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Progress and Prospects in High Energy Astrophysics* MARTIN J Proc. Nat. Acad. Sci. USA Vol. 72, No. 12, pp..4685-4690, December 1975 H. P. Robertson Memorial Lecture Progress and prospects in high energy astrophysics* MARTIN J. REES Institute of Astronomy, Madingley Road, Cambridge, England The traditional kind of astrophysicist is, in a sense, an "ap- pulsar; a compact neutron star spinning 30 times a second, plied" physicist, who computes models for stars and galaxies emitting a beam of radio and optical emission which swings based on relatively well-understood properties of atoms and through our line of sight once per revolution. Although neu- nuclei, Newtonian gravity, and other branches of classical tron stars remained hypothetical objects until the discovery physics. But recently radio and x-ray observations have re- of pulsars in 1968, their properties had been discussed by vealed some fascinating cosmic objects and phenomena Landau, Zwicky, Oppenheimer, Volkoff, and others in the where the inferred energies, densities, and gravitation field 1930s. Neutron stars are estimated to have masses of the strengths are so extreme that we cannot be confident that we order of that of the Sun, but radii of only 10 or 20 km. From know the relevant physics. The physical assumptions them- these numbers one can estimate their moment of inertia. selves, and not merely the astrophysical models, are then The Crab pulsar is observed to be slowing down, implying vulnerable to observational test; and the astrophysicist can that rotational energy is being lost; and, as was first empha- feel that he has a symbiotic rather than a parasitic relation- sized by Gold, the rate of loss of kinetic energy was just suf- ship with his physicist colleagues. My aim today will be to il- ficient, if channeled efficiently into relativistic particles, to lustrate this theme by describing, in an inevitably sketchy provide the inferred power input into the Nebula. way, some current ideas on neutron stars, black holes, and The discovery of pulsars has stimulated a great deal of de- the early universe. tailed study of the physics of nuclear matter and neutron stars. A slice through a neutron star would in some respects NEUTRON STARS AND PULSARS look rather like a slice through the Earth, in that it would I shall lead into this subject in what may seem to the astrono- display a crust, a liquid interior, and maybe a solid core (ii). mers present to be a very hackneyed way, by discussing the The outer crust would consist mainly of 56Fe, but at higher Crab Nebula-the expanding debris of a supernova explo- densities the main constituents would be progressively more sion witnessed and recorded by Chinese and Japanese as- exotic neutron-rich nuclei such as 62Ni, 82Ge, 124Nb, and tronomers in the year 1054 A.D. Supernova explosions sig- "18Kr. Though unstable in the laboratory, these nuclei are nify the violent endpoint of stellar evolution, when a star too prevented from decaying by the dense Fermi sea of elec- massive to become a white dwarf exhausts its available nu- trons. The nuclei would be in a crystalline lattice. As one clear energy. The core catastrophically implodes, and (by digs still deeper below the crust one reaches densities ap- processes whose details are poorly understood) the outer proaching 1014 g cm-3. Under these conditions the matter is layers are blown off. Filamentary ejecta such as are seen in mainly in the form of free neutrons. Moreover the neutrons the Crab Nebula play a crucial role in the "ecology" of the display superfluid properties, analogous to those displayed Galaxy, because the heavy elements in all stars like the Sun by terrestrial helium at temperatures of a few millidegrees. are believed to have been synthesized in stars that exploded The small fraction of the material in the form of protons early in galactic history, the solar system having condensed (and perhaps the electron gas as well) would constitute a su- from interstellar gas already contaminated by debris from perconductor. In the core of a neutron star, where the densi- such explosions. Supernovae are an important heat input ty may attain values of 1015 g cm-3, the character of the into the interstellar medium and are also believed to be per- material is uncertain, because normally unstable hyperons haps the predominant regions where galactic cosmic rays are may become quite stable under these conditions. Also, it is accelerated. still an important and controversial issue whether, at these The inner part of the Crab Nebula, and the space between extreme densities and pressures, a strongly interacting sys- the filaments, emits polarized continuum radiation, not only tem of baryons can turn into a nuclear solid. in the optical band, but also in the radio and x-ray parts of At the first sight one might fear that little could be the spectrum. (In this respect, the Crab Nebula is morpho- learned about the interior structure of a neutron star by as- logically different from the remnants of several other "his- tronomical observations. However, the timing precision of torical" supernovae-e.g., Tycho's supernova of 1572 and the Crab pulsar is so great that it is possible to detect very Kepler's supernova of 1604-which have a shell structure.) small irregularities in the rate at which it is slowing down. The continuum is interpreted as synchrotron radiation, On at least one occasion, in fact, it appears that the period implying the presence of relativistic electrons with energies underwent a sudden slight decrease. The most natural inter- up to 105 GeV and magnetic fields of a few times 10-4 G (a pretation of this is as follows. As a neutron star spins down it few times 10-8 tesla). The calculated synchrotron lifetimes adjusts its shape from an oblate spheroid to an increasingly are such that these electrons cannot all be survivors of the spherical configuration. (A rotation rate of >1000 revolu- explosion 900 years ago, but must be continually injected tions per second is required for centrifugal breakup, so even into the Nebula. How this could have occurred was a mys- for the Crab pulsar-the one with the shortest known pe- tery until the discovery that the faint central star, which ap- riod-the equatorial flattening is less than 1 part in 103.) If pears quite ordinary on a photographic plate, was actually a t This book contains several detailed up-to-date reviews relevant to neutron star theory and is recommended as a general source of * H. P. Robertson Memorial Lecture presented at the National further information on other topics briefly mentioned in this lec- Academy of Sciences, Washington, D.C., April 1975. ture. 4685 Downloaded by guest on September 27, 2021 4686 Robertson Memorial Lecture: Rees Proc. Nat. Acad. Sci. USA 72 (1975) the star were a fluid, this adjustment would take place con- fields, providing conditions that are propitious for accelerat- tinuously. However, since the crust (and possibly also the ing particles to high individual energies. core) of a neutron star can maintain a certain amount of All other known pulsars are spinning more slowly than the stress, this readjustment takes place in jerks, causing sudden Crab pulsar; they are presumably older, the remnants of the decreases in the moment of inertia. Such is the precision of supernovae that gave birth to them having long ago dis- the observations that a decrease in the equatorial radius of a persed and merged into the diffuse interstellar medium. neutron star by only about 10 gm is readily detectable in the From the estimated number of pulsars in the Galaxy, one timing data. There is some evidence for very small but per- can infer that a substantial fraction of supernovae must form sistent random jitter in the period of the Crab pulsar, and spinning neutron stars. Some supernovae, however, probably this is attributed by some theorists to "microquakes" in- do not; the fact that the remnants of the other "historical" duced by misalignment of the rotation axis and the symme- supernovae look so different from the Crab suggests that try axis-analogously to the Earth's Chandler wobble. Thus, these do not contain a continuing central source of power studies of these inferred starquakes can in principle provide injecting relativistic electrons into them. some of the same information about the interior of neutron It is plainly of great interest to determine the masses of stars as seismology provides about the interior of the Earth; neutron stars. So far, this has not been possible for any pul- it is perhaps in this way that we can best investigate the be- sar. However, two well-studied x-ray sources, Her X-1 and havior of matter at supernuclear densities. Cen X-3, are generally interpreted as neutron stars orbiting Neutron stars also appear to resemble the Earth in that around a (relatively ordinary) companion star, and in these they possess magnetic fields and a magnetosphere: the sur- cases one can in principle obtain mass estimates from the face magnetic field strength is about 1012 G (about 108 Newtonian dynamics of the orbit. In these objects, the emis- tesla). Processes occurring in the magnetosphere are be- sion mechanism for the x-rays is very different from that in lieved to be responsible for the pulsed radiation that we see the Crab pulsar. Instead of being associated with an electro- from pulsars, and also for accelerating the relativistic parti- magnetically-driven relativistic outflow powered by the ro- cles that keep the Crab Nebula shining in the optical and tational kinetic energy, the x-rays from Her X-1 and Cen x-ray continuum. (The magnetic field would be strong X-3 represent thermal radiation by the neutron star surface, enough to enforce corotation of the magnetosphere almost which is heated by the impact of infalling gas accreted from out to the "light cylinder" where the corotation velocity ap- the companion star.
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