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    {\small\rm\thepage\hfill
    \quad\quad ACTA ASTRONOMICA SINICA \hfill
    \hspace{2mm} Vol.44\vspace{2mm}}}} %%%%%%%
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    {\small\rm  suppl.\hfill X. Z. Zhang: A Search for Candidates of New Supernova Remnants  %%（作者：题目名称）
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\zihao{5}\songti

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    \\%%%[-0.5 true mm]
    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
A Search for Candidates of New Supernova Remnants    %%（文题）
%\\[1ex]
%$\!\!^*$
}

\vs\vs{\normalsize X.Z. Zhang}  %%（本文作者）

\vs
%%（本文作者单位）
 (\footnotesize \nao)
({\footnotesize {\it The Partner Group of MPIfR at NAO, CAS, Beijing }~100012}) \\
\vs\vs \ec

\normalsize
\begin{quotation}

 \no{\bf A{\scriptsize\bf BSTRACT }}~~~\small
Some results of searching for new supernova remnants (SNRs) using
the most sensitive radio surveys, such as NVSS, WENSS, Miyun  are
reported. IRAS data and ROSAT data were also used for
identifications. Four candidates are presented in this
contribution. They are G41.9+0.04, G47.8+2.03, G74.8+0.63, and
G93.2+2.63. All of these candidates show shell morphologies,
nonthermal spectra, linear
 polarized emission component, and
no strong infrared emission associated. We conclude that they are
probably new SNRs
\footnote{The project is supported by the
National Natural Science Foundation of China under Grant No.
10173015}

 \vn
 \no{\bf Key words}~~  supernova remnant; individual; structure; spectrum
\end{quotation}

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

\vs\vs The recent radio surveys with high sensitivities and space
resolution bring us high-quality images covering several parts of
the Galactic plane. This provides a new basis to search for new
candidates of supernova remnants (SNRs).  Four SNR candidates,
G41.9+0.04, G47.8+2.03, G74.8+0.63, and G93.2+2.63,  identified
from the radio surveys including NVSS (Condon et al. 1998), WENSS
( Rengelink et al 1997), Miyun (Zhang et al. 1997), Effelsberg
11cm survey (Reich et al. 1988, 1990; F\"urst et al. 1990; Duncan
et al. 1999 ), and GB6 (Gregory et al. 1996 ) are presented in
this contribution. The candidates are the results of a more
detailed examination of the radio survey data listed above,
together with
 IRAS 60/100\,$\mu$m infrared images and ROSAT
 images. The component of polarized emission was also examined for these
objects using NVSS 20\,cm and Effelsberg 11\,cm polarization data.

Morphology, linear polarization, nonthermal spectrum, and no strong infrared
emission associated are widly accepted as the criteria to identify new SNRs.
The tools to identify new SNRs were
completed  in recent years with X-ray data as well (Reich, 2002).
All the candidates listed above belong to small angular
objects. According to the statistical
 results the observed number of small angular SNRs is much less than that
expected by theory. On the other hand, there is  a gap in
SNR statistical study. One possible explaination  is that the small
angular SNRs are either very young or  far away. In general young SNRs are
usually  located in a very complex environment.
Mixture of thermal and nonthermal emission
in such area is a common phenomenon. So to separate
the thermal and nonthermal emission would be necessary before
calculating the spectral index. Discoveries of small angular
SNRs will serve to fill  the statistical gap. This is useful
on the study of  SNR evolution
and interstellar medium (ISM)  because young SNRs are at an important
evolution stage and interacting with their environment.

Survey data used in this study will be described
briefly in section 2 and a part of the results will be given in section 3.

\vspace*{1mm}
\para{\large\bf{2}~~The Surveys}

\vs\vs Several radio surveys have been used in this searching. The
most important one is NVSS (Condon et al. 1998). The NVSS was
carried out at 20cm band by NRAO with the VLA D configuration. The
synthesis beam of this survey is $45'' \times 45'' $. The noise
fluctuation of NVSS I map is about 0.45\,mJy/beam and about
0.29\,mJy/beam in Q and U maps. The largest angular size of D
 configuration at 20\,cm band which can be reasonably well imaged in
snapshot observation is about $450''$ (Perley 2000). The shortest
base-line of VLA D configuration is 35\,m. This allows us to
search SNR candidates smaller than $10'$ using the NVSS data
without serious missing large scale flux densities. This is also
helpful in reducing the effect of very large scale source and any
back ground structure. By reducing the confusion, it can
distinguish the  relatively small
 angular objects which are near a strong and large object.


WENSS is another important radio survey at a low frequency of
327\,MHz. Its
 synthesis beam is $54'' \times 54''csc\delta$ which is similar but
larger than  the NVSS beam. Its map fluctuation of $5\sigma $ is
about 18\,does not  match the NVSS well for this searching even
taking the SNR spectral property into account. WENNSS covers the
sky area of north $30^{o}$ which is smaller than the NVSS
coverage. The Miyun survey (Zhang et al. 1977) recorded about
34000 radio sources covering the sky area north of $30^{o}$. The
data were used for calculations of spectral indices  for a member
of  candidates.

At high frequencies the Effelsberg 1,cm survey (hereafter E11)
(Reich et al. 1988, 1990; F\"urst et al. 1990; Duncan et al. 1999
) and the Green Bank survey at 6 cm band (Gregory et al. 1996 )
were used to check the spectrum of the candidates found with low
frequency surveys. As the beams of E11 and GB6 are rather large
and different, the spectral indices obtained with them
 do not cover exactly the same  emission area as that of the NVSS and WENSS.
More analysis has to be done before  using the spectral information calculated
with them.

Since SNR's emission is of nonthermal nature, it is expected that no strong
infrared emission associate with it. This was confirmed by observations,
especially for the evolved SNRs. Some young SNRs are enclosed
by thermal emission because of the shock-heated dust.
It is quite complex to analyze the association of SNR and infrared emission.
Association with the IRAS 60/100 $\mu$m images (Beichmann et al. 1985) with
candidates presented in this paper have been checked.
The comparison method we used
 basically follows the method reported by F\"urst et al.
(F\"urst et al.  1987). The
main idea is to calculate the ratio of intensities of infrared and radio.
This is quite efficient in identifing SNR candidates, at least to separate the
thermal objects from the candidates.

\para{\large\bf{3}~~The New Candidates}

\vs\vs Table 1 lists the basic parameters of the four candidates
found from this
 searching. The searching started with examination of the total intensity
images in the  Galactic plane of $|b| \le 10^{o}$ covered by NVSS. Because
SNR emission is of synchrotron machnism, linear polarized
component  is expected to be found.  So the NVSS polarization
images, Q/U,  are  checked when some possible candidates were found
according to their total intensity morphologies.
The E11 polarization information is used
 when no NVSS polarized data available or in case there is  depolarization
at the relative low frequency. Finally the ratio, R, between infrared
and radio emission were calculated for further identification.

\bc{\small\bf

 Table 1~~   The basic parameters of the four candidates obtained from
the NVSS images
}\\[2mm]

\fns \tabcolsep 4mm
\begin{tabular}{llllll}
\hline\\[-3.6mm]\hline
 Name& Position& Angular size& Peak/integrated & Type & \\
     & (J2000)  &  ($' \times '$)       & intensity(mJy) &  &  \\
 \hline
G41.9+0.04& 19:07:51.0;08:00:59& $3 \times 3$&50/550  & S &   \\
 G47.8+2.03& 19:11:38.9;14:08:42& $7 \times 5$&6/56    & S &  \\
 G74.8+0.63& 20:17:39.1;36:46:07& $5 \times 5$&137/1460& S &   \\
 G93.2+2.63& 21:13:50.1;52:27:37& $4 \times 4$&31/260  & S &   \\
 \hline\\[-3.6mm]\hline%\\[-4.5mm]\hline
 \end{tabular}
 \ec

\vn{\bf{3.1~~G41.9+0.04}}

 This object is the smallest one of the
four candidates. The NVSS image reveals this object as a well
defined ring-like shell structure with the diameter of about $3'$.
Fig. 1 gives the NVSS total intensity map (left) and the
polarization map (right) images. Both WENSS and Miyun surveys do
not cover this sky area because of the low declination of this
object.
%The spectral index of $\alpha_{269}^{4850} = -0.6?$
% ($ S_\nu \propto \nu^{\alpha}$ ).
No obvious polarized emission related to the G41.9+0.04 was found
either from the NVSS or E11 data. IRAS 60\,$\mu$m  and the E11
data were used to calculate the ratio R following the method of
F\"urst et al. (1987). They defined a criterion of HII region as
R: $500 \sim 1000$. The R value of G41.9+0.04 is about 100.
Although it is lager than the values of most evolved SNRs it is
still smaller than 500. This may be common for small angular SNRs,
because they are usually not well evolved and may be associated
with thermal emission environment, the shock-heated dust (Vessey
1995). This is also potentially an important tool in separating
compact SNR candidates from the extragalactic synchrotron
population.

\begin{figure}[h]
\parbox[b]{7cm}
{\hspace{10mm}\includegraphics [bb=45 58 392 488,%angle=180,
scale=0.45]{G419I.eps}}

\vspace{-70mm}
 \ \hfill \
\parbox[b]{6cm}
{\includegraphics [bb=45 48 392 491,%angle=180,
scale=0.45]{G419.eps}}

   \parbox[tb]{14cm}
   %{\hspace{1cm}
   {%\center
   \footnotesize
Fig.\,1~~ The total intensity map (left) and polarization map of
G41.9+0.04 (right,grey map) obtained from NVSS}%}

    \end{figure}


\vn{\bf{3.2~~G47.8+2.03}}

 This object is the weakest one of the
four candidates. The NVSS image reveals this object as a well
defined  shell structure. Fig. 2 gives the NVSS total intensity
map and the E11 polarization map because there is no polarized
NVSS data available for this object.
 Both WENSS and Miyun surveys do not cover this sky area because of its low
declination
 Polarization emission from this object is evident
according to  the E11 polarized data. The morphology and the
polarization features suggest  G47.8+2.03 is probable a SNR. IRAS
60\,$\mu$m  and the E11 total intensity data were used to
calculate the ratio R.  The R value of G47.8+2.03 is about 40.
This also confirms that G47.8+2.03 is not a thermal object because
$R_{G47.8+2.03} $ is much less than that of a thermal object such
as HII region or planetary nebula(PN).

\begin{figure}[h]
\parbox[b]{6.4cm}
{\hspace{10mm}\includegraphics [bb=93 173 377 457,%angle=180,
scale=0.5]{G478I.eps}}

\vspace{-55mm}
 \ \hfill \
\parbox[b]{6.5cm}
{\includegraphics [bb=36 64 395 487,%angle=180,
scale=0.44]{G478P11CM.eps}}

   \parbox[tb]{14cm}
   %{\hspace{3.5cm}
   {%\center

   \footnotesize
Fig.\,2~~ The total intensity map of G47.8+2.03 (left) obtained
from NVSS image and the polarization map obtained from the
Effelsberg 11\,cm image}

    \end{figure}

\vn{\bf{3.3~~G74.8+0.63}}

 This candidate is resolved clearly by
NVSS showing a shell structure  of about $5'$ in diameter and it
is the strongest one of the four candidates. WENSS image also
shows the same morphology with  lower resolution of about $54''
\times 90''$. Fig. 3 gives the NVSS images of total intensity
(left) and the polarization intensity (right).  This object can
also be found with Miyun catalog giving the integrated flux
density of 1.9\,Jy at 232\,MHz (Zhang et al. 1997). The basic
paramters of this candidate can be found in Table 1. The spectral
indices  calculated with integrated flux densities at 232, 327,
and 1400\,MHz are $\alpha_{232}^{1400} = -0.66$ and
 $\alpha_{327}^{1400} = -0.62$ respectively.  Linear
 polarized emission of G74.8+0.63 was found with the NVSS Q/U maps.
Fig. 3 shows the polarized intensity map (right) and the total
intensity map (left) respectively. The polarization intensity is
about 1.5\,mJy level within the G74.8+0.63 region. The morphology,
nonthermal spectrum, and the linear polarized component make
G74.8+0.63 a very probable SNR.

\begin{figure}[h]
\parbox[b]{7cm}
{\hspace{10mm}\includegraphics [bb=93 173 377 457,%angle=180,
scale=0.52]{fig3i.eps}}

\vspace{-55mm}
 \ \hfill \
\parbox[b]{6cm}
{\includegraphics [bb=8 0 405 504,%angle=180,
scale=0.38]{G748.eps}}

   \parbox[tb]{14cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize

      Fig.\,3~~ The total intensity map of G74.8+0.63 (contour) and the
polarization intensity (grey map) obtained from NVSS images}

    \end{figure}

\vn{\bf{3.4~~G93.2+2.63}}

 This candidate is resolved clearly by
NVSS showing a shell structure, which is almost completely
circular with a diameter of about $4'$. WENSS image also shows the
same morphology in low resolution of about $54'' \times 68''$.
Fig. 4 gives the NVSS image. This object can also be found with
Miyun catalog giving the integrated flux density of 0.7\,Jy at
232\,MHz.
  The spectral indices
calculated with integrated flux densities at 232, 327, and
1400\,MHz are $\alpha_{232}^{1400} = -0.55$ and
 $\alpha_{327}^{1400} = -0.32$ respectively.  Linear
 polarized emission of G93.2+2.63 was found with the NVSS Q/U maps.
Fig. 4 shows the polarized intensity map (right). The polarization
intensity is about 2.0\,mJy level within the G93.2+2.63 region.
Further polarization
 observation with high sensitivity at high frequency
is needed to confirm the polarized component.

 The R value of G93.2+2.63 is about 80.  The morphology, spectral index,
presence of linear polarized emission, and no strong infrared
emission associated imply that G93.2+2.63 is propably a SNR.

\begin{figure}[h]
\parbox[b]{7cm}
{\hspace{10mm}\includegraphics [bb=39 89 395 480,%angle=180,
scale=0.48]{G932I.eps}}

\vspace{-65mm}
 \ \hfill \
\parbox[b]{6.5cm}
{\includegraphics [bb=39 90 395 479,%angle=180,
scale=0.48]{G932.eps}}

   \parbox[tb]{14cm}
   %{\hspace{3.5cm}
   {%\center
   %\vspace{-8mm}
   \footnotesize

      Fig.\,4~~ The total intensity map (left) and the polarization
intensity map (right) of G93.2+2.63 obtained from NVSS images}

    \end{figure}


\para{\large\bf{4}~~Conclusion and Discussion}

\vs\vs We conclude that the four candidates are probably new SNRs
according to their radio morphologies, radio spectra, linear
polarization, and no strong infrared emission associated.

\vs\vn{\bf ACKNOWLEDGEMENTS}~~ X. Zhang thanks for the support from
NAO, CAS and the hospitality from MPIfR during his visits in Bonn.

\newpage
\vspace*{1mm}
\bc {\bf References} \ec
 \vspace{-4mm} \no \fns

 \nref Beichmann C.A., Neugebauer G., Habing H.J., et al.,
        IRAS Explanatory Supp., 1985
\nref Condon J.J., Cotton W.D., Greisen E.W., et al., AJ, 1998,
115, 1963 (NVSS) \nref Condon J.J., Broderick J.J., Seielstad
G.A., AJ, 1989, 97, 1064 \nref Condon J.J., Broderick J.J.,
Seielstad G.A., et al., AJ, 1994, 107, 1829 \nref Duncan A.R.,
Reich P., Reich W et al., A\&A, 1999, 350, 447 \nref  F\"urst E.,
Reich W., Reich P., et al., A\&AS, 1984, 85, 691 \nref  F\"urst
E., Reich W., Sofue y., A\&AS, 1987,
 71, 63
\nref Gregory P.C., Scott W.K., Douglas k.,  ApJS, 1996, 103, 427
\nref Perley R.A.,  The VLA Observational Status Summary, 2000
\nref Reich W., F\"urst E., Steffen P., et al., A\&AS, 1990, 58,
197 \nref Reich W., F\"urst E., Reich P., et al., A\&AS, 1984, 85,
633 \nref Reich W.,  Radio Observations of Supernova Remnants,
       in proceedings of Xian Germany-Chinese Conference of ``Radio
       Studies of Galactic Objects, Galaxies, and AGNs'', 2002
\nref Rengelink R.B.,  Tang Y., Bruyn A.G., et al.,  1997,
 124, 259(WENSS)
\nref Vessey S.J.,  Ph.D.Thesis, University of Cambridge, 1995
\nref Zhang X., Zheng Y., Chen Y., et al., A\&AS, 1997, 121, 59


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