The Parkes Pulsar Timing Array

Detection and study of gravitational waves from astrophysical sources is a major goal of current astrophysics. Ground-based laser-interferometer systems such as LIGO and VIRGO are sensitive to gravitational waves with frequencies of order 100 Hz, whereas space-based systems such as LISA are sensitive in the millihertz regime. Precise timing observations of a sample of millisecond pulsars widely distributed on the sky have the potential to detect gravitational waves at nanohertz frequencies. Potential sources of such waves include binary super-massive black holes in the cores of galaxies, relic radiation from the inflationary era and oscillations of cosmic strings. The Parkes Pulsar Timing Array (PPTA) is an implementation of such a system in which 20 millisecond pulsars have been observed using the Parkes radio telescope at three frequencies at intervals of two -- three weeks for more than two years. Analysis of these data has been used to limit the gravitational wave background in our Galaxy and to constrain some models for its generation. The data have also been used to investigate fluctuations in the interstellar and Solar-wind electron density and have the potential to investigate the stability of terrestrial time standards and the accuracy of solar-system ephemerides.Comment: 9 pages, 6 figures, Proceedings of "40 Years of Pulsars: Millisecond Pulsars, Magnetars and More", Montreal, August 2007. Corrected SKA detection limi

Radio Emission Signatures in the Crab Pulsar

Our high time resolution observations of individual pulses from the Crab pulsar show that both the time and frequency signatures of the interpulse are distinctly different from those of the main pulse. Main pulses can occasionally be resolved into short-lived, relatively narrow-band nanoshots. We believe these nanoshots are produced by soliton collapse in strong plasma turbulence. Interpulses at centimeter wavelengths are very different. Their dynamic spectrum contains regular, microsecond-long emission bands. We have detected these bands, proportionately spaced in frequency, from 4.5 to 10.5 GHz. The bands cannot easily be explained by any current theory of pulsar radio emission; we speculate on possible new models.Comment: 26 pages, 10 figures, to appear in Ap

Observations of Six Glitches in PSR B1737-30

Six glitches have been recently observed in the rotational frequency of the young pulsar PSR B1737-30 (J1740-3015) using the 25-m Nanshan telescope of Urumqi Observatory. With a total of 20 glitches in 20 years, it is one of the most frequently glitching pulsars of the about 1750 known pulsars. Glitch amplitudes are very variable with fractional increases in rotation rate ranging from 10^{-9} to 10^{-6}. Inter-glitch intervals are also very variable, but no relationship is observed between interval and the size of the preceding glitch. There is a persistent increase in |\dot\nu|, opposite in sign to that expected from slowdown with a positive braking index, which may result from changes in the effective magnetic dipole moment of the star during the glitch.Comment: 7 pages, 10 figure

Timing measurements and proper motions of 74 pulsars using the Nanshan radio telescope

We have measured the positions of 74 pulsars from regular timing observations using the Nanshan radio telescope at Urumqi Observatory between 2000 January and 2004 August (MJD 51500 -- 53240). Proper motions were determined for these pulsars by comparing their current positions with positions given in pulsar catalogues. We compare our results to earlier measurements in the literature and show that, in general, the values agree. New or improved proper motions are obtained for 16 pulsars. The effect of period fluctuations and other timing noise on the determination of pulsar positions is investigated. For our sample, the mean and rms transverse velocities are 443 and 224 km/s respectively, agreeing with previous work even though we determine distances using the new NE2001 electron density model.Comment: 9 pages, 7 figures and 3 tables. Accepted by MNRA

On the peculiarities in the rotational frequency evolution of isolated neutron stars

The measurements of pulsar frequency second derivatives have shown that they are $10^2-10^6$ times larger than expected for standard pulsar spin-down law, and are even negative for about half of pulsars. We explain these paradoxical results on the basis of the statistical analysis of the rotational parameters $\nu$, $\dot \nu$ and $\ddot \nu$ of the subset of 295 pulsars taken mostly from the ATNF database. We have found a strong correlation between $\ddot \nu$ and $\dot \nu$ for both $\ddot\nu > 0$ and $\ddot\nu < 0$, as well as between $\nu$ and $\dot\nu$. We interpret these dependencies as evolutionary ones due to $\dot\nu$ being nearly proportional to the pulsars' age. The derived statistical relations as well as "anomalous" values of $\ddot\nu$ are well described by assuming the long-time variations of the spin-down rate. The pulsar frequency evolution, therefore, consists of secular change of $\nu_{ev}(t)$, $\dot\nu_{ev}(t)$ and $\ddot\nu_{ev}(t)$ according to the power law with $n \approx 5$, the irregularities, observed within a timespan as a timing noise, and the variations on the timescale larger than that timespan -- several tens of years.Comment: 4 pages, 3 figures. Accepted for publication in ApSS, in the proceedings of the conference "Isolated Neutron Stars: from the Interior to the Surface", London, April 2006; eds. S. Zane, R. Turolla and D. Pag

Constraining the coalescence rate of supermassive black-hole binaries using pulsar timing

Pulsar timing observations are used to place constraints on the rate of coalescence of supermassive black-hole (SMBH) binaries as a function of mass and redshift. In contrast to the indirect constraints obtained from other techniques, pulsar timing observations provide a direct constraint on the black-hole merger rate. This is possible since pulsar timing is sensitive to the gravitational waves (GWs) emitted by these sources in the final stages of their evolution. We find that upper bounds calculated from the recently published Parkes Pulsar Timing Array data are just above theoretical predictions for redshifts below 10. In the future, with improved timing precision and longer data spans, we show that a non-detection of GWs will rule out some of the available parameter space in a particular class of SMBH binary merger models. We also show that if we can time a set of pulsars to 10ns timing accuracy, for example, using the proposed Square Kilometre Array, it should be possible to detect one or more individual SMBH binary systems

Unusual glitch behaviours of two young pulsars

In this paper we report unusual glitches in two young pulsars, PSR J1825-0935 (B1822-09) and PSR J1835-1106. For PSR J1825-0935, a slow glitch characterised by a temporary decrease in the slowdown rate occurred between 2000 December 31 to 2001 December 6. This event resulted in a permanent increase in frequency with fractional size $\Delta\nu/\nu\sim31.2(2)\times10^{-9}$, however little effect remained in slowdown rate. The glitch in PSR J1835-1106 occurred abruptly in November 2001 (MJD 52220\pm3) with $\Delta\nu/\nu\sim14.6(4)\times10^{-9}$ and little or no change in the slow-down rate. A significant change in $\ddot\nu$ apparently occurred at the glitch with $\ddot\nu$ having opposite sign for the pre- and post-glitch data.Comment: Latex format, six files, 5 pages with 4 figues. accepted for MNRA