The possibility that the degeneracy pressure of neutrons might stabilize a self gravitating object was first noted in the 1930s by T. L.D. Landau and F.Zwicky. W.Baade and F.Zwicky emphasized that they might exist as the stellar remnant of a supernova explosion, interest in neutron stars was aroused by the discovery of X-ray stars in 1962. Because a neutron star is so small (radius of 10 km), in order to radiate energy at a rate comparable to the Sun it needs to have a much higher surface temperature, namely several million degrees. Radiation emitted by material at such a high temperature is X-radiation. It was thought at the time that some of the X-ray stars might be hot. cooling neutron stars. However the variable luminosity of these stars made this hypothesis inconclusive.

The situation changed dramatically with the discovery of PULUSARS (pulsing radio sources) in late- 1967 by astronomers at the Cavendish Laboratory. Cambridge. The first pulsar to be dis-covered. CP1919 (an abbreviation for Cambridge pulsar at 19 hr 19min right ascension), is a radio source which flashes regularly every 1.33730 seconds with each flash lasting only 50 milliseconds. In fact, the flashes are so regular that the pulsar could be used as a clock that is accurate to one part in a hundred million. Since then many more pulsars (about 200) have been dis-covered, with periods ranging from four seconds down to the pulsar in the Crab Nebula which has a period of 33 milliseconds. This last-mentioned pulsar is of special importance, since it proved beyond all reasonable doubt that pulsars are magnetized neutron stars ; the regularity of the pulsed radio signal is provided by the rapid rotation of such stars and only a neutron star can “rotate that rapidly

The Crab pulsar
The supernova that created the Crab Nebula was observed and recorded by oriental astronomers in 1054AD. The radio, optical and X-ray emission from the nebula itself is highly polarized, indicating that the radiation is emitted by the synchrotron process. In a given magnetic field, the more energetic an electron, the higher the frequency of the synchrotron radiation it emits and the faster it loses its energy. The high-energy electrons that give rise to the X-radiation from the nebula, lose their energy within 10 years. This means that there must be in the nebula a source of high-energy electrons that is still operative some 900 years after the supernova explosion. It is this source of energy which provides the necessary 1031 watts to keep the nebula alight.

One star in the remnant, known as Baade’s star and long thought to be of some significance on account of its odd spectrum, was identified as the pulsar in 1969. The star is also seen to pulse at optical frequencies and X-ray energies at the same period of 33 milliseconds. The period is slowly getting longer at a rate such that the interval between successive pulses is about one part in 1012 greater than the preceding one.

The minimum rotational period of a star is of a similar magnitude as its fundamental pulsational period. Both vary inversely as the square root of the star’s average density. Therefore, to get a short period (rapid rotation) we need a dense star. If a white dwarf rotates faster than about once a second, it will tear itself to pieces by centrifugal forces. We see therefore that the Crab pulsar must be a solid body which is over a thousand times denser than a white dwarf; in short it must be a neutron star. In fact, the energy lost by a neutron star that is rotating 30 times a second and that is slowing down with the observed timescale of a few thousand years is precisely the same as that which is required to keep the Crab Nebula glowing. Somehow the rotational energy of the Crab pulsar is being fed into the surrounding nebula, much of it in the form of fast particles which, together with the ambient magnetic field, produce synchrotron radiation.

Pulsar emission
The Sun has a present average surface magnetic field of about 10-4tesla, a rotation period of 26 days, and radius of 700000km. Consider its properties if it were forced to collapse to form a neutron star of radius 10km. Just as a pirouetting ice skater spins up as he brings in his arms, so the Sun would spin up until its rotation period was only 0.4ms. The magnetic field would be com¬pressed to a value of 5 X 105 tesla. The neutron stars that give rise to pulsars are thought to have magnetic fields as high as 108 tesla. As they emit radiation, they lose energy and slow down.

Such a strongly magnetized and rapidly rotating neutron star acts like a rotating bar magnet arid radiates electromagnetic waves at its rotation frequency. The rotating magnetic; field also generates such strong electric fields that electrons and protons can be dragged off the surface of the star, creating currents and setting up a magnetosphere. The particles in the magnetosphere rotate with the neutron star out to the distance at which their velocities approach the velocity of light . There are many un¬solved problems related to the studies of pulsars and it is safe to say that this is a branch of astronomy in which observations far out¬strip the theory. Somehow and somewhere in the rotating magneto-sphere, coherent (laser-like) radio emission is produced. This emission is concentrated into a narrow beam, and, like the flashing of a lighthouse, produces a radio pulse every time the sweeping beam crosses the earth. In addition, very fast (relativistic) par¬ticles are emitted.

The Crab pulsar, which has the shortest period known, is the only one from which pulses have been unambiguously detected right through the electromagnetic spectrum, from radio waves to gamma rays. This may be related to its comparative youth. Pulsed emission diminishes rapidly, particularly at optical and higher frequencies, as a pulsar ages and slows down. Because of this ageing process, most observed pulsars are intrinsically faint arid have been discovered only because they are relatively close to us. There are probably many tens of millions of undetected pulsars in the Galaxy, some of them active, yet with beams that never sweep the Earth and many that are now defunct, or so feeble as to be undetectable by present radio telescopes. Moreover, distant pulsars are difficult to recognize because their characteristic pulsa¬tions become increasingly smeared out by the free electrons in interstellar space.

Neutron starquakes
Because the emission from pulsars is poorly understood theoretically, they tell us little about the nature of neutron stars. Only those pulsars that show occasional abrupt changes in pulse period — termed GLITCHES – as observed in the Crab and Vela pulsars provide such information. These glitches are due to starquakes, or a sudden release of elastic strain in the crust or core of the neutron star. The glitches can be useful tools in revealing the structure of neutron stars, just as seismology tells us about the interior of the Earth.

The glitches in pulse arrival times as seen in several pulsars may be interpreted to reveal the inner structure of the underlying neutron star. The Crab pulsar, for example, probably consists of a thin crust encasing a superfluid interior. Initially this crust is deformed from a truly spherical shape because of the centrifugal forces associated with its rapid rotation. An equatorial bulge forms, just as in the case of the planets Earth and Jupiter. The subsequent spin-down due to the radiation of rotational energy then reduces the centrifugal forces. Being a solid lattice, the crust cannot follow these changes and cracks at some stage. Such star quakes then cause a readjustment of material in the neutron star and a change in the spin rate. The observer detects this as a change in pulse arrival times.

It is only the crust that initially readjusts, because it takes time for the change in spin rate to be spread throughout the star. We can visualize this if we recall that if a cup of coffee is suddenly rotated there is some delay before the coffee begins to rotate. Therefore the sudden glitch is followed by a slower change in pulse rate as the effects of the starquake spread themselves throughout the neutron star. A close study of this can reveal details of the nature of the superfluid region.

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