Showing papers by "W. van Straten published in 2010"

The International Pulsar Timing Array project: using pulsars as a gravitational wave detector

Abstract: The International Pulsar Timing Array project combines observations of pulsars from both northern and southern hemisphere observatories with the main aim of detecting ultra-low frequency (similar to 10(-9)-10(-8) Hz) gravitational waves. Here we introduce the project, review the methods used to search for gravitational waves emitted from coalescing supermassive binary black-hole systems in the centres of merging galaxies and discuss the status of the project.

The High Time Resolution Universe Pulsar Survey – I. System configuration and initial discoveries

Abstract: We have embarked on a survey for pulsars and fast transients using the 13-beam multibeam receiver on the Parkes Radio Telescope Installation of a digital backend allows us to record 400 MHz of bandwidth for each beam, split into 1024 channels and sampled every 64 μs Limits of the receiver package restrict us to a 340 MHz observing band centred at 1352 MHz The factor of 8 improvement in frequency resolution over previous multibeam surveys allows us to probe deeper into the Galactic plane for short-duration signals such as the pulses from millisecond pulsars We plan to survey the entire southern sky in 42 641 pointings, split into low, mid and high Galactic latitude regions, with integration times of 4200, 540 and 270 s, respectively Simulations suggest that we will discover 400 pulsars, of which 75 will be millisecond pulsars With ∼30 per cent of the mid-latitude survey complete, we have redetected 223 previously known pulsars and discovered 27 pulsars, five of which are millisecond pulsars The newly discovered millisecond pulsars tend to have larger dispersion measures than those discovered in previous surveys, as expected from the improved time and frequency resolution of our instrument

DSPSR: Digital Signal Processing Software for Pulsar Astronomy

Abstract: dspsr is a high-performance, open-source, object-oriented, digital signal processing software library and application suite for use in radio pulsar astronomy. Written primarily in C++, the library implements an extensive range of modular algorithms that can optionally exploit both multiple-core processors and general-purpose graphics processing units. After over a decade of research and development, dspsr is now stable and in widespread use in the community. This paper presents a detailed description of its functionality, justification of major design decisions, analysis of phase-coherent dispersion removal algorithms, and demonstration of performance on some contemporary microprocessor architectures.

The sensitivity of the Parkes Pulsar Timing Array to individual sources of gravitational waves

Abstract: We present the sensitivity of the Parkes Pulsar Timing Array to gravitational wavesemitted by individual super-massive black-hole binary systems in the early phases ofcoalescing at the cores of merged galaxies. Our analysis includes a detailed study of theeffects of fitting a pulsar timing model to non-white timing residuals. Pulsar timingis sensitive at nanoHertz frequencies and hence complementary to LIGO and LISA.We place a sky-averaged constraint on the merger rate of nearby (z<0.6) black-holebinaries in the early phases of coalescence with a chirp mass of 10 10 M ⊙ of less thanone merger every seven years. The prospects for future gravitational-wave astronomyof this type with the proposed Square Kilometre Array telescope are discussed.Key words: gravitational waves – pulsars: general. 1 INTRODUCTIONIn the era of ground- and space-based gravitational-wave(GW) detectors, GW astronomy is becoming increasinglyimportant for the wider astronomy and physics communi-ties. The ability of the current GW community to provide ei-ther limits on, or detections of, GW emission is of enormousimportance in characterising astrophysical sources of inter-est for further investigation. It is possible that GW detectionwill provide the only means to probe some of these sources.The sensitivity of existing and future observatories to indi-vidual GW sources, such as neutron-star binary systems andcoalescing black-hole binary systems, has been calculatedin the ∼kHz and ∼mHz frequency ranges. The sensitiv-ity curves of the Laser Interferometer Gravitational-WaveObservatory (Abbott et al. 2009)

The Commensal Real-Time ASKAP Fast-Transients (CRAFT) Survey

Abstract: We are developing a purely commensal survey experiment for fast (<5 s) transient radio sources. Short-timescale transients are associated with the most energetic and brightest single events in the Universe. Our objective is to cover the enormous volume of transients parameter space made available by ASKAP, with an unprecedented combination of sensitivity and field of view. Fast timescale transients open new vistas on the physics of high brightness temperature emission, extreme states of matter and the physics of strong gravitational fields. In addition, the detection of extragalactic objects affords us an entirely new and extremely sensitive probe on the huge reservoir of baryons present in the IGM. We outline here our approach to the considerable challenge involved in detecting fast transients, particularly the development of hardware fast enough to dedisperse and search the ASKAP data stream at or near real-time rates. Through CRAFT, ASKAP will provide the testbed of many of the key technologies and survey modes proposed for high time resolution science with the SKA.

Measuring the Mass of Solar System Planets Using Pulsar Timing

Abstract: High-precision pulsar timing relies on a solar system ephemeris in order to convert times of arrival (TOAs) of pulses measured at an observatory to the solar system barycenter. Any error in the conversion to the barycentric TOAs leads to a systematic variation in the observed timing residuals; specifically, an incorrect planetary mass leads to a predominantly sinusoidal variation having a period and phase associated with the planet's orbital motion about the Sun. By using an array of pulsars (PSRs J0437–4715, J1744–1134, J1857+0943, J1909–3744), the masses of the planetary systems from Mercury to Saturn have been determined. These masses are consistent with the best-known masses determined by spacecraft observations, with the mass of the Jovian system, 9.547921(2) ×10–4 M ☉, being significantly more accurate than the mass determined from the Pioneer and Voyager spacecraft, and consistent with but less accurate than the value from the Galileo spacecraft. While spacecraft are likely to produce the most accurate measurements for individual solar system bodies, the pulsar technique is sensitive to planetary system masses and has the potential to provide the most accurate values of these masses for some planets.

psrchive and psrfits: Definition of the Stokes Parameters and Instrumental Basis Conventions

Abstract: This paper defines the mathematical convention adopted to describe an electromagnetic wave and its polarisation state, as implemented in the psrchive software and represented in the psrfits definition. Contrast is made between the convention that has been widely accepted by pulsar astronomers and the IAU/IEEE definitions of the Stokes parameters. The former is adopted as the PSR/IEEE convention, and a set of useful parameters are presented for describing the differences between the PSR/IEEE standard and the conventions (either implicit or explicit) that form part of the design of observatory instrumentation. To aid in the empirical determination of instrumental convention parameters, well-calibrated average polarisation profiles of PSR J0304+1932 and PSR J0742–2822 are presented at radio wavelengths of approximately 10, 20, and 40 cm.

The Commensal Real-time ASKAP Fast Transients (CRAFT) survey

Abstract: We are developing a purely commensal survey experiment for fast (<5s) transient radio sources. Short-timescale transients are associated with the most energetic and brightest single events in the Universe. Our objective is to cover the enormous volume of transients parameter space made available by ASKAP, with an unprecedented combination of sensitivity and field of view. Fast timescale transients open new vistas on the physics of high brightness temperature emission, extreme states of matter and the physics of strong gravitational fields. In addition, the detection of extragalactic objects affords us an entirely new and extremely sensitive probe on the huge reservoir of baryons present in the IGM. We outline here our approach to the considerable challenge involved in detecting fast transients, particularly the development of hardware fast enough to dedisperse and search the ASKAP data stream at or near real-time rates. Through CRAFT, ASKAP will provide the testbed of many of the key technologies and survey modes proposed for high time resolution science with the SKA.

Giant pulses from the Crab pulsar: A wide-band study

Abstract: The Crab pulsar is well-known for its anomalous giant radio pulse emission. Past studies have concentrated only on the very bright pulses or were insensitive to the faint end of the giant pulse luminosity distribution. With our new instrumentation offering a large bandwidth and high time resolution combined with the narrow radio beam of the Westerbork Synthesis Radio Telescope (WSRT), we seek to probe the weak giant pulse emission regime. The WSRT was used in a phased array mode, resolving a large fraction of the Crab nebula. The resulting pulsar signal was recorded using the PuMa II pulsar backend and then coherently dedispersed and searched for giant pulse emission. After careful flux calibration, the data were analysed to study the giant pulse properties. The analysis includes the distributions of the measured pulse widths, intensities, energies, and scattering times. The weak giant pulses are shown to form a separate part of the intensity distribution. The large number of giant pulses detected were used to analyse scattering and scintillation in giant pulses. We report for the first time the detection of giant pulse emission at both the main- and interpulse phases within a single rotation period. The rate of detection is consistent with the appearance of pulses at either pulse phase as being independent. These pulse pairs were used to examine the scintillation timescales within a single pulse period.

DSPSR: Digital Signal Processing Software for Pulsar Astronomy

Abstract: DSPSR is a high-performance, open-source, object-oriented, digital signal processing software library and application suite for use in radio pulsar astronomy. Written primarily in C++, the library implements an extensive range of modular algorithms that can optionally exploit both multiple-core processors and general-purpose graphics processing units. After over a decade of research and development, DSPSR is now stable and in widespread use in the community. This paper presents a detailed description of its functionality, justification of major design decisions, analysis of phase-coherent dispersion removal algorithms, and demonstration of performance on some contemporary microprocessor architectures.

Observations and Modelling of Relativistic Spin Precession in PSR J1141-6545

Abstract: Observations of the binary pulsar PSR J1141-6545 using the Parkes radio telescope over 9.3 years show clear time-variations in pulse width, shape and polarization. We interpret these variations in terms of relativistic precession of the pulsar spin axis about the total angular momentum vector of the system. Over the nine years, the pulse width at the 50% level has changed by more than a factor of three. Large variations have also been observed in the 1400-MHz mean flux density. The pulse polarization has been monitored since 2004 April using digital filterbank systems and also shows large and systematic variations in both linear and circular polarization. Position angle variations, both across the pulse profile and over the data span, are complex, with major differences between the central and outer parts of the pulse profile. Modelling of the observed position angle variations by relativistic precession of the pulsar spin axis shows that the spin-orbit misalignment angle is about 110 deg and that the precessional phase has passed through 180 deg during the course of our observations. At the start of our observations, the line-of-sight impact parameter was about 4 deg in magnitude and it reached a minimum very close to 0 deg around early 2007, consistent with the observed pulse width variations. We have therefore mapped approximately one half of the emission beam, showing that it is very asymmetric with respect to the magnetic axis. The derived precessional parameters imply that the pre-supernova star had a mass of about 2 Msun and that the supernova recoil kick velocity was relatively small. With the reversal in the rate of change of the impact parameter, we predict that over the next decade we will see a reversed "replay" of the variations observed in the past decade.

Observations and modeling of relativistic spin precession in psr j1141–6545

Abstract: Observations of the binary pulsar PSR J1141–6545 using the Parkes radio telescope over 9.3 yr show clear time variations in pulse width, shape, and polarization. We interpret these variations in terms of relativistic precession of the pulsar spin axis about the total angular momentum vector of the system changing our view of the emission beam. Over those nine years, the pulse width at the 50% level has changed by more than a factor of 3, reaching a maximum value of nearly 13° in early 2007. Large variations have also been observed in the 1400 MHz mean flux density; this reached a peak of ~20 mJy in mid-2002 but over the past several years has been relatively steady at ~3 mJy. The pulse polarization has been monitored since 2004 April using digital filterbank systems and also shows large and systematic variations in both linear and circular polarization. Position angle (P.A.) variations, both across the pulse profile and over the data span, are complex, with major differences between the central and outer parts of the pulse profile. We interpret the outer parts as representing the underlying magnetic field and fit the rotating-vector model to these regions. Modeling of the observed P.A. variations by relativistic precession of the pulsar spin axis shows that the spin-orbit misalignment angle is about 110° and that the precessional phase has passed through 180° during the course of our observations. At the start of our observations, the line-of-sight impact parameter was about 4° in magnitude and it reached a minimum very close to 0° around early 2007, consistent with the observed pulse width variations. We have therefore mapped approximately one half of the emission beam, at least out to a radius of about 4°, showing that it is very asymmetric with respect to the magnetic axis. The derived precessional parameters imply that the pre-supernova (pre-SN) star had a mass of about 2 M ☉ and that the SN recoil kick velocity was relatively small, between 100 and 250 km s–1, depending on the assumed systemic velocity. With the reversal in the rate of change of the impact parameter, we predict that over the next decade we will see a reversed replay of the variations observed in the past decade.

The High Time Resolution Universe Pulsar Survey I: System configuration and initial discoveries

Abstract: We have embarked on a survey for pulsars and fast transients using the 13-beam Multibeam receiver on the Parkes radio telescope. Installation of a digital backend allows us to record 400 MHz of bandwidth for each beam, split into 1024 channels and sampled every 64 us. Limits of the receiver package restrict us to a 340 MHz observing band centred at 1352 MHz. The factor of eight improvement in frequency resolution over previous multibeam surveys allows us to probe deeper into the Galactic plane for short duration signals such as the pulses from millisecond pulsars. We plan to survey the entire southern sky in 42641 pointings, split into low, mid and high Galactic latitude regions, with integration times of 4200, 540 and 270 s respectively. Simulations suggest that we will discover 400 pulsars, of which 75 will be millisecond pulsars. With ~30% of the mid-latitude survey complete, we have re-detected 223 previously known pulsars and discovered 27 pulsars, 5 of which are millisecond pulsars. The newly discovered millisecond pulsars tend to have larger dispersion measures than those discovered in previous surveys, as expected from the improved time and frequency resolution of our instrument.

Status update of the Parkes pulsar timing array

Abstract: The Parkes Pulsar Timing Array project aims to make a direct detection of a gravitational wave background through the timing of millisecond pulsars. In this paper, the main requirements for that endeavour are described and recent and ongoing progress is outlined. We demonstrate that the timing properties of millisecond pulsars are adequate and that technological progress is timely to expect a successful detection of gravitational waves within a decade, or alternatively to rule out all current predictions for gravitational wave backgrounds formed by supermassive black-hole mergers.

ERRATUM: THE STATISTICS OF RADIO ASTRONOMICAL POLARIMETRY: BRIGHT SOURCES AND HIGH TIME RESOLUTION (2009, ApJ, 694, 1413)

Abstract: Section 4.3 of the published article presents the erroneous conclusion that the incoherent sum of covariant orthogonally polarized modes causes the variance of the total intensity to increase and that of the major polarization to decrease. This incorrect result is derived in Appendix C from the unsubstantiated expression for the cross-covariance matrix presented in Equation (33). Although this ansatz yields results that are consistent with previous work, such as Equations (5) and (6) of McKinnon & Stinebring (1998) and Equation (5) of Cordes et al. (1978), it does not take account of the covariances between the Stokes parameters as expressed by our Equation (28). The correct form of Equation (33) is derived by considering the incoherent superposition of two sources described by Stokes parameters A and B, the result of which is indistinguishable from a single stationary stochastic source of polarized radiation with mean Stokes parameters A + B. Substitution of S = A + B into Equation (28), yields where C A and C B are the covariance matrices of A and B as defined by Equation (28) and the rectified cross-covariance matrix is given by Here, the intensity correlation coefficient is reintroduced to generalize the result; it varies from zero (disjoint modes) to unity (superposed modes). To verify the above expression for the cross-covariance matrix, consider the superposition of orthogonally polarized modes; in this case, and, in the natural basis defined by A If the intensities, degrees of polarization, and degrees of freedom of the modes are equal (A 0 = B 0 = S 0/2, , and A = B = ), then the superposition of the modes will be unpolarized and the covariance matrix reduces to C = 2 S 2 0 I, where I is the 4 × 4 identity matrix, as is easily confirmed using the above equations. In the natural basis, it is readily seen that the cross-covariance terms due to the sum of orthogonally polarized modes contribute equally to the variances of the total intensity and the major polarization. That is, orthogonally polarized modes cannot explain the differences in the variances observed by McKinnon (2004) and summarized in Table 1. The differences in the variances can be explained by the coherence and/or obliquity of the modes. For example, the partially coherent sum of linearly polarized orthogonal modes would produce an additional elliptically polarized component that would increase the variances of the total intensity and the minor polarizations. Similarly, the superposition of non-orthogonal modes would add to the variances of the minor polarizations. In each case, the variance of the total intensity will exceed that of the major polarization, the distribution of the polarization vector will become ellipsoidal, and the major axis of the ellipsoid will no longer be parallel to the mean polarization vector. The rectified cross-covariance matrix does not affect the main conclusion of Section 4.4; that is, the excess dispersion of the polarization vector, first observed as modal broadening by Stinebring et al. (1984), is completely due to self-noise intrinsic to the pulsar signal. However, the correction does impact on the proposal to develop a new technique for producing mode-separated profiles. All of the equations presented in Appendix C must be replaced by a more general expression that includes the partial coherence and/or non-orthogonality of the modes. As before, this development is left for future work.

The Sensitivity of the Parkes Pulsar Timing Array to Individual Sources of Gravitational Waves

Abstract: We present the sensitivity of the Parkes Pulsar Timing Array to gravitational waves emitted by individual super-massive black-hole binary systems in the early phases of coalescing at the cores of merged galaxies. Our analysis includes a detailed study of the effects of fitting a pulsar timing model to non-white timing residuals. Pulsar timing is sensitive at nanoHertz frequencies and hence complementary to LIGO and LISA. We place a sky-averaged constraint on the merger rate of nearby ($z < 0.6$) black-hole binaries in the early phases of coalescence with a chirp mass of $10^{10}\,\rmn{M}_\odot$ of less than one merger every seven years. The prospects for future gravitational-wave astronomy of this type with the proposed Square Kilometre Array telescope are discussed.