Discovery Of Pulsar Research Articles

Neutron Stars in Interacting Binary Systems

The early history of the development of our understanding of neutron stars has been described by others, and we summarize it only briefly here. Following an early speculation by Baade & Zwicky ( 1934) soon after the discovery of the neutron in 1932, Oppenheimer & Volkoff(1939) presented a classic analysis of the theoretical viability of neutron stars, based on the general theory of relativity and the nuclear physics that was then known. A number of further theoretical studies were published during the next three decades, but it seemed that neutron stars might be virtually unobservable and thus remain little more than a theorist's curiosity piece. In a prophetic paper, Pacini (1967) noted that a rotating neutron star with a magnetic field whose axis was misaligned with the rotation axis should emit intense magnetic dipole radiation. Almost immediately thereafter (but quite independently), Hewish et al. (1968a,b) announced the discovery of radio pulsars. For a brief period, several theoretical models for the pulsar phenomenon were in contention. However, the discovery of the pulsar NP 0532 in the Crab Nebula (Staelin & Reifenstein i968, Comella et al. 1 969), followed by a notably straightforward set of observations and theoretical arguments, soon clinched the case for neutron stars (Gold 1969a).1 (For a review of these remarkable developments, see Gold 1969b.)

Cosmic-ray acceleration by pulsars

With the discovery of the millisecond pulsars, the expectation that pulsars exist that are spinning close to breakup was confirmed. Thus the nominal pulsar parameters for spin rate (≥ 6000 rad/sec) magnetization (≥ 10 13 gauss) and size (∼10 km) give pole-to-equator electric potentials ≳ 3 × 10 20 V, consistent with the highest observed cosmic ray energies. Accordingly, we reexamine the possibility that pulsars are themselves the primary source of cosmic rays. Early theoretical work suggested that the particle injection energy from pulsars was far less than the pole-to-equator potential. We describe recent developments that suggest that some particles actually are accelerated to much higher energies, which enhances the possible role of pulsars as a source of cosmic rays.

Implications of the millisecond pulsar for neutron star models

We examine the constraints imposed by the high spin rate of PSR 1937+214 on the masses and radii of nonrotating neutron stars that can be spun up to such a short period. Assuming uniform rotation, we find that none of the current viable nuclear equations of state can be ruled out. However, the discovery of a single pulsar with period < or approx. =0.5 ms would rule out all current equations of state. Assuming that PSR 1937+214 was spun up uniformly from a nonrotating progenitor requires that the maximum mass of a nonrotating star in general relativity cannot exceed 12.5 M/sub sun/, independent of any knowledge of the equation of state. This limit decreases linearly with the shortest known period of a pulsar.

General track of pulsar evolution

It is shown that large mulling, small luminosity and a large value of polarization angle variation are, together, a good indication of a pulsar's old age. Selection effects connected with the discovery of pulsars are analyzed. The general track of pulsar evolution, along which the diagram of pulsar emission and the angle between the magnetic and rotation axes decrease, is found.

The second millisecond pulsar

On the principle that no celestial object is unique, the discovery of another millisecond pulsar is overdue. That reported on page 417 has an unexpected interest.

Observations of G320.4–1.2 and Speculations on Supernova Remnant Morphology

The galactic radio source G320.4–1.2 (MSH15–52) consists of several components, the most prominent of which is situated in the north-west quadrant and is associated with the Hα nebula RCW89. Caswell et al. (1981) mapped the source at 1.4 GHz with a resolution of 50″ arc and concluded that it was a single supernova remnant with all components having spectral index α ≈ −0.34. This SNR has become more significant with the recent discovery (Seward and Harnden, 1982) of an X-ray pulsar of period 150 ms at the position (1950) R.A. 15h09m59s.5, Dec. −58°56′57″ near the centre of the remnant and the detection of this pulsar at radio frequencies (Manchester et al., 1982). The pulsar has some similarities to the Crab pulsar in that its period derivative is extremely high and hence its characteristic age low, ∼1570 years, comparable to that of the Crab pulsar. Timing observations (Manchester and Durdin, unpublished) indicate that the pulsar is not a member of a binary system and hence that the pulsed X-ray emission is powered by rotational energy, as in the Crab pulsar.

Corrections

November, page 36—Because of a composition error two lines were transposed from one column of text to another. The affected paragraphs should have read: A number of other processes in space are radio emitters—molecular rotational transitions excited by collisional impact and free–free transitions in ionized gas, to name twoand —radio astronomy as a consequence is both “high energy” and “low energy,” “relativistic” and “chemical,” with a phenomenology extremely rich compared to that of most bands of the electromagnetic spectrum. Radio waves have been detected from nearly every class of object known to astronomy at the time of Jansky's discovery—including the Sun, Moon, planets, stars, gaseous nebulae, supernova remnants and normal galaxies—and radio investigations have revealed entirely new objects without which modern astrophysics would hardly exist: neutron stars, molecular clouds, quasars, radio galaxies, and the cosmic microwave background. The Nobel Prize in physics has been awarded to four radioastronomers who participated in these discoveries—in 1974 to Martin Ryle for work on the extragalactic sources and to Anthony Hewish for the discovery of pulsars (neutron stars), and in 1978 to Arno Penzias and Robert W. Wilson for the discovery of the microwave background. As a result of these accomplishments, radioastronomy has grown into one of the main subdivisions of observational astronomy. Nearly 30% of the observational papers in the leading astronomical journals are based on radio data, and there are large radio observatories in over a dozen countries, with major new facilities under construction or planned by Australia, Britain, Canada, France, Germany, Japan and the United States. page 38—The photo in figure 2 is printed upside down, so that the companion of M51 is in the lower part of the photo instead of at the top. The editors regret these errors.

A millisecond pulsar

The radio properties of 4C21.53 have been an enigma for many years. First, the object displays interplanetary scintillations (IPS) at 81 MHz, indicating structure smaller than 1 arc s, despite its low galactic latitude (−0.3°). IPS modulation is rare at low latitudes because of interstellar angular broadening. Second, the source has an extremely steep (~v^(−2)) spectrum at decametric wavelengths. This combination of properties suggested that 4C21.53 was either an undetected pulsar or a member of some new class of objects. This puzzle may be resolved by the discovery and related observations of a fast pulsar, 1937+214, with a period of 1.558 ms in the constellation Vulpecula only a few degrees from the direction to the original pulsar, 1919+21. The existence of such a fast pulsar with no evidence either of a new formation event or of present energy losses raises new questions about the origin and evolution of pulsars.

Nuclear Compositions in the Inner Crust of Neutron Stars

It is likely that matter in a neutron star crust is compressed by accreting matter and/or by the slowingdown of its rotation after the freezing of thermonuclear equilibrium. The change of nuclear compositions, which takes place during the compression, has been investigated. If the initial species of nuclei is Fe, the charge and the mass number of nuclei decrease as a result of repeated electron captures and successive neutron emissions in the initial stage of compression. The nuclear charge and mass are then doubled by pycnonuclear reactions. The final values of the charge numbers of the nuclei in the inner crust at densities p<10 13• 7 g/cm' are less than 25, which are about one third of those for the conventional cold catalyzed matter. This result reduces the shear modulus of the crust to one half of the conventional value which makes the magnitude of star quakes weaker. § l. lntroduction After the discovery of pulsars, the nuclei in the neutron star matter have been investigated in detail by many authors_v~sl The nuclei may form a crystal lattice and breaking of that lattice may be the explanation of a sudden change in the period of pulsars. Langer et al.,v Bethe, Borner and Sato,'l Baym, Bethe and Pethick 3l (hereafter denoted as BBP), and recently Mackie and Baym4) in­ vestigated the nuclei in the neutron star matter with the aid of extended mass formulae of nuclei. Buchler and Barkat 5l and Barkat, Buchler and Ingber 6) cal­ culated the spatial distributions of neutrons and protons in the neutron star matter with the aid of a Thomas-Fermi model of nuclei. More complicated Hartree-Fock calculations were carried out by Ravenhall, Bennet and Pethick,7l and Negele and Vautherin. 8l Theyv~s) obtained nuclear species as a function of the density of the neutron star matter. However they assumed that the neutron star matter is in its absolute ground state. This state is realized only if the matter has had sufficient time in the preceding hot stages of the star to reach nuclear equilibrium and no density changes occur thereafter. Bisnovatyi-Kogan and Chechetkin 9) discussed the r­ process nucleosynthesis in the neutron star crust assuming the existence of abundant neutrons in its hot stage. They calculated how the r-process proceeds when the density is fixed. Their calculations are essentially a simplified version of the r­ process computations in a strongly degenerate electron sea, 10)~14) in which both of the temperature and the density vary.

A Survey for Sharply Pulsed Emissions

The discovery of pulsars in 1967 marked the watershed of interest in short-time-scale phenomena in radio astronomy. Ionospheric scintillation on time scales of seconds, interplanetary scintillations at tenths of seconds and solar bursts of similar duration had already been studied. But with pulsars individual pulses contained subpulses of width about 10 ms, and later observations of microstructure were to show that structure with scales of 10—100 μs were present. In other areas searches for the 10—100 ms radio pulses expected to accompany the gravitational wave events resulting from stellar collapses were made, and more recently searches have been made for the radio pulse accompanying the explosion of small black holes (Rees 1977). Work in this area is summarized by O’Sullivan et al. (1978) and Phinney and Taylor (1979).

Relativistic experiments in gravitational fields

Experimental investigations in gravitation and relativity theory are reviewed. The state of experiments that test the equivalence principle, the basis of the relativistic theory of gravitation, is discussed. The latest results of measurements of the classical effects of general relativity in the solar system are given and promising programs of similar experiments in the near future are analyzed. The new possibilities for testing gravitational theories provided by the discovery of the binary pulsar are described. Finally, the problem of searching for bursts of gravitational radiation using terrestrial Weber type antennas is discussed.

Did Chinese cosmology anticipate relativity?

Recent interest in historic novae and supernovae, stimulated by the discovery of pulsars, has shown the value of ancient Chinese astronomical observations. But the Chinese also have a long record of “cosmological” theorising; in some cases their old concepts regarding the Universe are remarkably similar to those developed in the West during the twentieth century.

Absorption of high energy heavy nuclei and γ rays at the surface of hot neutron stars

SINCE the discovery of pulsars and their association with rotating magnetic neutron stars, it has been realised that pulsars can efficiently accelerate particles to high energies and effectively contribute to the observed cosmic-ray density in the Galaxy. Acceleration of particles to very high energies by the low frequency electromagnetic waves beyond the light cylinder is one mechanism proposed1,2. Others3–5 have considered the possibility of accelerating particles by the electric field near the surface of the neutron star arising from nonzero E·B. High energy photons are also likely to be produced in association with these particles and these have been observed from the Crab pulsar. Presumably, younger pulsars may radiate larger numbers of more energetic γ rays.

The Origin of Cosmic Rays - New Interest in an Old Question

Abstract After briefly reviewing the experimental facts that seem of direct relevance for the question of the origin of cosmic rays, and after quickly going through the “classical” theories of cosmic ray origin, we discuss in some detail those theories that have been put forward over the past few years since the discovery of pulsars. This discovery brought the first observational evidence for the existence of super-strong magnetic fields in nature.

X-ray Astronomy

Many of the most interesting discoveries about the Universe, which have occurred in the last few decades, have resulted from observations of the sky in light not visible to our eye. To emphasize this point it is only necessary to recall the discovery of the microwave background radiation, still the most convincing evidence for the Big-Bang theory, apart from the discovery of the recession of the galaxies 50 years ago. The discovery of pulsars through radio observations established the existence of neutron stars, which until then had only existed as speculative objects predicted by theoretical astrophysicists to be the end point of stellar evolution for intermediate mass stars. Radio observations first revealed the existence of quasi-stellar objects, now generally believed to be objects at greater distance from us than any known, and therefore offering clues to the early stages of the formation of galaxies and their evolution.

Evolution of the Pulsar Magnetosphere

BEFORE the discovery of pulsars, Hoyle, Narlikar and Wheeler1 reasoned that the magnetic neutron star would have a very tenuous atmosphere. In 1969, Goldreich and Julian2 reached a different conclusion. They argued that the electromagnetic forces near the surface of a rotating magnetic neutron star are so large by comparison with the gravitational forces that charged particles must be pulled out of the star at a sufficient rate for a dense magnetosphere to be generated.

General Relativistic Theory of Rotating Neutron Stars – A Review

Except in cosmology, astrophysicists are used to thinking of general relativistic effects as small (e.g., light bending, perihelion advance, red shift) and have generally left such problems to general relativists. However, the discovery of pulsars (Hewish et al., 1968) may have changed this. Not only is general relativity necessary to treat rotating neutron stars, but relativity was also partly responsible for the elimination of pulsating white dwarfs as pulsar models.

Scattering of Gravitational Radiation by a Schwarzschild Black-hole

THE discovery of pulsars and the general conviction that they are neutron stars resulting from gravitational collapse have strengthened the belief in the possible presence of Schwarzschild black-holes—or Schwarzschild horizons—in nature, the latter being the ultimate stage in the progressive spherical collapse of a massive star. The stability of these objects, which has been discussed in a recent report1, ensures their continued existence after formation. Inasmuch as the infinite redshift associated with it and its behaviour as a one-way membrane make the Schwarzschild horizon at once elusive and intriguing, it is important to explore theoretically all possible modes in which the presence of such a black-hole manifests itself. In what follows, we present a partial summary of some results obtained from an investigation of the scattering of gravitational waves by a Schwarzschild horizon.

35. Stellar Constitution

During the period under review, theoretical work in the field of stellar constitution and evolution of stars has been effectively continued by several teams permanently working in Belgium, Denmark, France, German Democratic Republic, German Federal Republic, Italy, Japan, Poland, the United Kingdom, the U.S.A., the U.S.S.R. as well as by individual scientists in Argentina, Bulgaria, Czechoslovakia, India, Sweden and others. Most attention has been given during the past three years to analysis of late stages of stellar evolution, rotation, evolution of binaries, loss of mass on various stages of evolution, stellar stability, neutrino astrophysics, physical processes in the presupernovae stage. The discovery of pulsars has raised extremely the general interest for white dwarfs and neutron stars, and consequently the number of papers with new hypotheses concerning quasi-stellar sources has substantially dropped.Many new evolutionary sequences of stellar models for various masses and different chemical compositions have been computed using improved numerical techniques and large computing programs. It is regretted that many of these results are difficult to compare mainly because of the differences in the input physics and the way the numerical results are being published. This is a field in which a closer international co-operation is highly desirable, especially on the following points: exchange of ideas on the programs presently under way and the planned programs in order to avoid unnecessary duplications: agreement on the numerical results which should be published; exchange of technical informations: (input physics, computing methods, detailed results of computations, opacity tables, etc.). Several attempts in this direction have been undertaken during the period under review (for example, Commission 35 Circular Letters NN 1–7 containing information on current work, publication by the U.S.S.R. Academy of Sciences of the Cox and Stewart’s Opacity Tables for 23 mixtures, several joint international projects for using the same programs, etc.), but the problem still remains to be solved.

40. Radio Astronomy

Radio astronomy continues to develop at explosive pace. In addition to containing many notable pieces of observational work carried out with improved sensitivity, angular resolution and spectral detail – and aided by data-handling techniques of increasing sophistication – the last three years have seen a flow of new and sometimes startling results. These are exemplified by the discovery of organic matter in the galaxy, and by the first observations pertaining to what are almost certainly neutron stars. The latter, of course, came with the dramatic discovery of pulsars, which are the subject of an Invited Discourse at this General Assembly.Following the custom of this Commission, I have asked different members to review the work done in each of the main fields. On this occasion a special section has been added on pulsars; since the announcement of their discovery in February 1968, the rate of publication within so narrow a field could scarcely have been equalled in the history of science. As in the past, radio studies of the planets are incorporated in the Report to Commission 16.