Stephen M. Ord

Self-consistency of Relativistic Observables with General Relativity in the White Dwarf-Neutron Star Binary PSR J1141-6545

Here we report timing measurements of the relativistic binary PSR J1141-6545 that constrain the component masses and demonstrate that the orbital period derivative b = (-4 ± 1) × 10-13 is consistent with gravitational wave emission as described by the general theory of relativity. The mass of the neutron star and its companion are 1.30 ± 0.02 and 0.986 ± 0.02 M☉, respectively, suggesting a white dwarf companion and extending the range of systems for which general relativity provides a correct description. On evolutionary grounds, the progenitor mass of PSR J1141-6545 should be near the minimum for neutron star production. Its mass is 2 standard deviations below the mean of the other neutron stars, suggesting a relationship between progenitor and remnant masses.

A Search for Giant Pulses from Millisecond Pulsars

We have searched for microsecond-timescale broadband emission from a sample of 18 millisecond pulsars. Our study places strong limits on such emission from several millisecond pulsars and shows that it is only present in a small subset of millisecond pulsars. Giant pulses of up to 64 times the mean pulse energy were detected from PSR J1823-3021A in the globular cluster NGC 6624. In contrast to the giant pulses of PSR B1937+21, nearly all of the giant pulses from PSR J1823-3021A were distributed within the trailing half of the main-pulse component of the integrated pulse profile. The fact that no giant pulses were observed on the leading side of the main-pulse component suggests that giant pulses are preferentially emitted closer to the last open field line than ordinary emission. The correlation between giant pulse emissivity and spin-down luminosity in millisecond pulsars suggests that the high period derivative of PSR J1823-3021A is intrinsic and is not just an artifact of its acceleration in the gravitational potential of NGC 6624.

PSR J1909–3744: A Binary Millisecond Pulsar with a Very Small Duty Cycle

We report the discovery of PSR J1909-3744, a 2.95 ms pulsar in a nearly circular 1.53 day orbit. Its narrow pulse width of 43 μs allows pulse arrival times to be determined with great accuracy. We have spectroscopically identified the companion as a moderately hot (T ≈ 8500 K) white dwarf with strong absorption lines. Radial velocity measurements of the companion will yield the mass ratio of the system. Our timing data suggest the presence of Shapiro delay; we expect that further timing observations, combined with the mass ratio, will allow the first accurate determination of a millisecond pulsar mass. We have measured the timing parallax and proper motion for this pulsar, which indicate a transverse velocity of 140 km s-1. This pulsars stunningly narrow pulse profile makes it an excellent candidate for precision timing experiments that attempt to detect low-frequency gravitational waves from coalescing supermassive black hole binaries.

Geodetic precession in PSR J1141-6545

We present observations that show dramatic evolution of the mean pulse profile of the relativistic binary pulsar J1141-6545 over a period of 5 yr. This is consistent with the precession of the pulsar spin axis due to relativistic spin-orbit coupling. Observations made between 1999 and 2004 with a number of instruments at the Parkes radio telescope demonstrate a steady, secular evolution of the mean total intensity profile, which increases in width by more than 50% during the 5 yr period. Analysis of the changing position angle of the linearly polarized component of the mean profile suggests that our line of sight is shifting closer to the core of the emission cone. We find that the slope of the position angle swing across the center of the pulse steepens with time and use a simplified version of the rotating vector model to constrain the magnitude and direction of the change in our line-of-sight angle relative to the pulsar magnetic axis. The fact that we appear to be moving deeper into the emission cone is consistent with the nondetection of this pulsar in previous surveys.

PSR J1022+1001: profile stability and precision timing

We present an investigation of the morphology and arrival times of integrated radio pulses from the binary millisecond pulsar PSR J1022+1001. This pulsar is renowned for its poor timing properties, which have been postulated to originate from variability in its average pulse profile. Although a subclass of long-period pulsars is known to exhibit mode changes that give rise to very large deviations in their integrated profiles, this was the first millisecond pulsar thought to have an unstable mean profile. As part of a precision timing programme at the Parkes radio telescope, we observed this pulsar between 2003 January and 2004 March using a coherent de-dispersion system (the Second Caltech Parkes Swinburne Recorder). A study of morphological variability during our brightest observations suggests that the pulse profile varies by at most a few per cent, similar to the uncertainty in our calibration. Unlike previous authors, we find that this pulsar times extremely well. In 5-min integrations of 64-MHz bands, we obtain a weighted rms residual of just 2.27 μs. The reduced χ2 of our best fit is 1.43, which suggests that this pulsar can be timed to high accuracy with standard cross-correlation techniques. Combining relativistic constraints with the pulsar mass function and consideration of the Chandrasekhar mass limit on the white dwarf companion, we can constrain the inclination angle of the system to lie within the range 37° < i < 56°. For reasonable pulsar masses, this suggests that the white dwarf is at least 0.9 M⊙. We also find evidence for secular evolution of the projected semimajor axis.

The scintillation velocity of the relativistic binary pulsar PSR J1141-6545

We report a dramatic orbital modulation in the scintillation timescale of the relativistic binary pulsar J1141-6545 that both confirms the validity of the scintillation speed methodology and enables us to derive important physical parameters. We have determined the space velocity, the orbital inclination, and even the longitude of the periastron of the binary system, which we find to be in good agreement with that obtained from pulse-timing measurements. Our data permit two equally significant physical interpretations of the system. The system is either an edge-on binary with a high space velocity (~115 km s-1) or a more face-on binary with a much slower velocity (~45 km s-1). We favor the former, as it is more consistent with pulse timing and the distribution of known neutron star masses. Under this assumption, the runaway velocity of 115 km s-1 is much greater than is expected if pulsars do not receive a natal kick at birth. The derived inclination of the binary system is 76° ± 25, implying a companion mass of 1.01 ± 0.02 M☉ and a pulsar mass of 1.29 ± 0.02 M☉. Our derived physical parameters indicate that this pulsar should prove to be an excellent laboratory for tests of gravitational wave emission.

A neutral hydrogen distance limit to the relativistic binary PSR J1141-6545

We have obtained an HI absorption spectrum of the relativistic binary PSR J1141-6545 and used it to constrain the distance to the system. The spectrum suggests that the pulsar is at, or beyond, the tangent point, estimated to be at 3.7 kpc. PSR J1141-6545 offers the promise of stringent tests of general relativity (GR) by comparing its observed orbital period derivative with that derived from other relativistic observables. At the distance of PSR J1141-6545 it should be possible to verify GR to an accuracy of just a few per cent, as contributions to the observed orbital period derivative from kinematic terms will be a small fraction of that induced by the emission of gravitational radiation. PSR J1141-6545 will thus make an exceptional gravitational laboratory.

A Census of Southern Pulsars at 185 MHz

The Murchison Widefield Array (MWA), and its recently-developed Voltage Capture System (VCS), facilitates extending the low-frequency range of pulsar observations at high-time and -frequency resolution in the Southern Hemisphere, providing further information about pulsars and the ISM. We present the results of an initial time-resolved census of known pulsars using the MWA. To significantly reduce the processing load, we incoherently sum the detected powers from the 128 MWA tiles, which yields ~10% of the attainable sensitivity of the coherent sum. This preserves the large field-of-view (~450 deg2 at 185 MHz), allowing multiple pulsars to be observed simultaneously. We developed a WIde-field Pulsar Pipeline (WIPP) that processes the data from each observation and automatically folds every known pulsar located within the beam. We have detected 50 pulsars to date, 6 of which are millisecond pulsars. This is consistent with our expectation, given the telescope sensitivity and the sky coverage of the processed data (~17,000 deg2). For ten pulsars, we present the lowest-frequency detections published. For a subset of the pulsars, we present multi-frequency pulse profiles by combining our data with published profiles from other telescopes. Since the MWA is a low-frequency precursor to the Square Kilometre Array (SKA), we use our census results to forecast that a survey using Phase 1 of SKA-Low (SKA1-Low) can potentially detect around 9400 pulsars.

Drifting sub‐pulses in two newly discovered pulsars

We have detected the rare phenomenon of stable, drifting sub-pulse behaviour in two pulsars discovered in the recent Swinburne intermediate latitude pulsar survey. The pulsars, PSR and PSR J1919+0134, have approximate periods (P) of 1.873 and 1.6039xa0s respectively. n n n nBoth pulsars have multicomponent profiles, and distinct drifting is observed across them. We have identified a single drift mode in both pulsars: the drift rate for PSR being 5.4(1) ms P−1 and 5.8(2) ms P−1 for PSR 1919+0134. The drifting is linear across the profile with no departure from linearity at the edges within the sensitivity of our observations.