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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 N. Wang et al.: Pulsar Observations at Urumqi Observatory $-$
    Present and Future  %%（作者：题目名称）
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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
Pulsar Observations at Urumqi Observatory \\[2mm] --- Present and Future   %%（文题）
%\\[1ex]
%$\!\!^*$
}

\vs\vs{\normalsize  Na Wang\up{1},\hs\hs R. N. Manchester
\up{2},\hs\hs Jin Zhang\up{1},\hs\hs
 Xinji Wu\up{3}\hs\&\hs Ali Esamudin\up{1,3}}  %%（本文作者）

\vs
({\footnotesize 1~{\it National Astronomical Observatories, CAS, Urumqi}~ 830011}) \\%%（本文作者单位）
 (\footnotesize 2~{\it Australia Telescope National Facility, CSIRO, P.O. Box
76, Epping, NSW 1710, Australia})\\
 (\footnotesize 3~ \pka)\\

\vs\vs \ec

\normalsize
\begin{quotation}

 \no{\bf A{\scriptsize\bf BSTRACT }}~~~\small
We present the status and results of pulsar observations at Urumqi
Observatory. Observations commenced with a pulsar timing system at
the 25-m Urumqi Nanshan telescope in mid-1999; these were the
first regular and high-quality pulsar observations in China. The
center frequency of this system is 1540 MHz, and de-dispersion is
provided by a $2\times128\times2.5$ MHz filterbank/digitiser
system. Observations over more than one year have resulted in
updated positions and period parameters for 74 pulsars. Comparing
with earlier observations we showed that long-term period and
period-derivative fluctuations may be dominated by unseen
glitches. Taking advantage of the available telescope time, we are
also monitoring the variation of pulsar scintillation dynamic
spectra for a few strong pulsars. Scintillation parameters are
measured and their variations are under study. We are planning to
built a new system at a lower frequency so that
frequency-dependent pulsar properties can be investigated.

\end{quotation}

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

\vs\vs The first pulsar observation was made in late 1967, and
since then much progress has been made in understanding their
properties. Pulsars are also used as probes to investigate the
gravitational wave background and the interstellar medium. However
because of the lack of a suitably instrumented radio telescope,
China did not contribute much to the pulsar observations in the
past few decades, but this has now changed. This paper introduces
pulsar observations and results obtained with a 25-m radio
telescope operated by Urumqi Observatory. This telescope is
located near the geographic center of Asia with longitude
$87\deg$, latitude $+43\deg$ and altitude 2080m. The telescope was
initially constructed as a VLBI station; a back-end system for
pulsar studies in the 18-cm band was developed in mid-1999.  The
pulsar project at Urumqi is a collaboration between Urumqi
Observatory, Peking University, Hong Kong University, the
Australia Telescope National Facility and Jodrell Bank
Observatory.

De-dispersion is provided by a $2\times128\times2.5$~MHz
filterbank/digitizer system. The receiver was a room-temperature,
dual-polarization one, but a new more sensitive cryogenic receiver was
commissioned in July 2002.  However, the results discussed in this
paper were obtained between 2000 January and 2002 June with the
room-temperature receiver. This receiver allowed us to observe pulsars
with a flux density greater than about 4 mJy, so we monitored 74
detectable pulsars to study their timing properties. One reason for us
to do this is that the catalog for these stronger pulsars had not
been updated for many years, some of them even more than 30
years. Based on the timing system we started regular scintillation
observations for five pulsars from January 2001. We report the results
of pulsar timing observations in Sections 2 and 3 and scintillation
observation in Sections 4. In the last section we discuss the future
scope of pulsar programs at Urumqi Observatory.

\vspace*{1mm}
\para{\large\bf{2}~~ Timing Observations}

\vs\vn {\bf{2.1~~Updated Parameters and the Unseen Glitch Model}}

Frequent observations of 74 pulsars resulted in updated periods
and period derivatives, as discussed in detail by Wang et al.
(2001). The accuracy of period measurements is generally better
than 0.1~ns, and typical residuals are a few hundred microseconds.
By comparing the new measurements with the best previous
observation, we obtained the variations in periods and period
derivatives, i.e., $\Delta P$ and $\Delta \dot P$ respectively
over long time intervals, up to 30 years. The changes in periods
are in the range of $10^{-9}$ to $10^{-8}$~s, excluding few
pulsars which have glitched or those with errors in $\Delta P$
larger than 20~ns. We found a correlation between the period and
period-derivative changes which is shown in Figure~1. The period
and period-derivative changes tend to have the same sign and to be
correlated in amplitude. This correlation is improved when more
recent data are included, compared to that shown in Wang et al.
(2001). Random period noise would not produce such correlated
changes in $P$ and $\dot P$, however relaxation from unseen
glitches may do so.

\begin{figure}[h]
\center {\includegraphics [bb=0 168 408
458,scale=0.5]{dp_dp1.eps}}

   \parbox[tb]{8cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize

      Fig.\,1~~ Difference between observed and predicted period
derivative $\Delta \dot P$, plotted against difference in period,
$\Delta P$. The inset is an expanded version of the central
region.}

    \end{figure}

In most cases, the pulse frequency after a glitch can be described
by an exponential decay function:

\begin{equation}
\nu(t)=\nu_0(t)+\Delta\nu_g[1-Q(1- \exp(-t/\tau_d)] +
\Delta\dot\nu_p t\,,
%\label{eq:glmodel}
\end{equation}
where $\nu_0(t)$ is the value of $\nu$ extrapolated from before the glitch,
$\Delta\nu_g$ is the total frequency change at the time of the glitch,
$Q$ is the fractional part of $\Delta\nu_g$ which decays away exponentially,
$\tau_d$ is the decay time constant and $\Delta\dot\nu_p$ is the
permanent change in $\dot\nu$ at the time of the glitch.
The increment in frequency derivative is given by:

\begin{equation}
\Delta\dot\nu(t)=\frac{-Q\Delta\nu_g}{\tau_d}\exp(-t/\tau_d) +
\Delta\dot\nu_p\,.
%\label{eq:glnudot}
\end{equation}

We assume that the present results or the catalog values are
contaminated by an unseen glitch, and use the above glitch functions
to extrapolate back the period and period derivative changes caused by
the glitch relaxation process. In the analysis, the glitch epoch is
arbitrarily taken as 400 days before the present observation, the
relative change is $\Delta \nu/ \nu = 10^{-6}$, and time constant
$\tau_{\rm d}=100$~d. For post-glitch relaxation we considered the
situations of no decay, part decay and total decay of the period jump,
and a permanent increase, decrease or no change in the period
derivative at the time of the glitch. This model agrees well with the
observational results, in which the amplitude and sign in $\Delta P$
and $\Delta \dot P$ are correlated (see Figure~2). The model also
works for the very special Crab pulsar glitches, in which both $\Delta
P$ and $\Delta \dot P_{\rm p}$ are positive after the glitch. This
agreement suggests that long-term period and period derivative changes
are dominated by decay from unseen glitches.

\begin{figure}[h]
\center {\includegraphics [bb=66 81 385 482,scale=0.7]{gltmd.eps}}

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

      Fig.\,2~~ A model for the increments in pulse period and period derivative
due to a glitch occurring 400 days before the current time $t_0$.
A glitch magnitude $\Delta \nu_{\rm g}=10^{-6}$ and a post-glitch
decay time of 100 days are assumed. Changes for three different
decay parameters $Q$ and three different permanent changes in
period derivative $\Delta \dot P_{\rm p}$ are plotted. The groups
of three lines of one type are for different values of $Q$ and the
solid, dashed and dotted lines represent different values of
$\Delta \dot P_{\rm p}$. The post-glitch values are functions of
$Q$ and independent of $\Delta \dot P_{\rm p}$, so they overlap
for different $\Delta \dot P_{\rm p}$.}

    \end{figure}


\vs\vn{\bf{2.2~~Glitches}}

 Frequent observations at Urumqi have
revealed five glitches up to June 2002. Table~1 lists the glitch
pulsars and glitch epochs in the first two columns, and the glitch
sizes are given in the third column. Columns four and five list
the jump in frequency derivative at the glitch, $\Delta \dot \nu$,
and the permanent jump in frequency derivative, $\Delta
\dot\nu_{\rm p}$, respectively. The decaying part in frequency
jump, $\Delta \nu_{\rm d}$, is given in column six, and the decay
time constant $\tau_{\rm d}$ and fractional decay $Q$ are given in
the last two columns.

Figure 3 shows the variations in pulse frequency and frequency
derivative in the Crab pulsar over the MJD interval 51600 to 52420,
covering the two most recent glitches. Both glitches have a permanent
jump in $\Delta \dot\nu_{\rm p}$. Figure~3 doesn't show the detail of
the glitch decay process for the second glitch; this could be due to
the rapid post-glitch decay in this pulsar (Wong, Backer \& Lyne,
2001) combined with the large gap between the observations. PSR
B1737$-$30 is a well-known young pulsar which glitches about every 300
days (Shemar \& Lyne, 1996). In our observation, this pulsar was quite
stable for about 700 days, and then suddenly glitched three times in
about 120 days, with two small glitches followed by a larger
one. Figure~4 shows the phase residuals over these glitches - a slope
change indicates a jump in pulse period.  PSR J1835$-$1106 is a young
pulsar which has no previously reported glitch.

\bc{\small\bf

 Table 1~~   Glitches detected by Urumqi 25m radio telescope
}\\[2mm]

\fns \tabcolsep 1mm
\begin{tabular}{lllccccc}
\hline\\[-3.6mm]\hline
 PSR    & ~Epoch  & $\Delta \nu/\nu$ & $\Delta \dot \nu$ & $\Delta \dot
\nu_{\rm p}$  & $\Delta \nu_{\rm d}$ & $\tau_{\rm d}$ & $Q$   \\
       & ~(MJD)  &                  &($10^{-9}$)        & ($10^{-15}{\rm s}^{-2}$)
&($10^{-6}{\rm s}^{-1}$)  & (d) &  \\
 \hline
B0531+21   & 51740.8  & 23(15)  &  & $-37$(17) &  0.5(4)  &  4  & 0.8(8)\\
           & 52082(2) & 10.5(1) &  & $-62$(2)  &          &     &       \\
B1737$-$30 & 52234(2) & 5.3(9)  &  &           &          &     &       \\
           & 52268(2) & 13.2(9) &  &           &          &     &       \\
           & 52347.4  & 152.8(5)&$-$1.32(14) & &          &     &       \\
1835$-$1106& 52224(2) & 23(5)   &  &           & 0.020(6) & 100 &0.21(6)\\
 \hline\\[-3.6mm]\hline%\\[-4.5mm]\hline
 \end{tabular}
 \ec

\begin{figure}[h]
\center {\includegraphics [bb=76 164 382
459,scale=0.8]{crab_dnu1.eps}}

   \parbox[tb]{10cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize

      Fig.\,3~~ Two of the Crab pulsar glitches. (a) Variations of frequency
residual $\Delta\nu$ relative to the pre-glitch solution, and (b)
the variations of $\dot\nu$. The glitch parameters are given in
Table~1.}

    \end{figure}

\begin{figure}[h]
\center {\includegraphics [bb=0 169 408
458,scale=0.5,]{1740-3015_exp.eps}}

   \parbox[tb]{8cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize

      Fig.\,4~~ The pulse phase residual of PSR B1737$-$30 relative to the
  pre-glitch solution, the slope change indicate a glitch.}

    \end{figure}


\para{\large\bf{3}~~Proper Motion Measurements}

\vs\vs  The accuracy of position measurements is typically a few
milliarcseconds with two and a half years observation. By
comparing them with best previous measurements, we obtained the
position differences and hence the proper motions given in
Table~2. Column five gives transverse velocities based on
distances from the new distance model NE2001 (Cordes \& Lazio
2002) given in column four. Since we have only two and half years
data, these results may be contaminated by long-term timing noise
although the measurements seem significant. How reliable they are
remains to be checked from further observations.

\bc{\small\bf

 Table 2~~   Proper motion and pulsar velocity
}\\[2mm]

\fns \tabcolsep 4mm
\begin{tabular}{cclcl}
\hline\\[-3.6mm]\hline
 \multicolumn{2}{c}{PSR Name}  & Proper Motion  &  Distance  &   ~~~~V$_{\rm pm}$  \\
 (J2000)      &  (B1950)     & ~~~(mas/y)    &  (kpc)  &    ~~~(km/s)        \\
 \hline
0332+5434  &  0329+54   & ~~~~12(4)     &  0.972   &    ~~~~~55(20)        \\
 0358+5413  &  0355+54   & ~~~~16(5)     &  1.447   &    ~~~~110(40)       \\
 0454+5543  &  0450+55   & ~~~~73(8)     &  0.659   &    ~~~~228(25)      \\
 0630$-$2834  &  0628$-$28   & ~~~~58(7)     &  1.444   &    ~~~~400(50)       \\
 0738$-$4042 & 0736$-$40 & ~~~~57(4)     &  0.567  &     ~~~~157(5)    \\
 1136+1551  &  1133+16   & ~~~385(7)    &   0.333   &    ~~~~608(11)     \\
 1239+2453  &  1237+25   & ~~~116(7)    &   0.799   &    ~~~~441(27)   \\
 1935+1616  &  1933+16   & ~~~~20(1)     &  5.556   &    ~~~~531(25)   \\
 1948+3540  &  1946+35   & ~~~~18(2)     &  5.721   &    ~~~~480(60)      \\
 2257+5909  &  2255+58   & ~~~~27(7)     &  4.495   &    ~~~~570(160)    \\
 2313+4253  &  2310+42   & ~~~~33(8)     &  1.243   &    ~~~~200(50)  \\
 \hline\\[-3.6mm]\hline%\\[-4.5mm]\hline
 \end{tabular}
 \ec

\para{\large\bf{4}~~Scintillation Observations}

\vs\vs
%\label{sec:scint}
Regular scintillation observations for
five pulsars started in January 2001 with one observation about
every 10 days on average.  These pulsars are PSRs B0329+54,
B0823+26, B1929+10, B2020+28 and B2021+51. Each observation lasts
between two and six hours, mostly depending on the typical
correlation timescale for diffractive interstellar scintillation
(DISS), which is proportional to $D^{-3/2}$, where $D$ is the
pulsar distance. For example, for PSR B0329+54, each observation
is about three hours, but more nearby pulsars such as PSR B1929+10
are observed for six hours mostly. The 18 months of data have
shown major variations in the scintillation dynamic spectra for
each of these pulsars. The left side of Figure~5 shows such
changes for PSR B0329+54. We see the scintle size changes with
time, which indicates variations in the DISS time scale $\tau_{\rm
d}$ and the decorrelatin frequency scale $\Delta\nu_{\rm d}$.
Systematic drifts of the pattern are observed frequently for PSR
B0329+54, showing the effects of refractive scintillation (RISS).
The right side of Figure~5 is the secondary spectrum -- the
two-dimensional Fourier transform of the scintillation dynamic
spectrum. The extra power at higher frequencies shown in
observations at MJDs 52027.4, 52078.3, 52202.5 are due to the
finer fringes in their dynamic spectra.

These spectra have been corrected for the effects of interference in
the observation band. The interference may be narrow-band and
long-term or broad-band and short-term. The narrow band interference
is most serious at around 1480 MHz, 1550 MHz and 1620 MHz, normally
across few frequency channels. The broad-band interference occured
only occasionally and typically lasts one or two sub-intergration
times, which in our observation is four minutes. To provide a clean
data set for further analysis, we use a linear interpolation across
the bad data.

\begin{figure}[h]
\hspace{0.5cm}\parbox[b]{7cm} {\includegraphics [bb=22 0 401
504,scale=0.4]{0329_spc.eps}}

\vspace{-71mm}
 \ \hfill \
\parbox[b]{6.6cm} {\includegraphics [bb=25 0 400
504,scale=0.4]{0329_fft.eps}}

   %\parbox[tb]{14cm}
  %{\hspace{3.5cm}
   {\center
   \bc\footnotesize
Fig.\,5~~ Scintillation dynamic spectra (left) and secondary
spectra
  (right) for PSR B0329+54.
  \ec}

    \end{figure}

The parameters of diffractive scintillation are obtained from the
two-dimensional auto-correlation function (ACF) of the dynamic
spectrum (Cordes, 1986). We applied a two-dimensional Gaussian fit to
ACF, from which the scintillation time scale $\tau_{\rm d}$,
decorrelation frequency scale $\Delta \nu_{\rm d}$ and the tilt of the
ellipse hence the slope of the dynamic spectra are derived
(cf. Stinebring, Faison \& McKinnon, 1996). $\tau_{\rm d}$ is defined
as the half-width at 1/e of the maximum of the Gaussian in time, and
$\Delta \nu_{\rm d}$ to be the half-width at half-maximum of the
Gaussian in frequency.  The top two panels in Figure~6 show the
variation of $\tau_{\rm d}$ and $\Delta \nu_{\rm d}$. The third panel
is a derived parameter, the pulsar scintillation velocity $V$, which
is calculated based on a density fluctuation spectral slope of 4.0 in
the scattering screen (Gupta, Rickett \& Lyne, 1994).  Panel four is
the slope of the dynamic spectrum which indicates the drifting rate of
the pattern. Panel five is the mean flux density of each
observation. We checked the consistency of DISS parameters with the
dynamic spectra, and they are in good agreement. The variation of
adjacent points are also correlated, so the temporal changes of these
parameters in Figure~6 are realistic. The observed variations of
scintillation parameters indicates that the feature of the scattering
disk change significantly with time. The fluctuation of the flux density
also shows that due to the scintillation modulation, care has to be taken with the
measurement of this quantity. Many observations
over a long time span are necessary to obtain a reliable result,
especially for closer pulsars at higher frequencies.

\begin{figure}[h]
\center {\includegraphics [bb=0 157 408
461,scale=0.8]{scint_t.eps}}

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

      Fig.\,6~~ Time variation of scintillation parameters and derived
  parameters for PSR B0329+54.}

    \end{figure}

\para{\large\bf{5}~~Profiles and Flux Density Variation}

\vs\vs The grand mean pulse profiles of each pulsar are formed by
adding data corresponding to good timing points over the data
span. Calibration was done by choosing several distant pulsars
which have good flux density measurements in the catalogue. Based
on the digitiser counts, the timing analysis program TREDUCE
(developed at Swinburne University of Technology and ATNF) can
give a number which is proportional to the mean flux density. A
scale factor is then computed to convert the TREDUCE number into a
flux density. Some of the system effects such as systematic
changes of the receiver sensitivity are removed before this. We
consider the modulation index $m$ or r.m.s. variation/mean here.
Figure~7 shows a plot of modulation index versus the logarithm of
dispersion measure (DM); the dashed line is proportional to
DM$^{-1/3}$. The pulsars generally form two groups. The first
consists of the nearby pulsars where the DISS is the main effect
and the modulation is weak with $m<1$. The second group is the
larger DM pulsars, where DISS effect is weaker and RISS is
stronger. The observed relationship of $m\propto {\rm DM}^{-1/3}$
is expected from RISS theory, so the observed flux density
fluctuations are dominated by propagation effects rather than
intrinsic changes in the pulse emission.

\begin{figure}[h]
\center {\includegraphics [bb=0 169 408
457,scale=0.7]{flux_dm.eps}}

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

      Fig.\,7~~ Relative flux density fluctuation  versus  dispersion measure. The
  dashed line is proportional to DM$^{-1/3}$.}

    \end{figure}

\para{\large\bf{6}~~Prospect of Future Work}

\vs\vs
%\label{sec:future}
A new cryogenic receiver has been
designed and built at ATNF for Urumqi Observatory. This
state-of-the-art receiver was installed in July 2002 and it
greatly improves the system performance. It gives the system
temperature of about 22~K, allowing us to detect pulsars as weak
as 1~mJy, which means a great increase in the number of detectable
pulsars. With the larger sample of pulsars, our main interests
will remain in pulsar timing, scintillation and studies of the
pulse emission mechanism.

To broaden the pulsar study area and complement the present 18-cm
observations, a digital de-dispersing system at a lower frequency
such as 660\,MHz is planned. This system will be based on the
Jodrell Bank Observatory COBRA (Coherent Online Baseband Receiver
for Astronomy) system. The baseband system with coherent
dedispersion can give high time and frequency resolution,
enhancing the profile resolution and timing precision at these
lower frequencies. It also provides polarisation information
allowing studies of both individual pulses and mean pulse
profiles.  We plan to construct a new more sensitive receiver at
660\,MHz for this system.

We are also planning to develop a pulsar searching system at the
telescope. The availability of telescope time and the wide beam allows
us to search wide areas. Investigation of the associations of
unidentified gamma-ray sources with pulsars was the initial motivation
for this project.

\vs\vn{\bf ACKNOWLEDGEMENTS}~~This project is supported by The
National Natural Science Foundation of China (NNSFC) under the
projects of 10173020, 10210201147, 10210101093. NW thanks them for
their support. We also thank Michael Kramer for help with the
two-dimensional gaussian fitting program.

\bc {\bf References} \ec

\vspace{-4mm} \no \fns

 \no Cordes J.M., ApJ, 1986, 311, 183\\
 \no Cordes J.M.,
Lazio,  astro-ph/0207156 2002\\
\no Gupta Y., Rickett B.J., Lyne A.G., MNRAS, 1994, 269, 1035\\
\no Shemar S.L., Lyne A.G., MNRAS, 1996, 282, 677\\
 \no
Stinebring D.R., Faison M.D., McKinnon M.,  ApJ., 1996, 460, 460\\
\no  Wang N., Manchester R.N., Zhang J., et al.,  MNRAS, 2001,
328, 855\\
 \no Wong T., Backer D.C., Lyne
A.G.,  ApJ, 2001, 548, 447
%\end{CJK}
\end{document}
