Volume 16, Issue 2
June 1995, pages 69-325
pp 69-88 June 1995
pp 89-95 June 1995
pp 97-106 June 1995
Radio pulsars have long been established as having high velocities that are probably produced in the violence of their formation in Supernovae (Gunn & Ostriker 1970; Lyne, Anderson & Salter 1982). Three recent developments have resulted in a reassessment of their velocities: the adoption of a new distance scale (Taylor & Cordes 1993), many new determinations of proper motion (Harrison, Lyne & Anderson 1993; Bailes et al. 1989; Fomalont et al. 1992) and the realisation (Harrison & Lyne 1993) that estimates of speeds derived from scintillation measurements were systematically low by about a factor of 2. Taking into account a strong selection effect that makes the observed velocities unrepresentative of those acquired at birth, it seems that the mean space velocity of pulsars at birth is 450 ± 90 km s-1 (Lyne and Lorimer 1994), about a factor of 3 greater than earlier estimates. The general migration from the Galactic plane is consistent with birth in the supernova of massive Population I stars. An outstanding question is how such velocities are produced in the kinetics of supernova collapse. This large increase in birth velocity is likely to have a major impact upon our understanding of the retention of neutron stars in binary systems, globular clusters and the Galaxy as it exceeds or is comparable with all their escape velocities. The rapid spatial separation of fast and slow pulsars will have a profound effect upon calculations of the galactic population and birth rate, both of which have been underestimated in the past. Furthermore, the distribution of dead neutron stars will be more isotropic and may better match the distribution of the gamma-ray burst sources. A small number of pulsars are at a large distance from the Galactic plane, but moving towards it. The most likely origin of these objects lies in OB runaway stars.
pp 107-117 June 1995
Observations of mean or average pulse profiles and their polarization give us much information on the shape of pulsar beams. The observed polarization variations, profile symmetry and frequency dependence of profile shape strongly suggest that the emission beam is conical and emitted from the vicinity of a magnetic pole. Central and outer parts of the beam have somewhat different properties, but the evidence is that they are emitted by the same basic mechanism. Recent observations suggest that the highly polarized pulse components seen in young pulsars may be emitted at a large angle to the magnetic axis.
pp 119-135 June 1995
The difficulties of the pulsar magnetosphere problem are illustrated by two models for the axisymmetric magnetic rotator: (a) a classical model, in which the return current problem is linked with angular momentum dissipation’ through incoherent gamma radiation beyond the light-cylinder; and (b) a quantum model, in which electron positron pairs are produced near the star, and spin-down is primarily through the transport of angular momentum by an e+-e-wind. The first model has some pedagogic value, but it makes the embarrassing prediction that all the spin-down energy is emitted as gammaradiation. In the second model a predicted small but significant emission in gamma-and X-rays is again linked with the return current problem, but the bulk of the energy emission is through the wind. The presence of both a high-γ primary electron beam and a moderate-γ pair plasma is as required by most models of coherent microwave emission. Problems persist in ensuring that the macroscopic conditions on the electric field are locally consistent with the microphysics. The most promising picture is of a spontaneous local hydromagnetic instability that yields a large effective resistivity. Some remarks are made on future extension of the theory to oblique geometry. For agreement with observations of the Crab nebula, the predicted dominance in the wind of the Poynting flux must be reversed in the far magnetosphere.
pp 137-164 June 1995
The theory of pulsar radio emission is reviewed critically, emphasizing reasons why there is no single, widely-accepted emission mechanism. The uncertainties in our understanding of how the magnetosphere is populated with plasma preclude predicting the properties of the emission from first principles. Some important observational features are incorporated into virtually all the proposed emission mechanisms, and other observational features are either controversial or fail to provide criteria that clearly favor one mechanism over others. It is suggested that the criterion that the emission mechanism apply to millisecond, fast young, and slow pulsars implies that it is insensitive to the magnetic field strength.
It is argued that coherent emission processes in all astrophysical and space plasmas consist of emission from many localized, transient subsources, that any theory requires both an emission mechanism and a statistical theory for the subsource, and, that this aspect of coherent emission has been largely ignored in treatments of pulsar radio emission. Several specific proposed emission mechanisms are discussed critically: coherent curvature emission by bunches, relativistic plasma emission, maser curvature emission, cyclotron instability and free electron maser emission. It is suggested that some form of relativistic plasma emission is the most plausible candidate although one form of maser curvature emission and free electron maser emission are not ruled out. Propagation effects are discussed, emphasizing the interpretation of jumps between orthogonal polarizations.
pp 165-171 June 1995
Two investigations of millisecond pulsar radiation are discussed: average total intensity pulse morphology and individual pulse to pulse fluctuations. The average emission profiles of millisecond pulsars are compared with those of slower pulsars in the context of polar cap models. In general the full widths of pulsar emission regions continue to widen inversely with periodP as P-(0.30-0.5) as expected for dipole polar cap models. Many pulse components are very narrow. The period scaling of pulsar profiles -separations and widths -can tell us about the angular distribution of radiating currents. An investigation of individual pulses from two millisecond pulsars at 430 MHz shows erratic pulse to pulse variations similar to that seen in slow pulsars. PSR B1937+21 displays occasional strong pulses that are located in the trailing edge of the average profile with relative flux densities in the range of 100 to 400. These are similar to the giant pulses seen in the Crab pulsar.
pp 173-178 June 1995
Various observations of γ-ray pulsars are summarized briefly and related to outer-magnetosphere accelerator models.
pp 179-190 June 1995
One of the most remarkable properties of radio pulsars is their rotational stability which allows many uses as clocks, For instance they enable us to determine the shapes and sizes of binary orbits, to study general relativistic effects in strong gravitational fields, to demonstrate the existance of gravitational radiation from binary systems, to permit the detection of extra solar planets, and also to put limits on the long period gravitational wave background. However, some display timing imperfections which tell us about the insides of neutron stars. This review describes the basic physics of slowdown and how period instabilities seem to be related to the rate of slowdown and the presence of internal superfluid liquid. Careful studies of glitches and the subsequent rotational behaviour of the pulsars can provide valuable information on the internal structure of neutron stars.
pp 191-206 June 1995
It is shown that if neutron stars contain dynamically de-coupled components, most plausibly discrete superfluid zones, then it is possible for the spin axes of these components o become slightly misaligned with respect to the crustal spin following a series of glitches. The crust will then undergo Lense-Thirring precession about the total angular moment with a period of ∼ 6–7.5P (assuming a crustal superfluid) and ∼ 3–6P (if the core superfluid is not tightly coupled to the crust). The precise precessional period is diagnostic of the mass distribution within each component. The implications of recent observational inferences concerning glitching pulsars are discussed. The conditions necessary for precession to be observable are analysed phenomenologically and a search of pulse-timing data for evidence of a Lense-Thirring modulation within the period range ∼ 3–8P is proposed.
pp 207-216 June 1995
Interactions among a neutron star’s superfluid neutrons, superconducting protons, and solid crust cause predictable spin-down and spin-up driven crustal motion and magnetic field changes. Applications and unsolved problems are discussed for millisecond pulsar evolution and properties, glitches and post-glitch responses, and transcient gamma-ray emission.
pp 217-232 June 1995
Observational evidence, and theoretical models of the magnetic field evolution of neutron stars is discussed. Observational data indicates that the magnetic field of a neutron star decays significantly only if it has been a member of a close interacting binary. Theoretically, the magnetic field evolution has been related to the processing of a neutron star in a binary system through the spin evolution of the neutron star, and also through the accretion of matter on the neutron star surface. I describe two specific models, one in which magnetic flux is expelled from the superconducting core during spin-down, via a copuling between Abrikosov fluxoids and Onsager-Feynman vortices; and another in which the compression and heating of the stellar crust by the accreted mass drastically reduces the ohmic decay time scale of a magnetic field configuration confined entirely to the crust. General remarks about the behaviour of the crustal field under ohmic diffusion are also made.
pp 233-244 June 1995
Recent searches for millisecond and binary pulsars are reviewed, with particular emphasis on the nearly complete Parkes southern survey. Correlations between several of the major parameters of these systems are discussed.
pp 245-253 June 1995
We have obtained deep 6, 20 and 90 cm observation of two of the richest globular clusters in the galaxy, Terzan 5 and Liller 1. In addition to the well known eclipsing pulsar, PSR1744-24A, our 20 cm image of Terzan 5 displays numerous point sources within 30″, or 4 core radii of the cluster center. The density of these objects rises rapidly toward the core, where an elongated region of emission is found. This feature may be due to the presence of a higher density of point sources than can be resolved by the 1″ resolution of our data. Observations at 6 and 90 cm show that the brightest individual sources, as well as the extended emission, possess the steep spectra expected of pulsars. Furthermore, we find that the strength of the sources in the 20 cm image agrees well with the standard pulsar luminosity function. The total luminosity and number of objects observed in Terzan 5 suggest that this cluster may contain 100 pulsars — probably more than any other Galactic cluster. Although we do not resolve individual sources in Liller 1, comparison of our images with other observations suggests that we are again observing the sum of emission from a number of objects. The 90 cm flux densities of Terzan 5 and Liller 1 are 35 and 9 mJy, respectively, implying that a number of bright pulsars in these clusters have been hidden from pulsed searches, perhaps by a combination of dispersion smearing, scattering and doppler broadening.
pp 255-288 June 1995
The evolution of high-and low-mass X-ray binaries (HMXB and LMXB) into different types of binary radio pulsars, the ‘high-mass binary pulsars’(HMBP) and ‘low-mass binary pulsars’ (LMBP) is discussed. The HMXB evolve either into Thorne-Zytkow objects or into short-period binaries consisting of a helium star plus a neutron star (or a black hole), resembling Cygnus X-3. The latter systems evolve (with or without a second common-envelope phase) into close binary pulsars, in which the companion of the pulsar may be a massive white dwarf, a neutron star or a black hole ( some final systems may also consist of two black holes). A considerable fraction of the systems may also be disrupted in the second supernova explosion. We discuss the possible reasons why the observed numbers of double neutron stars and of systems like Cyg X-3 are several orders of magnitude lower than theoretically predicted. It is argued that the observed systems form the tip of an iceberg of much larger populations of unobserved systems, some of which may become observable in the future. As to the LMBP, we consider in some detail the origins of systems with orbital periods in the range 1–20 days. We show that to explain their existence, losses of orbital angular momentum (e.g., by magnetic braking) and in a number of cases: also of mass, have to be taken into account. The masses of the low-mass white dwarf companions in these systems can be predicted accurately. We notice a clear correlation between spin period and orbital period for these systems, as well as a clear correlation between pulsar magnetic field strength and orbital period. These relations strongly suggest that increased amounts of mass accreted by the neutron stars lead to increased decay of their magnetic fields: we suggest a simple way to understand the observed value of the ‘bottom’ field strengths of a few times 108 G. Furthermore, we find that the LMBP-systems in which the pulsar has a strong magnetic field (> 1011 G) have an about two orders of magnitude larger birth rate (i.e., about 4 × 10-4 yr-1 in the Galaxy) than the systems with millisecond pulsars (which have B < 109 G). Using the observational fact that neutron stars receive a velocity kick of ∼450 km/s at birth, we find that some 90% of the potential progenitor systems of the strong-field LMBP must have been disrupted in the Supernovae in which their neutron stars were formed. Hence, the formation rate of the progenitors of the strong-field LMBP is of the same order as the galactic supernova rate (4 × 10-3 yr-1). This implies that a large fraction of all Supernovae take place in binaries with a close low-mass (< 2.3 M⊙) companion.
pp 289-305 June 1995
I review our understanding of the evolution of the spin periods of neutron stars in binary stellar systems, from their birth as fast, spin-powered pulsars, through their middle life as accretion-powered pulsars, upto their recycling or “rebirth” as spin-powered pulsars with relatively low magnetic fields and fast rotation. I discuss how the new-born neutron star is spun down by electromagnetic and “propeller” torques, until accretion of matter from the companion star begins, and the neutron star becomes an accretion-powered X-ray pulsar. Detailed observations of massive radio pulsar binaries like PSR 1259-63 will yield valuable information about this phase of initial spindown. I indicate how the spin of the neutron star then evolves under accretion torques during the subsequent phase as an accretion-powered pulsar. Finally, I describe how the neutron star is spun up to short periods again during the subsequent phase of recycling, with the accompanying reduction in the stellar magnetic field, the origins of which are still not completely understood.
pp 307-325 June 1995
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