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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 A. Esamdin et al.: Observations of PSR 0329+54  %%（作者：题目名称）
    \vspace{1.5mm}
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\zihao{5}\songti

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    %第44卷~第1期  %%%第44卷~第1期
    %\hfill~~{\zihao{5}天\quad 文\quad 学\quad 报}  \hfill
    %Vol.44~ No.1
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    Vol.44~ Suppl.
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      Feb.,~2003 \\[-1.5 true mm] %%January
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\vs\vs\vs \bc {\Large\bf\zihao{2}\heiti
Two-Frequency Simultaneous Observations of \\[2mm]Mode-changing Phenomena of PSR0329+54    %%（文题）
%\\[1ex]
%$\!\!^*$
} %\footnotetext {2001-03-08收到原稿，2001-05-14收到修改稿   %%（文章收到似的日期）

\vs\vs{\normalsize\zihao{4}\fangsong Ali Esamdin\up{1,2,3},\hs\hs X.J. Wu\up{1,2} \hs\&\hs X.Z. Zhang\up{4}}  %%（本文作者）

\vs
({\footnotesize 1~{\it Urumqi Astronomical Observatory, NAO, CAS, Urumqi}~ 830011}) \\%%（本文作者单位）
 (\footnotesize 2~ \pka)\\
 (\footnotesize 3~ \bac)\\
 (\footnotesize 4~ \nao)
\vs\vs \ec

\normalsize
\begin{quotation}

 \no{\bf A{\scriptsize\bf BSTRACT }}~~\small\zihao{-5}
Two-frequency simultaneous monitoring of pulsars were carried out
using Urumqi 25-m telescope at 327\,MHz and 610\,MHz from 11th July to
21st August 2000. Two mode-changing data of PSR0329+54 were
obtained. In this paper, the observed results of PSR0329+54 are
presented and spectral behaviors in its normal and abnormal modes
are briefly investigated.

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

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

\vs\vs The individual pulses of pulsars are highly variable in
shape and intensity. Nevertheless, the integrated profiles of
pulsars are well known for their long-term stability. Studies of
the stability of integrated profiles have been carried out
systematically (Helfand et al., 1975; Rankin et al., 1995). These
studies show that a number of a few thousand pulses folded
together often leads to a very stable profile. Despite this
stability of pulsar integrated profiles, some pulsars shows
distinct profile changes on time scales of minutes. Following the
first identification of the profile changes in PSR 1237+25, such a
phenomenon was first noticed by Backer (1970) and is now
known as mode-changing. During a mode change, the pulsar switches
from one stable profile to another on a time scale of less than a
pulse period and remains in that mode for typically hundreds of
periods before it returns back to the original. Pulsar
mode-changing is often associated with a sudden change in pulse
intensity (Rankin, 1986; Suleymanova et al., 1996).

Mode-changing is often observed for sources which exhibit rather
complex profiles showing both the ``cone'' and ``core'' components
(Rankin, 1983; Rankin, 1986; Lyne et al., 1988). The mode-changing
phenomenon implies a reorganization of core and cone emission. So
the whole pulsar profile is affected by the mode-changing rather
than only certain pulse longitudes. There is no clear explanations
for the origin of mode-changing, sometimes interpreted as a
re-arrangement in the structure of the emitting region, e.g. a
possible relationship between mode-changing and change in emission
height (Bartel et al., 1982). Maybe it is related to the
properties of the magnetosphere above the polar cap (Zhang et al.,
1997). Further observations are very important to understand such
a behavior of some of the pulsars.

The intensities of integrate pulsar profiles are changing all the
time because of the interstellar scintillation (Rickett et al.,
1977; Romani et al., 1986). There are two types of scintillation,
the DISS and the RISS. The time scale of DISS ranges from seconds
to minutes. The time scale of RISS ranges from days to months. So
the best way of measuring the spectrum of pulsars is simultaneous
observation at several frequencies. From 11th July to 21st August
2000, the simultaneous monitoring was carried out at two
frequencies (327\,MHz and 610\,MHz) using Urumqi 25\,m radio
telescope. A sample of 18 strong pulsars was observed. Two
abnormal profiles and ten normal profiles of PSR0329+54 (all with
high signal to noise ratio) were obtained. In this paper, the
spectrum behaviors of PSR0329+54 in its normal mode and abnormal
mode are briefly investigated.

\para{\large\bf{2}~~Observations and Data Reduction}

\vs\vs The data presented in this paper were obtained during
two-frequency simultaneous monitoring of pulsars at 327\,MHz and
610\,MHz with the Urumqi 25-m radio telescope of NAO located at
Nanshan observatory, Urumqi. System temperatures are 130\,k and
110\,k respectively.

At each frequency, the signal was sampled 2\,ms intervals in a
duration of 2000\,s by a single-channel, single polarization data
collecting system and recorded on hard disk. Then using our own
compiled computer software the data were period-folded and the
integrated pulse profiles at each frequency were obtained. The
bandwidths of receivers were restricted to 1\,MHz at 327\,MHz and
2\,MHz at 610\,MHz because of dispersion of pulsar radio signal by
interstellar medium.

\begin{figure}[h]
\vspace{-12mm}
\center {\includegraphics [bb=0 0 331 251,scale=1]{figure1.eps}}
   \parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize
      Fig.\,1~~ One of the two-frequency profiles
of PSR0329+54 in abnormal mode. Data were simultaneously obtained
at 327\,MHz and 610\,MHz at 22:56 in 18th July 2000(BT) using
Urumqi 25-m telescope. The top panel of each diagram shows 250
individual pulses. The bottom panel presents the integrated
profiles gotten at each frequency.}

    \end{figure}
\begin{figure}[h]
\vspace{-12mm}
\center {\includegraphics [bb=0 0 315 251,scale=1]{figure2.eps}}
   \parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize Fig.\,2~~ Two-frequency profiles of PSR0329+54 in
normal mode. Data were simultaneously obtained at 327\,MHz and
610\,MHz using Urumqi 25-m telescope at 00:17 in 12th July
2000(BT).}
    \end{figure}

   \para{\large\bf{3}~~Results}

\vs Twelve high signal to noise ratio integrated profiles of
PSR0329+54 were obtained during our observation, among them two
profiles in abnormal modes were found. Abnormal and normal
profiles obtained during our simultaneous observations are
displayed in Figure 1 and 2.

In one observational duration of 2000\,s, nearly 2800 individual
pulses are gotten. In Figure 1 and 2, the top panel of each
diagram shows 250 individual pulses of them. The bottom panel
presents the integrated profile that is obtained by summing all
individual pulses gotten in one observational duration. The
horizontal axis of bottom panel is phase of pulse expressed in
degrees and the vertical axis is relative flux density in
arbitrary units.

The observed and derived parameters at two frequencies, pulse
widths, relative flux densities and relative spectral indices of
leading and trailing component are listed in Table 1. Listed are
time of observation (Beijing time), pulse widths at 50\% of the
peak amplitude ($w_{50}$), relative intensities of three
components, relative spectral indices of leading and trailing
components. We set the peak intensities of all profiles at two
frequencies are 100. That means we simply set all relative
spectral indices to zero.
% Two column table 2
%-------------------------

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

\vs Figure 3 displays the observed relative spectral indices of
leading and trailing components of PSR0329+54 when all of the
spectral indices of the central components of twelve observations
are set to be 0.

Figure 3 shows that, when PSR0329+54 is in its normal mode, the
spectrum of leading and trailing component are a little bit
flatter than the spectrum of the central component, and the
spectrum of trailing component is steeper than the leading one.
The spectrum of leading component get a little bit steep and the
spectrum of trailing component get flat in abnormal mode.

Because leading and trailing component of PSR0329+54 are mainly
contributed by cone emission and central component are due to core
emission, such a result implies core emission of PSR0329+54 may
affect the trailing component in normal mode. In abnormal mode,
such affection vanishes and the leading component is affected by
core emission instead. However, because of the large errors of our
observed data, further observations are needed.

\newpage
\bc{\small\bf

 Table 1~~   Observed and derived parameters of twelve observations
}\\[2mm]

\fns \tabcolsep 3mm
\begin{tabular}{ccccccccccccc}
\hline\\[-3.6mm]\hline
 Time& $W_{50}$&C&$L_{327}$&$L_{610}$&$\alpha_{L}$&$F_{327}$&$F_{610}$&$\alpha_{F}$\\
(BT)&(Deg)    & &         &         &            &         &         &            \\
 \hline
Jul.12\ 00:17 & 3.9/3.2 & 100 & 5.6 & 8.5 & 0.67 & 11.6 & 12.9 & 0.18\\
 Jul.16\ 19:16 & 3.9/3.2 & 100 & 6.1 & 9.9 & 0.78 &  11.5 & 13.3 & 0.21\\
 $\star$ Jul.18\ 22:56  & 3.9/3.1 & 100 & 12.5 & 16.1 & 0.41 & 6.6 & 9.1 & 0.52\\
 Jul.19\ 01:53 & 3.8/3.2 & 100 & 5.4  & 8.6 & 0.74 & 11.4 & 12.3 & 0.12\\
 Jul.23\ 01:11 & 4.0/3.2 & 100 & 6.5 & 9.5 & 0.59 &  11.8 & 13.7 & 0.23\\
 Jul.24\ 15:14  & 3.9/3.1 & 100 & 5.9 & 8.7 & 0.63 &  12.1 & 12.9 & 0.11\\
 Jul.28\ 18:24 & 3.9/3.2 & 100 & 5.6 & 8.5 & 0.65 & 11.7 & 13.1 & 0.18\\
 $\star$ Jul.29\ 16:58  & 3.9/3.1 & 100 & 12.4  & 16.5  & 0.43 & 6.2 & 8.8 & 0.56\\
 Jul.30\ 23:26 & 3.8/3.2 & 100 & 5.1 & 8.5 & 0.81 & 10.3 & 12.6 & 0.32\\
 Jul.31\ 08:29 & 3.9/3.2 &  100 & 5.1 & 8.3 & 0.78 & 11.5 & 12.5 & 0.15\\
 Aug.15\ 13:21 & 4.1/3.2 &  100 & 5.7 & 8.5 & 0.63 & 10.9 & 12.2 & 0.18\\
 Aug.16\ 23:47 & 4.1/3.1 &  100 & 5.7 & 8.7 & 0.66 & 11.4 & 13.1 & 0.22\\
 \hline\\[-3.6mm]\hline%\\[-4.5mm]\hline
 \end{tabular}
 \ec
 \vspace{-2mm}{\footnotesize
 The symbol $\star$ indicate the integrated
profiles of PSR0329+54 in abnormal mode.
L = Leading component,\\
\vspace{-0.5mm}C
= Central component, F = Following component.\\
 \vspace{-0.5mm}$\alpha_L$ and
$\alpha_F$ are relative spectral indices of leading and trailing
component
respectively.\\
 \vspace{-0.5mm}In column 2, the number above and below the slash are the
parameters at 327\,MHz and 610\,MHz respectively.
 }

\begin{figure}[tbph]
\vspace{-14mm}
\center {\includegraphics [bb=0 0 315 234,scale=1]{figure3.eps}}
   \parbox[tb]{12cm}
   %{\hspace{3.5cm}
  { %\center
   \footnotesize
  \vspace*{-2mm} Fig.\,3~~ The relative spectral indices of
leading (top panel) and trailing components (bottom panel) of
PSR0329+54 when all of the central component's spectral indices of
twelve data are zero. The black-square points are the results of
normal mode and the star points are the results of abnormal mode.}
    \end{figure}

%\newpage\vspace*{1mm}
\vspace{-3mm}\bc
{\bf References}
\ec

\vspace{-4mm}
\no \fns

 \nref Backer D C. Nature, 228: 1297
   \nref Bartel N, Morris D, Siber W, et al. AJ, 1982, 258: 776
   \nref Helfand D J, Manchester R N, Taylor J H. AJ, 198: 661
   \nref Lyne A G, Manchester R N.  MNRAS,  1988, 234: 477
   \nref Rankin J M, Rathnasree N. AJ, 1995, 452: 814
   \nref Rankin J M. AJ, 1983, 274: 333
   \nref Rankin J M. AJ, 1986, 301: 901
   \nref Rickett B J. ARA\&A, 1977, 15: 479
  \nref Romani R W, Narayan R, Blandford R. MNRAS, 1986, 220: 19
   \nref Suleymanva S, Izvekova V, Rankin J M.  In: Johnston S, Walker M A eds.
   IAU Colloquium 160, Pulsar: Problem and Progress. San Francisco: Astronomical
   Society of the Pacific, 1996. 223
   \nref Zhang B, Qiao G J, Lin W P, et al. AJ,  1997, 478: 313

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