© Winnie Lim, 1995.THE REVEALING PULSES OF PULSARS Introduction Pulsars are some of the most unusual structures in the universe. They proclaim their existence by emitting beams of electromagnetic radiation that appear as pulses, which occur at regular intervals, called periods. No one might have noticed the first known pulsar were it not for the split-second accuracy of its period. Since then, many other pulsars have been discovered. In this research paper, I have drawn on several examples of pulsars that have yielded the most information by their pulses alone, from which much has been learned about the nature of neutron stars, stellar evolution, and the universe itself. These pulsars include the first pulsar discovered, CP1919, the binary pulsar PSR1913+16, the Vela Pulsar (PSR0833-45), and the Crab Pulsar in the Crab Nebula (PSR0531+21), as well as numerous millisecond pulsars that have been discovered in the last decade. The discovery of pulsars Pulsars were first detected - by accident - in 1967 by then-research student Jocelyn Bell (now Dr Burnell), who was scanning the skies above Cambridge, England, with a radio telescope for another cosmic phenomenon, the scintillation (or "twinkling" effect) of quasars and other distant stellar objects. The receiving antenna, adjusted to receive relatively long wavelengths of 3.7m so as to be able to detect faint radio sources, picked up a source of radio energy that appeared in the form of bursts of short, regularly timed pulses. The period between the pulses was a constant 0.715 seconds. The remarkable uniformity of the pulses at first raised the suspicion that they were radio signals from a distant extraterrestrial civilization and were nicknamed LGM (for Little Green Men). However, as Bell and her professor, Antony Hewish, began to detect more sources of these signals, the LGM possibility was discarded in favor of a natural explanation. The sharpness and amplitude of the pulses showed variations of less than 0.0001 s, meaning that the angular size of the pulsing bodies had to be small, less than 30km across; a larger body would emit radiation from different parts of its surface, blurring the pulse signal. Pulsars were initially thought to be rotating white dwarf stars, but it was quickly pointed out that white dwarfs could not spin as rapidly as the radio sources did without flying apart. Dr Franco Pacini and Professor T. Gold, working separately, theorized that neutron stars (the existence of which was predicted in 1933 by Russian physicist Lev T. Landau) were the sources of the pulsing radio energy. Pulsars = neutron stars A pulsar is a rotating neutron star that emits powerful beams of electromagnetic radiation from its two magnetic poles. As the pulsar spins on its axis, the beams sweep over the sky like a lighthouse beacon. If the Earth were in the path of one of the beams, the pulsar would appear to an Earthbound observer to "pulse" - appear and disappear. The nature of a neutron star A neutron star has the mass of the sun, but a radius of only about 10km across, making it one of the densest bodies in the universe. The crushing pressure generated by the star's contraction forces the protons and electrons of the atoms in the star to combine, forming neutrons. These neutrons, further pressed together by the contraction of the star, form a degenerate Fermi gas which generates extremely strong Fermipressure that counteracts the star's gravity and keeps it from falling in completely on itself. A neutron star is believed to have a solid crust made up of a crystalline lattice a few hundred meters thick. Beyond that limit, crushing pressures in the star's interior causes the internal matter to exist in the form of a superconducting "neutron liquid". Neutron liquid is, strictly speaking, not a liquid, but a "superfluid", made up of bound neutron pairs, all of which have the same energy state, which is the lowest (because, as neutrons, they have no charge and therefore no actual energy state to speak of). This superfluid behaves like a liquid, but its behavior is governed by quantum mechanics - since all its bound neutron pairs, or particles, have the same energy states, the particles do not move by exchanging energies (as ordinary liquid particles do) and therefore the superfluid has no resistance, or viscosity. Due to this property, the neutron liquid, when spun, moves in a specialized manner. The superfluid contains a number of regularly-spaced cylinder-shaped vortices that run parallel to the pulsar's spin axis. These vortices repel each other and are arranged in a honeycomb-like array and, all together, cause the superfluid to act like a normal rotating liquid. The theory for the existence of the vortices was based on observations of the Crab pulsar's pulse arrival times - the slow variations detected in this pulsar's signals "are believed to be oscillations of its whole superfluid core relative to the surrounding outer shell"1, according to Russian theorist V.K. Tkachenko, who predicted that superfluids would behave in this manner. The neutron liquid also contains free electrons and protons, that are believed to be the source of the synchrotron radiation in the Crab Nebula. Pulsars with larger masses, such as the Vela Pulsar, may have solid cores at their center. It is thought that solid cores may exist in denser neutron stars with a density of roughly 1015gcm^-3. The superfluid interior of neutron stars are believed to be connected to the variations in period that pulsars sometimes exhibit. The pulse variation most closely associated with changes in the interior is the glitch, which is a sudden speed-up in the rotation of the neutron star. The first recorded pulsar glitch occurred in the Vela pulsar in 1969 and had the effect of advancing the arrival time of pulses by 0.2 microseconds, a few days after which it returned to its regular routine. Some explanations for the cause of glitches include "starquakes", and the movement of the liquid interior relative to the crust of the neutron star. The 1969 Vela glitch was attributed to a starquake, in which the outer crust cracked due to the strain placed on it by the star's diminishing centrifugal force caused by its gradual spin-down. In order to conserve angular momentum, the broken crust speeds up, skimming over the superfluid underneath and transferring its excess energy to the liquid. The other explanation, used to account for later glitches in the Crab pulsar, involves the vortices in the neutron liquid; the vortices nearer the crystalline lattice of the crust get "pinned" to nuclei in the crust while the star's rotation is slowing down. After some time, the trapped vortices break away abruptly from the crust, and the resulting transfer of angular momentum to the crust causes the star's rotation rate to accelerate. Given the small size of neutron stars, their existence may not have been known were it not for the fact that some of them are pulsars. The ultra-precise periods of the pulsars drew attention to their existence. The electromagnetic radiation pulses of a pulsar originate from the neutron star's poles. As the original star collased and shrank, its magnetic field was intensified enormously. This magnetic field, combined with the star's rapid spinning, effectively turns the star into a powerful dynamo that creates a very strong electrical field around the pulsar. It is this dynamo action that fuels the production of the electromagnetic radiation beams from the poles of the neutron star. The electrical field pulls charged particles out of the neutron star's surface, accelerating them to high velocities and discharging them in narrow beams that radiate out of the pulsar's poles. The origin of a neutron star Walter Baade and Fritz Zwicky predicted in 1933 that supernovae might leave behind core remnants massive enough to collapse into neutron stars. A supernova begins with the collapse of the core of the doomed star, followed by the inner layers that cause a shock wave to start moving out from the core. The outgoing shock wave blasts the outer layers of the star into space, resulting in the explosion that is the supernova. Normally a supernova would destroy its progenitor. However, massive stars of about 10 to 100 solar masses may leave behind a remnant of mass that continues to contract (due to a "backlash" of the explosions that blew out the star's outer layers), resulting in either a neutron star or a black hole. For a supernova remnant to become a neutron star, it has to have a mass of at least 1.4 to 2 - 3 solar masses, or it would collapse into a black hole due to the gravitational pressures generated by the matter of the remnant itself. The process by which a supernova remnant becomes a neutron star takes place over the space of hundreds of years. The first confirmation that neutron stars are formed from the remnant of a supernova came in the form of the Crab pulsar. In 1968, a pulsar with a period of 33 milliseconds was discovered in the Crab Nebula. After further calculations, this pulsar was deduced to be in the exact same spot where, in 1054, Chinese astrologers had seen a "Guest Star", later identified as a supernova, in the constellation Taurus - more than nine hundred years before. The timeframe and the apparent age of the pulsar, based on its period, confirmed that this pulsar was indeed the solid remnant of the exploding "Guest Star" of 1054. Astronomers are currently searching for a pulsar in the debris of the recent supernova SN1987A, but efforts to detect it have been unsuccessful so far. Supernovae are not the only sources of neutron stars, however. A neutron star may also be born in a binary star system. In such a system, the larger of the two stars evolves faster than its partner. As such, the larger star reaches the white dwarf stage first. It then draws material from its still-evolving companion, and, according to Sir Francis Graham Smith in Sky and Telescope (September 1990): the [white dwarf] could gain sufficient material to exceed the so-called Chandrasekhar limit. Above this mass value, about 1.4 Suns, the star cannot support itself against its own gravity and collapses to a neutron star. In some cases, the collapse may occur without a dramatic supernova explosion and without leaving an obvious remnant nebula." (243-244) This theory was developed following the realization that supernovae could not possibly be the reason behind all 400 to 500 currently known pulsars. The spin of things Pulsars are essentially rapidly rotating neutron stars. Most pulsars are characterized by the speed of their spin; this gives rise to some particularly interesting specimens, of which the fastest ones are millisecond pulsars. A pulsar's spin rate was generated when it was first formed. As a body is compressed, its rotational energy increases in order to conserve its rotational angular momentum. Therefore, as the pulsar's progenitor collapses in on itself, the speed of its spin increases. At the same time, the newly-forming pulsar develops a strong magnetic field as a result of the conservation of angular momentum and magnetic flux that went on as the star collapses. Pulsar periods vary widely - the slowest known pulsar, PSR0525+21, has a period of 3.75 seconds. Until recently, the fastest known pulsar was the pulsar PSR0531+21, in the Crab Nebula, with a period of 0.033s, but the recent discovery of so-called "millisecond pulsars", with periods of less than 10 ms, have broken the speed record set by the Crab pulsar. The fastest pulsar discovered so far is the millisecond pulsar PSR1937+21 with a period of 1.56 x 10^-3s. A typical millisecond pulsar has a longer life than a normal pulsar, and its rate of slowdown is also much lower. Many of them occur in binary systems. F. Graham-Smith has speculated that the solitary millisecond pulsars were once part of binary systems themselves, but the partner star was either destroyed or knocked out of orbit by the initial supernova that created the pulsar. Most millisecond pulsars have been found to be located in globular clusters. Millisecond pulsars have been "accused" of causing the evacuation of gas from globular clusters. The failure to detect neutral hydrogen, ionized gases and intracluster molecular gas in galactic globular clusters have led astronomers to speculate that there may be a removal mechanism operating within the galactic disk. In a letter to Nature, D.N. Spergel speculated that relativistic winds generated by the rapid spin of millisecond pulsars may interact with the dense gas in the globular cluster, "sweeping" away the gas generated by stellar mass loss in the cluster. A pulsar's spin will eventually slow down as the pulsar loses energy - that is, its period will increase over time. This has already been observed in several pulsars; however, the rate of slow-down in a pulsar's rotation rate is so small that it goes unnoticed for a very long time. Pulsars and their partners There are 500 known pulsars, and most of them are isolated. However, observations have indicated that about 3% of known pulsars exist in binary systems. The binary nature of these systems is revealed by the variations in their Doppler shifts caused by their orbit around the barycenter of their system. There is a proven link between binary systems and pulsars that emit X-rays. In these systems, the pulsar's partner is a larger star in the final stages of its evolution, with an expanding gaseous envelope. The atmosphere of this giant star is pulled rapidly towards the neutron star by the smaller star's powerful gravitational field. As the gaseous matter falls towards the neutron star, it becomes very hot and produces tremendous energies that are detected by Earth-based telescopes as X-rays.2 The study of binary systems of pulsars has also been instrumental in testing the theory of General Relativity. General Relativity states the existence of gravitational waves (also known as gravitational radiation, or gravitons) that are emitted by any rotating body. These gravitational waves produce an effect known as a gravitational redshift, where "light emitted by atoms in a gravitational field is redshifted to longer wavelengths or lower frequencies."3 Pulsar binary systems prove an ideal testing ground for this theory, since the undeviating periods of pulsars make them excellent timekeepers, thus providing a stable frame of reference on which observers can base their calculations. The gravitational wave emission from the binary system also causes the binary system to lose energy at a constant and predictable rate. These effects of gravitational radiation have been confirmed by observations of the binary pulsar PSR1913+16. Planets around pulsars? In July 1991, Andrew Lyne, a British astronomer, announced that he and his colleagues had observational data indicating the presence of a planet the size of Uranus orbiting the pulsar PSR 1829-10, a neutron star near the center of the Milky Way, with a radius of 0.72 a.u. every six months. They based this conclusion on the discovery of a "small, cyclic variation in the pulse arrival times from the neutron star"4, which they found after analyzing the pulses from the pulsar. A planet orbiting a pulsar would cause the pulsar to move around a center of mass, thus making the pulsar alternately recede from and approach a stationary observer. This causes its pulses to appear to speed up, and then slow down, at regular intervals. However, the astronomers had neglected to allow for the Earth's motion around the sun. The receding and approaching effect was actually caused by the Earth's orbit around the sun, movement which was erroneously attributed to the pulsar. Lyne retracted the news of his discovery at a meeting of the American Astronomical Society in January 1992. However, at that same meeting, Aleksandr Wolszczan announced that he and his colleague Dale A. Frail had discovered evidence of the existence of not one, but two planets orbiting the pulsar PSR 1257+12 - and they had already taken into account the Earth's orbit. PSR 1257+12 is a millisecond pulsar in the Virgo constellation, with an average period of 6.2 milliseconds. Wolszczan noticed that the pulses were not as regular as they should have been, with some pulses arriving early and some later. Frail helped him to determine that the pulse arrival time variations were not random, but had a fairly distinct pattern. Further analysis of this revealed that the pulsar had two definite periods, one 66.6 days and the other 98.2 days. The only explanation they could find was that there were two planetary-mass companions orbiting the pulsar with those same periods. The influence of the planets' motions made the pulsar "execute its own tiny orbit around the system's barycenter, or center of mass."5 On February 25, 1994, Wolszczan announced that it was a "clear-cut case" that the planets existed and were altering each other's orbits by exerting gravitational attraction on each other. There was also additional evidence indicating that there was a third, moon-size object orbiting near the pulsar, and possibly still more orbiting objects.
NOTES 1. Sir Francis Graham-Smith, "Pulsars Today," Sky & Telescope September 1990: 243. 2. Sir Francis Graham-Smith, "The binary and millisecond pulsars," Contemporary Physics 33.3 (1992):165-173. 3. Kaufmann, William J. III, Cosmic Frontiers of General Relativity, 79 4. Richard Tresch Fienberg, "Pulsars, Planets and Pathos," Sky and Telescope, May 1992: 493-495. 5. Ibid.WORKS CITED Aller, Lawrence H. Atoms, Stars and Nebulae. 3rd ed. Cambridge: Cambridge University Press, 1991. Backer, D. and S. Sallmen. Letter. Nature Jul. 2 1992: 24-25. Blandford, R.D. "A brief history of pulsar time." Nature Oct 22 1992: 675. "Distant Planets Confirmed." Omaha World-Herald 26 Feb. 1994: 3. Fienberg, Richard Tresch. "Pulsars, Planets and Pathos." Sky & Telescope May 1992: 493-495. Gibilisco, Stan. Black Holes, Quasars and other Mysteries of the Universe. Pennsylvania: Tab Books Inc, 1984. van den Heuvel and J. van Paradijs. "Fate of the companion stars of ultra-rapid pulsars." Nature Jul. 21 1988: 227-228. Hewish, A., S.J. Bell, J.D.H. Pilkington, P.F. Scott, R.A. Collins. "Observation of a Rapidly Pulsating Radio Source." Nature 24 Feb. 1968: 709-713. Katz, J.I. "Pulsar Structure." Nature 25 May 1989: 263-264. Kauffman, William J. III. Cosmic Frontiers of General Relativity. Link, Bennett, Richard I. Epstein and Kenneth A. Van Riper. "Pulsar glitches as probes of neutron star interiors." Nature 15 Oct. 1992. Lipunov, Vladimir M. Astrophysics of Neutron Stars. Trans. R.S. Wadhwa. New York: Springer-Verlag New York, 1992. Longair, Malcolm S. "Modern Cosmology." Manchester, R.N. and J.H. Taylor. Pulsars. San Francisco: W.H. Freeman and Company, 1977. Manchester, R.N. "Pulsars miss a beat." Nature 25 Jan. 1990: 317-318. Mitton, Simon. The Crab Nebula. London: Faber and Faber Ltd., 1979. Smith, Francis Graham. Pulsars. Cambridge: Cambridge University Press, 1977. ---, "Pulsars Today." Sky and Telescope Sept. 1990: 240-244. ---. "The binary and millisecond pulsars." Contemporary Physics 33.3 (1992):165-173. Spergel, D.N. "Evacuation of gas from globular clusters by winds from millisecond pulsars." Nature 18 Jul. 1991: 221. Walker, Phil and Nina Morgan. "Nuclear matter in a spin." New Scientist 11 Jan. 1992: 34-37.
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