Individual Pulsars Research Articles

Constraining the luminosity function parameters and population size of radio pulsars in globular clusters

AbstractThe luminosity distribution of Galactic radio pulsars is believed to be log-normal in form. Applying this functional form to populations of pulsars in globular clusters, we employ Bayesian methods to explore constraints on the mean and standard deviation of the function, as well as the total number of pulsars in the cluster. Our analysis is based on an observed number of pulsars down to some limiting flux density, measurements of flux densities of individual pulsars, as well as diffuse emission from the direction of the cluster. We apply our analysis to Terzan 5 and demonstrate, under reasonable assumptions, that the number of potentially observable pulsars is in a 95.45% credible interval of 133+101−58. Beaming considerations would increase the true population size by approximately a factor of two.

Modeling pulsar time noise with long term decay modulated by short term oscillations of the magnetic fields of neutron stars

AbstractWe model the evolution of the magnetic fields of neutron stars as consisting of a long term power-law decay modulated by short term small amplitude oscillations. Our model predictions on the timing noise of neutron stars agree well with the observed statistical properties and correlations of normal radio pulsars. For individual pulsars our model can effectively reduce their timing residuals, thus offering the potential of more sensitive detections of gravitational waves with pulsar timing arrays. Finally our model can also re-produce their observed correlation and oscillations of second derivative of frenquency, as well as the “slow glitch” phenomenon.

The hunt for new pulsars with the Green Bank Telescope

AbstractThe Green Bank Telescope (GBT) is the largest fully steerable radio telescope in the world and is one of our greatest tools for discovering and studying radio pulsars. Over the last decade, the GBT has successfully found over 100 new pulsars through large-area surveys. Here I discuss the two most recent—the GBT 350 MHz Drift-scan survey and the Green Bank North Celestial Cap survey. The primary science goal of both surveys is to find interesting individual pulsars, including young pulsars, rotating radio transients, exotic binary systems, and especially bright millisecond pulsars (MSPs) suitable for inclusion in Pulsar Timing Arrays, which are trying to directly detect gravitational waves. These two surveys have combined to discover 85 pulsars to date, among which are 14 MSPs and many unique and fascinating systems. I present highlights from these surveys and discuss future plans. I also discuss recent results from targeted GBT pulsar searches of globular clusters and Fermi sources.

The optimal schedule for pulsar timing array observations

In order to maximize the sensitivity of pulsar timing arrays to a stochastic gravitational wave background, we present computational techniques to optimize observing schedules. The techniques are applicable to both single and multi-telescope experiments. The observing schedule is optimized for each telescope by adjusting the observing time allocated to each pulsar while keeping the total amount of observing time constant. The optimized schedule depends on the timing noise characteristics of each individual pulsar as well as the performance of instrumentation. Several examples are given to illustrate the effects of different types of noise. A method to select the most suitable pulsars to be included in a pulsar timing array project is also presented.

The Parkes Observatory Pulsar Data Archive

AbstractThe Parkes pulsar data archive currently provides access to 144044 data files obtained from observations carried out at the Parkes observatory since the year 1991. Around 105 files are from surveys of the sky, the remainder are observations of 775 individual pulsars and their corresponding calibration signals. Survey observations are included from the Parkes 70 cm and the Swinburne Intermediate Latitude surveys. Individual pulsar observations are included from young pulsar timing projects, the Parkes Pulsar Timing Array and from the PULSE@Parkes outreach program. The data files and access methods are compatible with Virtual Observatory protocols. This paper describes the data currently stored in the archive and presents ways in which these data can be searched and downloaded.

Observational features of pulsar glitches

Pulsar glitches are sudden increases in the rotation rate which probably result from angular momentum transfer within the neutron star. We review the observational features of the 39 glitches detected at Nanshan from 2000 to 2008, including several events which appear to be slow glitches. A wide variety of post-glitch behavior is observed with very little recovery in some pulsars and over-recovery in others. Analysis of the whole sample of known glitches shows that fractional glitch amplitudes are correlated with characteristic age with a peak at about 105 years, but there is a spread of two or three orders of magnitude at all ages. For individual pulsars with many glitches, the time until the next glitch is sometimes proportional to the fractional glitch amplitude.

SUPERFLUID VORTEX UNPINNING AS A COHERENT NOISE PROCESS, AND THE SCALE INVARIANCE OF PULSAR GLITCHES

The scale-invariant glitch statistics observed in individual pulsars (exponential waiting-time and power-law size distributions) are consistent with a critical self-organization process, wherein superfluid vortices pin metastably in macroscopic domains and unpin collectively via nearest-neighbor avalanches. Macroscopic inhomogeneity emerges naturally if pinning occurs at crustal faults. If, instead, pinning occurs at lattice sites and defects, which are macroscopically homogeneous, we show that an alternative, noncritical self-organization process operates, termed coherent noise, wherein the global Magnus force acts uniformly on vortices trapped in a range of pinning potentials and undergoing thermal creep. It is found that vortices again unpin collectively, but not via nearest-neighbor avalanches, and that, counterintuitively, the resulting glitch sizes are scale invariant, in accord with observational data. A mean-field analytic theory of the coherent noise process, supported by Monte Carlo simulations, yields a power-law size distribution, between the smallest and largest glitch, with exponent a in the range −2 ⩽ a ⩽ 0. When the theory is fitted to data from the nine most active pulsars, including the two quasi-periodic glitchers PSR J0537−6910 and PSR J0835−4510, it directly constrains the distribution of pinning potentials in the star, leading to two conclusions: (1) the potentials are broadly distributed, with the mean comparable to the standard deviation; and (2) the mean potential decreases with characteristic age. Fitting the theory to the data also constrains the pinned vortex fraction and the rate of thermal creep. An observational test is proposed to discriminate between nearest-neighbor avalanches and coherent noise: the latter process predicts a statistical excess of large glitches ("aftershocks") following a large glitch, whereas the former process does not. Its discriminatory power is discussed under various microphysical scenarios.

Constraining A General-Relativistic Frame-Dragging Model for Pulsed Radiation from a Population of Millisecond Pulsars in 47 Tucanae using GLAST LAT

Although only 22 millisecond pulsars (MSPs) are currently known to exist in the globular cluster (GC) 47 Tucanae, this cluster may harbor 30-60 MSPs, or even up to ~200. In this Letter, we model the pulsed curvature radiation (CR) gamma-ray flux expected from a population of MSPs in 47 Tucanae. These MSPs produce gamma-rays in their magnetospheres via accelerated electron primaries which are moving along curved magnetic field lines. A GC like 47 Tucanae containing a large number of MSPs provides the opportunity to study a randomized set of pulsar geometries. Geometry-averaged spectra make the testing of the underlying pulsar model more reliable, since in this case the relative flux uncertainty is reduced by one order of magnitude relative to the variation expected for individual pulsars (if the number of visible pulsars N=100). Our predicted spectra violate the EGRET upper limit at 1 GeV, constraining the product of the number of visible pulsars N and the average integral flux above 1 GeV per pulsar. GLAST/LAT should place even more stringent constraints on this product, and may also limit the maximum average accelerating potential by probing the CR spectral tail. For N=22-200, a GLAST/LAT non-detection will lead to the constraints that the average integral flux per pulsar should be lower by factors 0.03-0.003 than current model predictions.

Avalanche Dynamics of Radio Pulsar Glitches

We test statistically the hypothesis that radio pulsar glitches result from an avalanche process, in which angular momentum is transferred erratically from the flywheel-like superfluid in the star to the slowly decelerating, solid crust via spatially connected chains of local, impulsive, threshold-activated events, so that the system fluctuates around a self-organized critical state. Analysis of the glitch population (currently 285 events from 101 pulsars) demonstrates that the size distribution in individual pulsars is consistent with being scale invariant, as expected for an avalanche process. The measured power-law exponents fall in the range -0.13 ⩽ a⩽ 2.4, with a ≈ 1.2 for the youngest pulsars. The waiting-time distribution is consistent with being exponential in seven out of nine pulsars where it can be measured reliably, after adjusting for observational limits on the minimum waiting time, as for a constant-rate Poisson process. PSR J0537–6910 and PSR J0835–4510 are the exceptions; their waiting-time distributions show evidence of quasi-periodicity. In each object, stationarity requires that the rate λ equal −/⟨Δν⟩, where is the angular acceleration of the crust, ⟨ Δ ν ⟩ is the mean glitch size, and is the relative angular acceleration of the crust and superfluid. Measurements yield ⩽ 7 × 10−5 for PSR J0358+5413 and ⩽ 1 (trivially) for the other eight objects, which have a < 2. There is no evidence that λ changes monotonically with spin-down age. The rate distribution itself is fitted reasonably well by an exponential for λ ⩾ 0.25 yr−1, with ⟨ λ ⟩ = 1.3+ 0.7−0.6 yr−1. For λ < 0.25 yr−1 the exact form is unknown; the exponential overestimates the number of glitching pulsars observed at low λ, where the limited total observation time exercises a selection bias. In order to reproduce the aggregate waiting-time distribution of the glitch population as a whole, the fraction of pulsars with λ > 0.25 yr−1 must exceed ~70%.

Revised Pulsar Spin‐down

We address the issue of electromagnetic pulsar spin-down by combining our experience from the two limiting idealized cases that have been studied thoroughly in the past: that of an aligned rotator, in which ideal MHD conditions apply, and that of a misaligned rotator in vacuum. We construct a spin-down formula that takes into account the misalignment of the magnetic and rotation axes, and the magnetospheric particle acceleration gaps. We show that near the death line, aligned rotators spin down more slowly than orthogonal ones. In order to test this approach, we use a simple Monte Carlo method to simulate the evolution of pulsars and find a good fit to the observed pulsar distribution in the P- diagram without invoking magnetic field decay. Our model may also account for individual pulsars spinning down with braking index n 3, and that the older pulsar population has preferentially smaller magnetic inclination angles. We discuss possible signatures of such alignment in the existing pulsar data.

Gravitational radiation of slowly rotating neutron stars

The gravitational rotation of slowly rotating neutron stars with rough surfaces is examined. The source of the gravitational waves is assumed to be the energy transferred to the crust of the star during irregular changes in its angular rotation velocity. It is shown that individual pulsars whose angular velocity regularly undergoes glitches will radiate a periodic gravitational signal that can be distinguished from noise by the latest generation of detectors. Simultaneous recording of a gravitational signal and of a glitch in the angular velocity of a pulsar will ensure reliable detection of gravitational radiation.

The Green Bank Telescope Pulsar Spigot

ABSTRACTWe describe the design, implementation, and usage of a new facility pulsar search and timing instrument for the 100 m Robert C. Byrd Green Bank Telescope (GBT). This instrument, known as the Pulsar Spigot, makes use of the extremely flexible digital correlator at the GBT to perform rapid autocorrelations (as fast as 2.56 μs) across wide bandwidths (up to 800 MHz) with large numbers of lags (up to 4096), using as many as four simultaneous frequency bands. Successful observations using this instrument have been conducted over the past year and have already resulted in a number of newly discovered pulsars and detailed studies of individual pulsars.

High precision pulsar astrometry and its applications

Very long baseline interferometry (VLBI) is a radio astronomy technique that allows imaging and astrometry at milliarcsecond or better resolution. The last decade has seen improvements in VLBI techniques and equipment which have enabled precision astrometry of pulsars. Most notably, VLBI arrays have become more sensitive and calibration has become more accurate. Pulsar astrometry using VLBI can readily yield a proper motion and in many cases a trigonometric parallax as well. These observables have have numerous applications which justify the difficulty of the measurements. The goals of pulsar astrometry include: 1. determination of the distribution of pulsar birth velocities, a critical probe of supernova asymmetry; 2. improvement in understanding of the ISM through observations of pulsar bow shocks, interstellar scattering, and timing dispersion measures; and 3. understanding characteristics of individual pulsars though knowledge of their distance and velocity. I will discuss these science cases, some recent results, and prospects for better astrometry in the near future.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Gamma‐Ray Luminosity and Death Lines of Pulsars with Outer Gaps

We reexamine the outer-gap size by taking the geometry of the dipole magnetic field into account. Furthermore, we also consider that instead of taking the gap size at half of the light cylinder radius to represent the entire outer gap, it is more appropriate to average the entire outer-gap size over the distance. When these two factors are considered, the derived outer-gap size f (P, B, [r] (alpha)) is a function not only of the period P and magnetic field B of the neutron star but also of the average radial distance to the neutron star, [r], which depends on the magnetic inclination angle alpha. We use this new outer-gap model to study the gamma-ray luminosity of pulsars, which is given by L-gamma = f(3) (P, B, [r] (alpha))L-sd, where L-sd is the pulsar spin-down power, and to study the death lines of gamma-ray emission of the pulsars. Our model can predict the gamma-ray luminosity of an individual pulsar if its P, B, and alpha are known. Since different pulsars have different alpha, this explains why some gamma-ray pulsars have very similar P and B but very different gamma-ray luminosities. In determining the death line of gamma-ray pulsars, we have used a new criterion based on a concrete physical property, i.e., that the fractional size of the outer gap at the null-charge surface for a given pulsar cannot be larger than unity. In an estimate of the fractional size of the outer gap, two possible X-ray fields are considered: (1) X-rays produced by neutron star cooling and polar-cap heating, and (2) X-rays produced by the bombardment of relativistic particles from the outer gap onto the stellar surface ( the outer gap is called a outer gap''). Since it is very difficult to measure alpha in general, we use a Monte Carlo method to simulate the properties of gamma-ray pulsars in our Galaxy. We find that this new outer-gap model predicts many more weak gamma-ray pulsars, which have a typical age between 0.3 and 3 Myr, than does the old model. For all simulated gamma-ray pulsars with self-sustained outer gaps, the gamma-ray luminosity L-gamma satisfies L-gamma proportional to L-sd(delta), where the value of delta depends on the sensitivity of the gamma-ray detector. For EGRET, delta similar to 0.38, whereas delta similar to 0.46 for GLAST. For gamma-ray pulsars with L(sd)less than or similar toL(sd)(crit), delta similar to 1, and L-sd(crit) = 1.5 x 10(34)P(1/3) ergs s(-1) is determined by f([r] similar to r(L)) = 1. These results are roughly consistent with the observed luminosity of gamma-ray pulsars. These predictions are very different from those of the previous outer-gap model, which predicts a very flat relation between L-gamma and L-sd.

Finding Pulsars at Parkes

AbstractThere are many reasons why it is important to increase the number of known pulsars. Not only do pulsar searches continue to improve statistical estimates of, for example, pulsar birthrates, lifetimes and the Galactic distribution, but they continue to turn up interesting and, in some cases, unique individual pulsars. In the early days of pulsar astronomy, the Molonglo radio telescope led the world as a pulsar detection instrument. However, the Parkes radio telescope, with its frequency versatility and greater tracking ablility, combined with sensitive receivers and powerful computer detection algorithms, is now the world’s most successful telescope at finding pulsars. The Parkes multibeam survey, begun in 1997, by itself will come close to doubling the number of known pulsars. Parkes has also been very successful at finding millisecond pulsars (MSPs), especially in globular clusters. One third of the known MSPs have been found in just one cluster, 47 Tucanae.

Glitches in southern pulsars

Timing observations of 40 mostly young pulsars using the ATNF Parkes radio telescope between 1990 January and 1998 December are reported. In total, 20 previously unreported glitches and ten other glitches were detected in 11 pulsars. These included 12 glitches in PSR J1341$- $6220, corresponding to a glitch rate of 1.5 glitches per year. We also detected the largest known glitch, in PSR J1614$-$5047, with $\Delta\nu_g/\nu \approx 6.5 \times 10^{-6}$ where $\nu = 1/P$ is the pulse frequency. Glitch parameters were determined both by extrapolating timing solutions to inter-glitch intervals and by phase-coherent timing fits across the glitch(es). Analysis of glitch parameters, both from this work and from previously published results, shows that most glitches have a fractional amplitude $\Delta\nu_g/\nu$ of between $10^{-8}$ and $10^{-6}$. There is no consistent relationship between glitch amplitude and the time since the previous glitch or the time to the following glitch, either for the ensemble or for individual pulsars. As previously recognised, the largest glitch activity is seen in pulsars with ages of order 10$^4$ years, but for about 30 per cent of such pulsars, no glitches were detected in the 8-year data span. There is some evidence for a new type of timing irregularity in which there is a significant increase in pulse frequency over a few days, accompanied by a decrease in the magnitude of the slowdown rate. Fits of an exponential recovery to post-glitch data show that for most older pulsars, only a small fraction of the glitch decays. In some younger pulsars, a large fraction of the glitch decays, but in others, there is very little decay.

New Direct Observational Evidence for Kicks in SNe

We present an updated list of direct strong evidence in favour of kicks being imparted to newborn neutron stars. In particular we discuss the new cases of evidence resulting from recent observations of the X-ray binary Circinus X-1 and the newly discovered binary radio pulsar PSR J1141–6545. We conclude that the assumption that neutron stars receive a kick velocity at their formation is unavoidable (van den Heuvel &amp;amp; van Paradijs 1997).This assumption explains a large variety of observations, ranging from direct observed properties of individual binary pulsars and Be/X-ray binaries to the observed birth rates and dynamical properties of the populations of LMXBs, binary recycled pulsars as well as the motion and distribution of single pulsars. Below we give an updated list in favour of kicks based on the compilation given by van den Heuvel &amp;amp; van Paradijs (1997) – see references therein for details.

Diffractive Interstellar Scintillation Timescales and Velocities

We derive general relationships between the observed timescale of diffractive interstellar scintillations and the physical velocities of the observer, the source, and the scattering medium. Our treatment applies exclusively to saturated scintillations of point sources in the strong scattering regime. We show how scintillation observations may be combined with other observables (proper motion and dispersion measure) to yield (1) improvements in galactic models for the free-electron density and (2) estimates of the distance and transverse velocity of individual pulsars. We explicitly consider cases of current astrophysical interest, including hypervelocity pulsars too far above the Galactic plane to allow distance estimates from dispersion measures alone. We also briefly consider scintillations of extragalactic sources, including gamma-ray burst sources at great distances from the Galaxy.

Intrinsic Kicks at Birth Are Required to Explain the Observed Properties of Single and Binary Neutron Stars

The single assumption that neutron stars receive a kick velocity at their formation explains a large variety of observations, ranging from observed properties of individual binary radio pulsars and Be/X-ray binaries to the observed birth rates and dynamical properties of the populations of low-mass X-ray binaries and recycled radio pulsars. The rejection of this one assumption by Iben & Tutukov compels them to introduce a considerable number of ad hoc hypotheses, each explaining a only small part of the observational results. We conclude that the choice of the single powerful assumption that neutron stars receive a kick velocity at their formation is highly preferable.

THE AGES, SPEEDS AND OFFSPRING OF PULSARS

ABSTRACT The first part of this thesis is devoted to the study of the cooling of low mass helium white dwarfs such as are found in binaries with millisecond pulsars. We describe in detail the construction of a numerical model to calculate accurate cooling ages for these stars, using the most recent input physics available and including our own calculations of low temperature opacities. We use the cooling ages we infer to determine the ages of individual millisecond pulsars in such binaries, which in turn allows us to place constraints on the initial spin periods and accretion histories of the neutron stars. We also provide low mass white dwarf mass-radius relations for those cases where a spectroscopic gravity can be measured. For PSR J1012+5307, this information, combined with radial velocity measurements, constrains the neutron star mass to be > 2 solar mass. Chapters 2 through 4 deal with these calculations. In chapter 5 we study the spallation of atmospheric helium by the pulsar wind and it's effect on the white dwarf cooling. The final chapter (chapter 7) deals with the velocity distribution of young pulsars.