No abstract

It is conjectured that cold quark matter with very high baryon density could be in a solid state, and strange stars with low temperatures should thus be solid stars. The speculation could be close to the truth if no peculiar polarization of thermal X-ray emission (as in, e.g., RX J1856) or no gravitational wave in postglitch phases are detected in future advanced facilities or if spin frequencies beyond the critical ones limited by r-mode instability are discovered. The shear modulus of solid quark matter could be ∼10 32 ergs cm Ϫ3 if the kilohertz quasi-periodic oscillations observed are relevant to the eigenvalues of the center star oscillations.

Timing observations from the Parkes 64-m radio telescope for 165 pulsars between 1990 and 2011 have been searched for period glitches. Data spans for each pulsar ranged between 5.3 years and 20.8 years. From the total of 1911 years of pulsar rotational history, 107 glitches were identified in 36 pulsars. Out of these glitches, 61 have previously been reported whereas 46 are new discoveries. Glitch parameters, both for the previously known and the new glitch detections, were measured by fitting the timing residual data. Observed relative glitch sizes ∆ν g /ν range between 10 −10 and 10 −5 , where ν = 1/P is the pulse frequency. We confirm that the distribution of ∆ν g /ν is bimodal with peaks at approximately 10 −9 and 10 −6 . Glitches are mostly observed in pulsars with characteristic ages between 10 3 and 10 5 years, with large glitches mostly occurring in the younger pulsars. Exponential post-glitch recoveries were observed for 27 large glitches in 18 pulsars. The fraction Q of the glitch that recovers exponentially also has a bimodal distribution. Large glitches generally have low Q, typically just a few per cent, but large Q values are observed in both large and small glitches. Observed time constants for exponential recoveries ranged between 10 and 300 days with some tendency for longer timescales in older pulsars. Shorter timescale recoveries may exist but were not revealed by our data which typically have observation intervals of 2 -4 weeks. For most of the 36 pulsars with observed glitches, there is a persistent linear increase inν (i.e., decrease in the slow-down rate |ν|) in the inter-glitch interval. Where an exponential recovery is also observed, the effects of this are superimposed on the linear increase inν. In some but not all cases, the slope of the linear recovery changes at the time of a glitch. Theν values characterising the linear changes inν are almost always positive and, after subtracting the magnetospheric component of the braking, are approximately proportional to the ratio of |ν| and the inter-glitch interval, as predicted by vortex-creep models.

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 λ 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.