A Study of Multi-frequency Polarization Pulse Profiles of Millisecond
  Pulsars
  We present high signal-to-noise ratio, multi-frequency polarization pulse
profiles for 24 millisecond pulsars that are being observed as part of the
Parkes Pulsar Timing Array (PPTA) project. The pulsars are observed in three
bands, centred close to 730, 1400 and 3100 MHz, using a dual-band 10 cm/50 cm
receiver and the central beam of the 20 cm multibeam receiver. Observations
spanning approximately six years have been carefully calibrated and summed to
produce high S/N profiles. This allows us to study the individual profile
components and in particular how they evolve with frequency. We also identify
previously undetected profile features. For many pulsars we show that pulsed
emission extends across almost the entire pulse profile. The pulse component
widths and component separations follow a complex evolution with frequency; in
some cases these parameters increase and in other cases they decrease with
increasing frequency. The evolution with frequency of the polarization
properties of the profile is also non-trivial. We provide evidence that the
pre- and post-cursors generally have higher fractional linear polarization than
the main pulse. We have obtained the spectral index and rotation measure for
each pulsar by fitting across all three observing bands. For the majority of
pulsars, the spectra follow a single power-law and the position angles follow a
$\lambda^2$ relation, as expected. However, clear deviations are seen for some
pulsars. We also present phase-resolved measurements of the spectral index,
fractional linear polarization and rotation measure. All these properties are
shown to vary systematically over the pulse profile.
