

\section{Introduction}

As part of a program to study the geometrical constraints on the
emission mechanism of pulsars, we have recorded a set of total
intensity average profiles at a number of frequencies from 25 to
4800\,MHz at the Arecibo Observatory. The data were time-tagged 
so that through use of a suitable timing model for the pulsar, we 
were able to shift the arrival times of the pulses, modulo one stellar 
rotation period, to the solar system barycenter. Then by assuming 
a dispersion measure {\it DM}, we shifted all the profiles to infinite 
frequency arrival time. In many cases the dispersion measure was 
the only free parameter for alignment.  The techniques used were 
similar to those of Hankins \& Rickett (1986)\nocite{hr86}. In addition 
we have observed a large overlapping group of pulsars at selected 
frequencies with full Stokes' parameters. We present these two sets 
of observations in a coordinated fashion, so that they can be used 
for studies of the pulsar magnetospheric structure.


\section{Observations}
The two main programs of time-aligned total-power (P868) and 
polarimetric (P1260) observations presented here were made at 
the Arecibo Observatory during a number of sessions between 
1988 and 1992.  A few observations from previous polarimetric 
surveys (P110 at 430 MHz and P467 at 21 cm), however, are also 
presented in order to extend the completeness of our coverage. 

\subsection{Multiple frequency total intensity profiles}

For the total intensity observation program, circularly polarized feeds 
were used at 430, 1400, 2380, and 4800\,MHz and crossed-dipole 
linearly polarized feeds (with circular hybrids) were used at 130/111, 
49, and 26\,MHz. All profiles were obtained using the then existing 
Arecibo 40-MHz Correlator (Hagen 1987); the autocorrelation 
functions (ACFs) were accumulated into typically 1024 
pulsar-synchronous phase bins spanning one pulsar period, and 
then dumped to tape every two minutes.
 
These ACFs were then Fourier transformed off-line to obtain power
spectra, and the appropriate dispersion delays between adjacent
spectral channels were removed. The profiles from all spectral 
channels were then co-added to produce a dedispersed average 
profile. The two-minute averages were edited to remove interference 
and other corruption, then synchronously added together to form (in 
many cases) a high-signal-to-noise ratio (hereafter S/N) average 
profile. The time-tag of the first sample was used to relate the times 
of arrival to the solar system barycenter in terms of the pulsar phase 
at infinite frequency.  Thus if the dispersion delays strictly obey the 
cold plasma dispersion law, then the pulses align in pulse phase 
exactly as they would if the observer were in the immediate pulsar 
neighborhood.  The dispersion measure {\it DM} is, however, a free 
parameter in the phase alignment, and we found that adjustments 
from the tabulated values (Taylor, Manchester \& Lyne 1993, 
Manchester 2002) \nocite{tml93,man2002} were required for optimal 
alignment for many of the pulsars. The dispersion measures and 
timing models used for alignment are listed in Table \ref{table1}.

To test and monitor the timing stability of the system during each
observing session, we recorded a reference profile from a strong 
pulsar using the same sampling parameters each time.  From these 
profiles systematic timing offsets could be determined and applied 
to data from other pulsars. Since it was not possible to observe 
throughout the whole range of frequencies in a single observing 
session, we recorded reference profiles for each pulsar, usually at 
430 MHz, to confirm its tabulated timing model.  In a number of cases 
we had to make minor adjustments in the timing model to achieve 
alignments among profiles recorded at the same frequency. After 
any timing issues were resolved, the dispersion measure was 
adjusted to obtain pulse-phase alignment among profiles at 
different frequencies. Since the change in pulse phase $\phi$ as 
a function of radio frequency $\nu$, $d\phi/d\nu \propto \nu^{-3}$, 
we used the lowest frequency pairs with good S/N to determine 
any adjustment to {\it DM}. 

The fiducial point of a pulse profile to be used for inter-frequency
alignment depends upon the pulse shape. Usually aligning core
components works well for pulsars with odd numbers of components. 
We aligned either the peak of the core component or the (symmetrical)
half-power points for these pulsars. For double profiles, where the
separation between the main components is clearly frequency 
dependent, we used the centroid between the two components.

\subsection{Polarimetry}

Most of the primary polarimetry observations were carried out during 
a set of sessions in October 1992; a few, however, come from earlier 
observations in March 1992 and January 1990.  All used the then 
existing Arecibo 40-MHz Correlator, such that the average polarimetry
was carried out in a continuous mode, whereas the pulse-sequence
observations entailed use of special programs which gated the
correlator synchronously with the pulsar.  In both cases the basic data
recorded at the telescope were the auto- and cross-correlation functions
of the left- and right-hand channel voltages.  The 21-cm observations 
used a 20-MHz bandwidth and the lower a maximum of 10-MHz at 430 
MHz, 2.5 MHz at 111/130 MHz, and 625 kHz at 49 MHz. A minimum of 
32 correlation lags were retained in order to reduce the dispersion 
delay across the bandpass to usually negligible levels.  The resolution 
was then often essentially the correlator dump time for the 
pulse-sequence observations and the averaging-bin size for the 
profile-polarization measurements.  These parameters are tabulated in 
Table \ref{table_polarization}.  The Arecibo 40-MHz Correlator is 
described by Hagen (1987)\nocite{hag1987} and the continuous and 
gated observing software by Perillat (1988, 1992)\nocite{per1988,per1992}.  
The measured correlation functions were scaled, 3-level-sampling 
corrected, and Fourier transformed to produce raw Stokes parameters, 
which were in turn corrected (channel by channel) for dispersion, 
Faraday rotation, instrumental delays, and all of the known feed 
imperfections as determined by full-sky tracks of pulsar B1929+10 and 
other sources.  These methods are more completely described in the 
Appendix.  During the course of our analyses, we discovered that the 
instrumental polarization is highly frequency dependent, particularly at 
430 MHz; therefore, the most recent observations represent some of 
the best calibrated ever made at the Arecibo Observatory.  For some 
few, however, not all of the calibration information was available for 
one reason or another.  We found, for instance, that interference rather 
easily corrupted our continuum-source observations, which are needed 
to determine the relative left- and right-hand channel gains, so as to 
calibrate the circular polarization $V$.  In any case, Table \ref{table_polarization}
indicates this gain calibration with a ``c'' when a continuum source 
was used and and ``n'' when computed from the off-pulse noise 
level.  The correction of the Stokes parameters for cross-coupling in 
the feed is denoted by a ``p''.
  
Profile and single-pulse polarimetry were also carried out during these
three sessions at 111.5/130 MHz and near 50 MHz.  The techniques were
virtually identical to those described above except that additional
corrections were required for the changing ionospheric Faraday rotation
and many fewer of our efforts to obtain reliable feed cross-coupling data 
were successful.
  
The older 430-MHz observations were carried out in the early 1970s with
a single-channel polarimeter using various bandwidths and integration
times as indicated by the particular star being observed; these are
tabulated in Table \ref{table_polarization}.  The polarimetry scheme is
described in Rankin {\it et al.}\thinspace\ (1975)\nocite{rcs1975}; and while the nominal 
0.25\% voltage amplitude of the feed crosscoupling---which can produce 
spurious circular polarization at typical levels of about 10\% of the linear 
polarization---was known from radar observations, correction of the Stokes 
parameters was then impossible because the cross-coupling phase was 
unknown.
  
The older 1400-MHz observations were carried out in October 1981, 
again with a single-channel, adding polarimeter.  Here, the bandwidths 
ranged up to 40 MHz, with these and the integration times chosen to 
provide a resolution of about a milliperiod. A serious effort was made for 
the first time to correct the measured Stokes parameters for instrumental
cross-coupling distortion using the ``orthogonal'' approximation described 
by Stinebring {\it et al.}\thinspace\ (1984).

In all cases we make no effort to give absolute polarization position angles.  
Therefore, the position angles in our polarization displays are arbitrary 
within a constant value.  
  
\section{Results}

Our results are summarized in the tables and figures. Table \ref{table1} 
gives the period, period derivative, timing epoch and dispersion-measure 
value used for the multi-frequency alignments, and Tables~\ref{table_polarization} 
\& \ref{table_polarization_a} give the observations date, configuration, 
resolution and rotation-measure value used in the polarimetric analyses. 
Comments on Tables \ref{table1} \& \ref{table_polarization} are given in 
Table~\ref{table2}.  

Rotation-measure values were generally taken from Manchester /etal/ (2002); 
however, those given in boldface with errors in Tables~\ref{table_polarization} 
\& \ref{table_polarization_a} were determined in the course of our polarimetric 
analyses by fitting the polarization position-angle swing across the available 
bandwidth.  The errors in the rotation-measure values were computed from 
the position-angle errors which in turn were computed from the off-pulse noise 
level and represent two standard deviations.  

\input{table1}
\input{polar_tableV2.5}
\input{polar_tableV2.5a}
\input{table2}

The combined multi-frequency and polarimetry results are presented in 
approximate Right Ascension order in Figures \ref{b1} through \ref{b17} 
in order to facilitate intercomparison.  The multi-frequency profiles are 
normally positioned in the lefthhand columns and the 430- and 1400-MHz 
polarization displays in the central (C) and righthand (R) columns, where 
they are available---and in the half-dozen cases where both 111/130- and  
50-MHz polarimetry is also available, these are given in the center and 
righthand columns of the following row. Details of the multi-frequency 
alignments and polarimetry are given in Tables~\ref{table1} \& 
\ref{table_polarization}.  In several cases where no polarimetry was 
available, the multi-frequency results are interpolated into the sequence 
(Fig.~\ref{b8} (top row), Fig.~\ref{b10} center row, and Fig.~\ref{b11} bottom left).  
Finally, Figures \ref{p1}-\ref{p2} give polarimetry results for those stars 
where only single-frequency observations were carried out, and these 
in turn are described in Table~\ref{table_polarization_a}.  The time 
resolution of the multifrequency profiles is indicated by a set of three 
horizontal bars plotted on the left side of the profiles. The upper bar denotes 
the phase shift which would result from a change of dispersion measure of 
0.01, 0.1, or 1.0 pc cm$^{-3}$.  The middle bar, labeled ``BW'' shows the 
time resolution limited by the dispersion sweep 
time, $\tau_{\it DM}=\mbox{{\it DM}}\,\Delta\nu/(1.205\times 10^{-16}\nu^3)$, 
across the receiver bandwidth, $\Delta\nu$, tuned to frequency $\nu$. 
The bottom bar shows the integration time constant used for the plot. On 
the left side of the multifrequency profile plots an error bar is plotted to 
show the range of 2 standard deviations of the off-pulse noise level.

For the polarization plots the total intensity, linear and circular
polarizations are depicted by solid, dashed and dotted lines,
respectively. The solid vertical bars indicate the range of 3 standard
deviations of the off-pulse noise. Where present, the time resolution,
including the effects of dispersion across the receiver band and the
post integration time constant are shown by a horizontal bar. In the
lower part of each polarization plot the average linear polarization
position angle is shown wherever the linear polarization exceeds two
times the off-pulse noise level. 
Error bars for the position angle are shown for $\pm2\sigma$
uncertainties due to the estimation error of the linear polarization.
For many of the pulsars individual pulse records were available. For
these pulsars the position angle of each sample which exceeded 2 times
the off-pulse noise is plotted as a dot. This type of plot allows study
of polarization moding behavior (Manchester, Taylor \& Huguenin 1975, 
Backer \& Rankin 1980\nocite{mth1975,br1980}). 


\acknowledgements
We wish to acknowledge Dan Stinebring's assistance in making many of
the profile observations as well as making available some of our 1981
polarimetry and that of Amy Carlow, Vera Izvekova, N.\ Rathnasree,
Svetlana Suleymanova, and Kyriaki Xilouris in carrying out some of the
1990/1992 observations.  It is a pleasure to thank Phil Perillat without 
whose remarkable software for the Arecibo 40-MHz Correlator this work 
would have been impossible. We thank Mark McKinnon of NRAO for 
determining timing models for many of these pulsars, Dipanjan Mitra
for alignment of some of the profiles, the Pushchino pulsar group of
the Lebedev Physical Institute for 61 and 102-MHz profiles, and
J.\ A.\ Phillips for the 25-MHz profiles. The Arecibo Observatory is part
of the National Astronomy and Ionosphere Center, which is operated by
Cornell University for the National Science Foundation. This material
is based upon work supported by the National Science Foundation under
Grant  AST-8917722, AST-9618408, AST-9986754 and AST-0139641.



