The current situation regarding pulsar surveys is briefly reviewed. Most of the known pulsars have been found by major radio surveys that were unbiased in the sense of more-or-less uniformly covering a given area of sky. The results from two recent such surveys, the Green Bank 390 MHz survey of Stokes et al. (1985) and the Jodrell Bank 1400 MHz survey of Clifton and Lyne (1986) are compared. Conclusions are drawn regarding the effect of observing frequency on the results of pulsar surveys and the galactic distribution of pulsars and interstellar electrons.

In 1982 a new class of pulsars was defined by the discovery of a star with a millisecond rotation period, 1.6 ms. In the past 3.5 years two additional pulsars with millisecond periods have been discovered. The rapid spin of these pulsars is attributed to mass transfer in a low-mass binary progenitor system. This hypothesis is supported by the presence of companions in two of the three millisecond pulsars. These recent discoveries have led both to a deeper understanding of the final stages of stellar evolution in binary systems, and to closer ties between the observational study of neutron stars by radio, optical and X-ray techniques. In addition the millisecond pulsars provide precise astrophysical clocks that can be used to improve the solar-system ephemeredes and to search for a background of gravitational waves that may have been produced in the early stages of the visible universe. Old and ongoing searches for new millisecond pulsars are described in this paper.

Pulsars have a galactic radial distribution similar to that of many galactic populations such as HII regions, massive stars and supernova remnants. However they are generally much further from the plane of the Galaxy than these objects. Proper motion measurements sho that this is because they are typically moving with high velocities. The measurements also indicate that most pulsars were formed a few million years ago close to the plane, within the normal Population I regions. Some pulsars will escape from the Galaxy, although the majority will end up in a halo population. The origin of the high velocities is not clear at present but may be due either to some asymme try in the formation event or to the disruption of a close binary system.

The interstellar scintillation technique for measuring neutron star speeds is described and results are given for 71 radio pulsars. The mean transverse neutron star speed is 100 km s−1 and the distribution extends to 300 km s−1. The transverse speed correlates with the z velocity derived independently using distance from the galactic plane, consistent with most neutron stars having been born near the galactic plane. A correlation of transverse speed with the quantity PP ∝ (magnetic moment)2 is a general property of the neutron star population. Monte Carlo simulations of the progenitors of neutron stars show that the velocity distribution is inconsistent with the disruption of binary systems solely by symmetric supernova explosions. Either explosions are asymmetric or there are additional accelerations of neutron stars after their formation.

A new survey of the Galactic plane for pulsars has now been completed at Jodrell Bank at the relatively high frequency of 1400 MHz (Clifton, T.R. and Lyne, A.G. 1986, Nature, 320, 43). A further 8 new objects have been detected. The properties of these are summarized below.

Following up on an earlier paper on the local pulsar population (Blaauw 1985; Paper I), in which it was shown that the majority of their progenitors must be sought among the local field stars between 6 and 10 solar masses (types B2, B3) - the local OB associations accounting for a minority only of the pulsar population - we consider implications of two further restrictions: raising the lower limiting mass for neutron star formation, Mn, above 6 solar masses, and raising the beaming factor, f, above 0.3.

The relationship between radio luminosity L and magnetic field B is re-investigated under the assumption that the new spindown mechanism proposed by Huang et al. (1982) becomes dominant at large periods (P ∝ P2). Magnetic dipole braking is still assumed to provide the main torque at short periods (P ∝ P−1). Both spindown torques depend on B2 (assuming proportionality between internal and external fields) so that the resultant magnetic field in this hybrid model is given by (note the difference with the standard expression): A value of 3.6 is used for γ (Pineault 1986). For the radio luminosity we use the values tabulated by Manchester and Taylor (1981).

In usual statistical analyses, because of diversities of proper parameters of pulsars, some interesting features might be smeared. In order to remove these diversities, we use the mean values for all quantities of pulsars, instead of values of individual pulsar, to do statistical analyses. logP/P3 - log τ and logL - logτ have been plotted, here τ P/2P and L denote the characteristic time scale and the radio luminosity of pulsars respectively. The most striking feature is that after its initial dropping to a dip at about τ∼106 yrs, the radio luminosity of pulsar appears to grow up evidently and then redrop again. This feature is difficult to be understood in usual models. However, two tentative interpretations have been given in this paper.

Huang et al have classified pulsars into two types according to the role played by the superfluid neutron vortexes in the interior of neutron stars. They have also presented the formule of P for these two types of pulsars. In this short remark, we shall compare the conclusions about the evolutions of these two types of pulsars with the observational distributions. The consequences obtained are summarized as follws.

Polarized, pulsar profiles consist of two distinct types of emission units, core and conal components. Various configurations of these two basic units then underlie the 5 principal species of profile: conal and core single, conal double, triple, and multiple (5 comp.) profiles. Conal single profiles typically evolve through bifurcation into conal double forms at low frequency; whereas core single profiles evolve into triple forms at high frequency by adding pairs of conal “outriders”. Conal components and outrider pairs exhibit polarization properties compatible with a hollow-conical emission zone which spreads weakly at low frequency. The central core components in each profile species, by contrast, often exhibit circular polarization of symmetrically alternating sense. The triple profile is then most generally prototypical of pulsar emission, and the physical distinction between its outriding conal components and its central core emission feature are primary–implying two distinct corresponding radiation mechanism. (Ap. J. 274, 333/359).

The isolated core components of stars with core single profiles exhibit little ordered modulation, nor do they null or evince of any sort of mode changing; many have featureless fluctuation spectra, while others exhibit low-frequency (P3 = 15–50 periods/cycle) features with which no orderly “drift” is apparently associated. The core components in triple, double (“saddle” region), and multiple profiles also exhibit these stationary, low-frequency fluctuations. “Drifting” subpulses are then an exclusively conal phenomenon. Systematic subpulse modulation (P3 = 2–15 periods/cycle) is associated with the conal components of stars with double, triple, and multiple profiles, but progressive, orderly drifting is observed only in conal single stars. The ensemble P3 values associated with the conal components of the various profile species seem indistinguishable, again suggesting that these species are geometry-specific manifestations of a single physical configuration. Mode changing and pulse nulling are exclusively associated with neither core nor conal emission, but rather connect the two. Mode changing is typically manifest as a reorganization of the conal constituents of emission about the profile's core component. Finally, pulse nulling is not so closely associated with spindown age as has been thought. Pulsars which null are members of older profile species to be sure, but within a given species, those stars which null are no older than those that do not. (1986 Ap. J. 301, 901)

By considering a simple model of slowly rotating neutron stars (for which the Schwartzschild metric could be used properly) whose emission is confined to circular antipolar caps, Pechenick, Ftaclas and Cohen (1983) studied the gravitational effects on the light curves. They found that for typical neutron star parameters, the light curves flattened out due to the bending of light in the neutron stellar gravity.

We estimate the polarization limiting radius, rp1, as a function of rotation period P, magnetic field strength B, and radio frequency v, in radio pulsars assuming plasma parameters that are typical of polar-cap pair-creation models of pulsars. We find that rpl ⋍ 9 × 108(P/1s)0.4 cm, for a surface magnetic field strength of 1012 G, and radio frequency of 109 Hz. For short rotation periods, rpl approaches the light cylinder radius, rlc. Here the magnetic field becomes more azimuthal, and the excursion in position angle over a pulse is less, on average, than when rpl ≪ rlc. With the assumption of a vacuum magnetic field we calculate the polarization position angle as a function of pulse longitude, and the angles i (the angle between the magnetic moment and the rotation axis) and α (the angle between the line of sight and the rotation axis). We calculate the average change in polarization angle as a function of pulsar period, assuming a circular beam, and find consistency with the polarization data summarized by Narayan and Vivekand (1983). We conclude that the evidence is consistent with beams that are roughly constant in shape, providing an alternative to the evolving elliptical beam model of Narayan and Vivekand (1983). This interpretation is further supported by the frequency dependence of the polarization angle in the Crab Pulsar, the frequency of pulsars with double and multiple pulse components, the frequency of pulsars with interpulses, and the absence of pulsars in plerions. See Barnard (1986) for further details.

The polarization angle swing observed in some pulsars, e.g. PSR 0833-45, may be understood as the pattern which results when our line of sight cuts through the magnetic dipole field of a pulsar. In such a geometry the polarization angle χ is a function of μ, the angle between rotation axis and magnetic field axis and Δ, the angle between magnetic field and line of sight to the observer.

A mechanism for generation of two concentric pulsar beams corresponding to the core and conal pulsar emission is proposed. The inner beam originates close to the star, where the radius of curvature of the dipolar magnetic field lines is suitable for coherent curvature emission at pulsar radio frequencies. Further from the star where the outer beam originates, the radius of curvature of dipolar lines is probably too large, but the actual curvature can be dominated by the toroidal component of field lines twisted back due to the pulsar rotation. One can show that for even moderate twisting the actual radius of curvature leads again to pulsar radio frequencies. Thus, the pulsar emission is a superposition of two beams originating at widely radially separated locations. When the observer's line-of-sight cuts both beams, a three-component profile should be observed. Because of the retardational time delay, the inner (core) component should appear late with respect to the profile midpoint, that is, closer to the traling component. Such an asymmetry is indeed observed in complex profile pulsars. In pulsar magnetosphere, the radius of curvature of dipolar field lines depends on the radius of the emission region in the opposite way than that of the toroidal lines. This explains why core and conal components dominate the mean profile at low and high frequencies, respectively.