-ray is different from that in the radio
 band.

\begin{figure}[h]
\center {\includegraphics [scale=0.63]{pn1604.ps}}

   %\parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {\center
   \footnotesize

   Fig.\,1~~ The
images of 1604+159 at different resolutions}

    \end{figure}

 \para{\large\bf{3}~~The structure variation of the pc scale jet in some
radio-loud\\[2mm] AGNs}

\vs 0420-014 is an EGRET detected source with the superluminal
motion. Based on the multiepoch VLBI results and radio light
curves, we found the flares in the radio bands could be classified
as two types according to their geometric origin (Zhou et al.
2000). One type flare in the radio light curves may be due to the
ejection of new component from the core. There are time lags of
the flare observed at the different frequencies, and the peak time
of the flare at low frequency is delayed compared with that at
high frequency, because of the opacity effects of the synchrotron
self-absorption. The second type of the flare is probably
introduced by the sharply bending of the jet. When a component
moves via the bending position, the flux density of this component
will vary because of the variation of the Doppler boosting factor,
in which case the flare time will be nearly the same at different
radio frequencies.


The new VLBA observations of the radio source J1625+4134 at 22 and
15\,GHz show a short and bending jet which has about 270 degree
difference in position angle with the northern jet detected at
lower frequencies. The new results, combined with the data
available in the literature, allow us to identify one of the
compact components as the VLBI core based on its flat spectrum.
Relative to this core component, the jet appears to be
bi-directional at VLBI scale. The proper motion measurement of the
component and the estimate of the Doppler boosting factor suggest
that the orientation of the jet is close to the line of sight. The
projection effect of an intrinsically bending jet within a few mas
from the core or the erratic change in the nozzle direction of the
jet can explain the uncommon bi-directional structure of the jet
(Jiang et al. 2002).



\para{\large\bf{4}~~EVN observations of Intermediate BL Lac Objects}

\vs\vs The BL Lac Objects have been discovered through radio or
X-ray survey, historically they were divided into two classes,
namely radio selected BL Lac objects (RBLs) and X-ray selected BL
Lac objects (XBLs). The XBLs have higher ratio of optical to radio
luminosity than RBL, but both have the similar X-ray luminosity.
The smaller and weaker degree of activity of XBL was attributed to
the orientation effects. The emission from the XBL was less beamed
than that of RBL, and XBLs are probably observed at larger
inclination to the jet axis (Marashi et al. 1986). In the recent
years, many works (Giommi \& Padovani 1994, Fossati et al. 1998)
based on the investigations of the broad band energy distribution
suggested that the BL Lacs were most likely form one class, with a
continuous distribution of syncrotron emission peak energies
(frequency), of which HBLs (XBLs) and LBLs (RBLs) represent the
extreme ends respectively. Laurent-Muehleisen et al.(1999)
presented the RGB sample of intermediate BL Lacs. The results
indicate that the simple unified scheme which postulates HBLs and
LBLs differ solely by orientation may be in need of revision.
\begin{figure}[h]
\center {\includegraphics [bb=0 86 408 481,scale=0.5]{j1542.eps}}

   \parbox[tb]{10cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize Fig.\,2~~ EVN map of 1542+614 at 5\,GHz; the restoring beam is 2.24
   $\times$1.43\,mas at P. A. =$ 7.^{o}9$; the contour levels are (-1, 1, 2,
   4, 8, 16, 32, 64)$\times$ 0.69\,mJy/beam; the peak flux density
   is 94.4\,mJy/beam.}

    \end{figure}

To explore the possible difference between the intermediate BL
Lacs and the RBL, the information about the compact structure of
the intermediate BL Lacs is important to understand if the
orientation effects play the important role. Kollgaard et al.
(1996) investigated 4 XBL objects using the VLBI technique, and
found their core-jet morphology. The radio flux of most HBLs are
too weak for VLBI observation, so we selected 7 intermediate BL
Lacs from the RGB sample for EVN 5\,GHz observation. Their peak
frequencies of the broad band spectral energy distributions are in
the range 8$\times10^{14}$ to 1.2$\times 10^{16}$\,Hz and the VLA
core fluxes are higher than 0.1\,Jy. The observation was carried
out on 22 November 1999. At the VLBI scale, the 7 IBLs all have
the one-sided core-jet structure and there is no evidence of the
counter-jet emission, which suggests that the jet emission at the
VLBI scale in these IBLs is Doppler boosted. Fig.2 is the EVN
image of 1542+614 at 5\,Hz. The model-fitting results of the VLBI
data show that the brightness temperatures of the VLBI core in the
source rest frame are relatively low, from 4$\times 10^{9}$ to
3$\times 10^{10}$\,K. The core size may be limited by the VLBI
angular resolution, the brightness temperature of the core is
probably just a lower limit, it needs high resolution observations
to confirm. If the brightness temperature of the VLBI core in
these sources is intrinsically low, the possible explanation is
that the emitting particle and magnetic field in the emission
region are far from the energy equipartition, with lower electron
number density and/or higher magnetic field.

\para{\large\bf{5}~~EVN Observations of Broad Absorption Line Quasars}

\vs\vs About 10\% of optically selected quasars display broad
absorption troughs, which extends up to 0.1\,c, in the blueward of
high ionization resonant lines (Weymann et al.  1991). Another ten
percent of them also show absorption troughs but in low ionization
lines.  The high ionization level and continuous absorption over a
large velocity range indicate that the absorption lines are
intrinsic to the nucleus. The blueshift of lines implies that
absorbing gas is flowing outward from the nucleus. A general
paradigm has been established that Broad Absorption Line (BAL)
region exists in both BAL and non-BAL quasars, and BAL quasars are
normal quasars seen along a particular line of sight. The
spectropolarimetry results can be best explained by an equatorial
BAL region which intersects the line of sight in a BAL QSOs
(Goodrich \& Miller 1995; Hines \& Wills 1995; Cohen et al. 1995).
Hence, the absorbing material can be a wind from an acrretion disk
or torus which is accelerated by radiation pressure, and BAL QSOs
are seen at large inclination. The BAL QSOs were found preferently
in radio quiet quasars, but not as radio loud until FIRST quasar
survey was carried out. The survey discovered more than 20 BAL
QSOs which can be formally classified as radio-loud quasars based
on the radio to optical flux ratio as well as on the radio power
(Becker et al. 1997, 2000, Menou et al. 2001). Brotherton et al.
(1998) discovered 5 radio-loud BALs detected by NRAO VLA Sky
Survey (NVSS). Most of them are unresolved on the VLA resolution
at 20\,cm with either flat or steep radio spectra. Based on the
large scatter in radio spectral indices, Becker et al. (2000)
argue that BAL quasars are not viewed at any particular
orientation. An alternative picture is that BALs are an early
stage in the development of new or refueled quasars. Voit et al.
(1993) suggested that the low-ionization BAL quasars are the
nascent QSOs embedded in a region with a high rate of massive
start formation.
\begin{figure}[h]
\center {\includegraphics [bb=0 123 397
502,scale=0.5]{j0957evna.eps}}

   \parbox[tb]{10cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize Fig.\,3~~ EVN map of J1312+2319 at 1.6\,GHz; the restoring beam is 24.5
   $\times$19.3\,mas at P. A. =$ -69.^{o}2$; the contour levels are (-1, 1, 2,
   4, 8, 16, 32, 64, 128, 256)$\times$ 0.34\,mJy/beam; the peak flux density
   is 131\,mJy/beam.}

    \end{figure}

The High resolution radio observations are important in the BALs
study, the radio morphology can serve as an inclination indicator
of these QSOs, therefore, yields direct test of the orientation
model. It is general expected that the axis of accretion disk is
aligned with the radio jets. If the BAL region is equatorial, we
would expect that we will see two side jets in these objects. The
EVN 18\,cm observation of 3 BALs was carried out in May 2001. The
phase referencing technique was used, since the radio fluxes are
weak. We obtained the preliminary results, two sources have
point-like structure and one source has two side jet structure. An
EVN image of J0957+2319 at L band is shown in Fig.3. If the
compact and bright components in the 2 unresolved sources are the
core ( this needs the spectral index to confirm), this means that
we observed directly the base of the jet and the jet orientation
may not be near the line of sight. The EVN images suggest that the
accretion disk or torus (if it exists) should not be edge-on, and
the absorption gases may not locate in the equatorial plane at
least for the 2 unresolved sources. Since the number of sources is
small, more observations of a large sample are needed for the
statistic study. This ongoing project collaborates with Wang
Tinggui.

\vn{\bf ACKNOWLEDGEMENTS}~~
      This work was supported by the Chinese fund NKBRSF (G1999075403).
      The European VLBI Network is a joint facility of European and Chinese
      and other radio astronomy institutes funded by their national
      research councils.

\bc {\bf References} \ec

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