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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 J. L. Han: Highly Polarized Radio Sources -- an On-going Project  %%�����ߣ���Ŀ���ƣ�
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    Vol.44~ Suppl.
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    \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 Highly Polarized Radio Sources -- an On-going Project  %%�����⣩
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\vs\vs{\normalsize J. L. Han}  %%���������ߣ�

\vs

(\nao) \vs\vs \ec

\normalsize
\begin{quotation}

 \no{\bf A{\scriptsize\bf BSTRACT }}~~~\small
The NRAO VLA Sky Survey detected about 1.8 million radio sources,
for the first time, with linear polarization information. The
global polarization percentage of any type of objects, except for
pulsars, can rarely be more than 20\% to 25\%.  We construct a
sample of highly polarized radio sources from the NVSS catalog,
with polarization percentage $>30\%$. We have conducted the pulsar
searching on these targets. However, except the a dozen known
pulsars, no new pulsars have been found. Superposition of the
sources over optical pictures revealed several sources being 2MASS
infrared-strong objects. It is intriguing to understand why these
sources are so highly polarized.

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

%%%% ����

\vs\vs

Detailed studies of radio sources have been one of the most important
approaches for the major discoveries. For example, in 1960s, deliberate
measurements of fluctuations of radio sources in scintillation studies
directly resulted in the discovery of pulsars (Hewish et al.
1968). Optical identifications of radio sources led to the
discovery of radio galaxies (e.g. Baade \& Minkowski 1954)
and quasars (e.g. 3C273 by Schmidt 1963), which are
powered by a black hole with an accretion disk. In 1980s, the first
millisecond pulsar was discovered during the very detailed investigation
of a compact low-latitude radio source with very steep spectrum and
interplanetary scintillations (Backer et al. 1982).

The NRAO VLA Sky Survey (NVSS; Condon et al. 1998) is a great
project, which mapped 82\% of the celestial sphere at 1.4\,GHz and
detected 1.8 million radio sources with flux density greater than
2.5\,mJy. Most grandly, it is the first time to have linear
polarization information for such a large complete sample of radio
sources.

The polarization of synchrotron emission in a perfectly regular
field can reach a maximum of 70\% (assuming an electron spectral
distribution index of $-$2). An intrinsic polarization percentage of
30\% implies that $>40\%$ of magnetic energy is stored in a
relatively large-scale regular field.

Previously very few studies on highly polarized sources have been
published. The anomalously high-linearly polarized radio
source near the spiral galaxy NGC 891 was ever found to be a
very luminous binary millisecond pulsar by Navarro et al. (1995).
PSR B1951+32 was first identified as a polarized
steep-spectrum point source in CTB 80 (Strom 1987).
The exotically polarized object, J06587-5558, discovered
by Liang et al. (2001) near a cluster of galaxies, is an extended
non-pulsar extragalactic object with the highest polarization (54\%)
up to now and a very steep spectrum ($\alpha\sim-1$). It is not
clear yet what it is.

We are going to work on a sample of radio sources with highest
polarization detected by the NVSS. All sources in the NVSS catalog
with polarization percentage, $p_p$, greater than 30\% have been
checked. Some of them have been observed as pulsar candidates
with Parkes and Jodrell bank telescopes, two dozen of them have
been imaged with VLA and ATCA for higher resolution polarization map;
a number of sources have been observed for optical spectra.

\begin{figure}[tbph]
\vspace{-10mm} \center
{\includegraphics[scale=0.4]{0739.eps}\hspace{3mm}\includegraphics[scale=0.4]{0910.eps}}
 \center {\includegraphics [scale=0.4]{1044.eps}\hspace{3mm}\includegraphics [scale=0.4]{1111.eps}}
 \center {\includegraphics[scale=0.4]{1546.eps}\hspace{3mm}\includegraphics[scale=0.4]{1741.eps}}
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   %{\hspace{3.5cm}
   {%\center
   \footnotesize

      Fig.\,1~~ Some highly polarized NVSS sources have optical identifications,
most likely associated with galactic nuclei, as clearly shown by
the higher resolution FIRST map superimposed onto the DSS images.}

    \end{figure}

We cross-check all known pulsars in pulsar catalog (the
updated version of Taylor, Manchester \& Lyne 1993)
with our sample by position coincidence. 10 sources are found
to be known pulsars.

Among 52 sources with good FIRST survey images (Becker, White \& Helfand
1995), 19 sources have optical
identifications.
According to the profiles of optical images obtained from DSS-FITS file,
almost all of them are  galactic nuclei (maybe active!). The high resolution
FIRST maps of most of them show good extended emission. It is not clear why
they generate radio emission with high polarization. It is somewhat
astonishing that 5 of 6 known objects are very red objects, either
detected in 2MASS or IRAS surveys.



\bc{\small\bf

 Table 1~~   Some highly polarized sources are very red objects
}\\[2mm]

\fns \tabcolsep 1mm
\begin{tabular}{cccccl}
\hline\\[-3.6mm]\hline
 Source name& $p_p\pm\sigma_{p_p}$& \mult{3}{c}{USNO
results (in mag)} &NED results \\
             &    (\%)   &  B2  & R2   &  I2  &  \\
\hline 073904$+$1804& 60.5$\pm$16.7&  10.93& 10.32& 10.57& 2MASXi
J0739043+18042
5: IrS \\
111151$+$2711& 30.7$\pm$~6.3&  11.64& 10.57& 10.49& 2MASXi
J1111515+27115
5: IrS\\
131015$+$1056& 31.6$\pm$~6.4&      &      & 11.71& MAPS-NGP
O-498-116998
8: G\\
142945$-$0009& 61.4$\pm$14.8&  20.38& 19.84&      & IRAS 14
272+0003;z=0.03; IrS\\
162558$+$5651& 52.6$\pm$13.9& 17.15& 15.54& 15.07& 2MASXi
J1625591+56512
3: IrS         \\
163221$+$1551& 36.5$\pm$~8.0& 17.24& 16.23& 16.53& IRAS
F16300+1558;
 LINER; z=0.241; IrS\\
 \hline\\[-3.6mm]\hline%\\[-4.5mm]\hline
 \end{tabular}
 \ec

\vs\vn{\bf ACKNOWLEDGEMENTS}~~ The research work described here is
supported by the National Natural Science Foundation of China
(19903003 and 10025313), the National Key Basic Research Science
Foundation of China (G19990754) and the partner group of MPIfR at
NAOC.

\bc {\bf References} \ec

\vspace{-4mm} \no \fns

 \no Baade W., Minkowski R., ApJ, 1954, 119: 206\\
\no Backer D.C., Kulkarni S.R., Heiles C., et al, Nature, 1982,
300: 615\\
\no Becker R. H., White R. L., Helfand D. J., ApJ, 1995, 450: 559\\
\no Condon J.J., Cotton W.D., Greisen E.W., et al, AJ, 1998, 115:
1693\\
\no Hewish A., Bell S.J., Pilkington J.D.H., et al, Nature, 1968,
217: 709\\
\no Liang H., Ekers R.D., Hunstead R.W., et al,  MNRAS, 2001, 328:
L21\\
\no Navarro Jose, de Bruyn A.G., Frail D.A., et al,  ApJ, 1995,
455: L55\\
\no Schmidt M., Nature, 1963, 197: 1040\\
\no Strom R.G., ApJ, 1987, 319: L103\\
\no Taylor J.H., Manchester R.N., Lyne A.G, ApJS, 1993,  88: 529\\

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