Aidan W. Hotan

PSRCHIVE and PSRFITS: An Open Approach to Radio Pulsar Data Storage and Analysis

A new set of software applications and libraries for use in the archival and analysis of pulsar astronomical data is introduced. Known collectively as the psrchive scheme, the code was developed in parallel with a new data storage format called psrfits, which is based on the Flexible Image Transport System (FITS). Both of these projects utilise a modular, object-oriented design philosophy. psrchive is an open source development environment that incorporates an extensive range of c++ object classes and pre-built command line and graphical utilities. These deal transparently and simultaneously with multiple data storage formats, thereby enhancing data portability and facilitating the adoption of the psrfits file format. Here, data are stored in a series of modular header-data units that provide flexibility and scope for future expansion. As it is based on FITS, various standard libraries and applications may be used for data input, output, and visualisation. Both psrchive and psrfits are made publicly available to the academic community in the hope that this will promote their widespread use and acceptance.

The mass of a millisecond pulsar

We report on nearly 2 years of timing observations of the low-mass binary millisecond pulsar PSR J1909-3744 with the Caltech-Parkes-Swinburne Recorder II, a new instrument that gives unprecedented timing precision. Daily observations give a weighted rms residual of 74 ns, indicating an extremely low level of systematic error. We have greatly improved on the previous parallax and proper motion measurements of PSR J1909-3744, yielding a distance of 1.14 kpc and transverse velocity of 200 km s-1. The systems orbital eccentricity is just (1.35 ± 12) × 10-7, the smallest yet recorded. Since their discovery, the masses of the rapidly rotating millisecond pulsars have remained a mystery, with the recycling hypothesis arguing for heavy objects, and the accretion-induced collapse of a white dwarf more consistent with neutron stars less than the Chandrashkar limit. Fortuitously, PSR J1909-3744 is an edge-on system, and our data have allowed the measurement of the range and shape of the Shapiro delay to high accuracy, giving the first precise determination of a millisecond pulsar mass to date, mp = 1.438 ± 0.024 M☉. The mass of PSR J1909-3744 is at the upper edge of the range observed in mildly recycled pulsars in double neutron star systems, consistent with the recycling hypothesis. It appears that the production of millisecond pulsars is possible with the accretion of <0.2 M☉.

Long-term variations in the pulse emission from PSR J0737-3039B

Analysis of 20 months of observations at the Parkes radio telescope shows secular changes in the pulsed emission from J0737-3039B, the 2.77 s pulsar of the double-pulsar system. Pulse profiles are becoming single-peaked in both bright phases of the orbital modulation, although there is no clear variation in overall pulse width. The shape of the orbital modulation is also varying systematically, with both bright phases shrinking in longitude by ~7° yr-1. However, the combined span of the two bright phases is relatively constant, and together they are shifting to higher longitudes at a rate of ~3° yr-1. We discuss the possible contributions of geodetic precession and periastron advance to the observed variations.

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.

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.

The Mean Pulse Profile of PSR J0737–3039A

General relativity predicts that the spin axes of the pulsars in the double-pulsar system PSR J0737-3039A/B will precess rapidly, in general leading to a change in the observed pulse profiles. We have observed this system over a 1 yr interval using the Parkes 64 m radio telescope at three frequencies: 680, 1390, and 3030 MHz. These data, combined with the short survey observation made 2 years earlier, show no evidence for significant changes in the pulse profile of PSR J0737-3039A, the 22 ms pulsar. The limit on variations of the profile 10% width is about 050 yr-1. These results imply an angle δ between the pulsar spin axis and the orbit normal of 60°, consistent with recent evolutionary studies of the system. Although a wide range of system parameters remain consistent with the data, the model recently proposed by F. A. Jenet & S. M. Ransom can be ruled out. A nonzero ellipticity for the radiation beam gives slightly but not significantly improved fits to the data, so that a circular beam describes the data equally well within the uncertainties.

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.

Discovery of Five Recycled Pulsars in a High Galactic Latitude Survey

We present five recycled pulsars discovered during a 21 cm survey of approximately 4150 deg2 between 15° and 30° from the Galactic plane using the Parkes radio telescope. One new pulsar, PSR J1528-3146, has a 61 ms spin period and a massive white dwarf companion. Like many recycled pulsars with heavy companions, the orbital eccentricity is relatively high (~0.0002), consistent with evolutionary models that predict less time for circularization. The four remaining pulsars have short spin periods (3 ms < P < 6 ms); three of these have probable white dwarf binary companions and one (PSR J2010-1323) is isolated. PSR J1600-3053 is relatively bright for its dispersion measure of 52.3 pc cm-3 and promises good timing precision thanks to an intrinsically narrow feature in its pulse profile, resolvable through coherent dedispersion. In this survey, the recycled pulsar discovery rate was 1 per 4 days of telescope time or 1 per 600 deg2 of sky. The variability of these sources implies that there are more millisecond pulsars that might be found by repeating this survey.

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.