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The search for a pulsar in supernova 1987A
The search for a pulsar in supernova 1987A G. Chanmugam According to conventional astronomical theory a supernova - a star that explodes, releasing enormous energy - leaves behind a pulsar or a black hole, or is completely disrupted. This article reviews the properties of neutron stars, pulsars, and supernovae and possible mechanisms for their formation.
The discovery of a supernova visible to the naked eye in the Large Magellanic Cloud (LMC), a nearby galaxy, by the Canadian astronomer I. Shelton in 1987 was an extraordinarily fortuitous astronomical event (figure l), since such a bright supernova had not been seen since Kepler’s discovery of one in our own Milky Way galaxy nearly four centuries ago. The detection of neutrinos, from SN 1987A, established the precise time of the explosion and implied that a neutron star was probably formed in the event. Astronomers therefore subsequently searched at various wavelengths of the electromagnetic spectrum for the pulsar, or spinning neutron star, such an explosion was expected to produce. The reported discovery [l] in 1989 of a pulsar emitting visible light with a period of 0.5 milliseconds in the remnant of the supernova stimulated a flurry of theoretical activity in order to understand its rapid spin. However, these observations were recently found to be an instrumental effect due to a television camera used in the observations. Thus the search for the pulsar continues in earnest. Here, after a review of the properties of neutron stars, pulsars, and supernovae we discuss the implications of what the discovery of a pulsar in SN 1987A would have for theories of pulsars and the physics of neutron stars. Neutron
understanding of the basic constituents of matter. Prior to that time only three elementary particles were known, the negatively charged electron, the positively charged proton, and the neutral and massless carrier of electromagnetic radiation the photon. When news of the discovery of the neutron reached the Niels Bohr Institute in Copenhagen, several of its members met to discuss its
impact on physics. During the same evening one of them, the Soviet physicist L. D. Landau, conceived of the idea that there may exist in the universe stars composed predominantly of neutrons. The theory of such stars was examined in more detail by J. R. Oppenheimer and G. M. Volkoff in 1939 who showed how such stars, held together by gravity, could exist [2].
stars and pulsars
J. Chadwick’s discovery of the neutron in 1932 added considerably to our Ganesar Chanmugam, B.A., BSc., Ph.D. Was born’in Colombo in 1939 and has BSc. and B.A. degrees from the University of Ceylon and Cambridge University respectively, and a Ph.D. in Physics from Brandeis University. He is Professor of Physics and Astronomy at Louisiana State University, Baton Rouge. His research, which is particularly concerned with white dwarfs and pulsars, is supported by the National Science Foundation.
EURO-ARTICLE. (see page ii) This article is published in association with Bild der Wissenschat?, Stuttgart. Endowour. New Series, Volume 14, No. 2.1990. 0160-9327190$3.00 + 0.00. Pergamon Press pk. Printed in Great Britain.
Figure 1 This beautiful colour image shows Supernova 1987A and the regions of the Large Magellanic Cloud surrounding it on 27 February 1987 -just three days after the supernova was first detected. The supernova is the bright spot of light with ‘spikes’ radiating from it above and to the right of centre. (The spikes are not real; they are ‘diffraction effects’ caused by the telescope when such a very bright point of light is photographed.) The reddish clouds in the image are large regions of star formation in which the gas glows, powered by ultraviolet radiation from hot, massive stars within them. The largest one in the photograph -the diamondshaped cloud at upper-left - is called the ‘Tarantula Nebula’ (because of its shape) or the ‘30 Doradus Nebula’ (after a hot, bright star within it). It is the largest such cloud in the Local Group - the cluster of about two dozen galaxies that includes our Milky Way and the Large Magellanic Cloud. The Large Magellanic Cloud (including the supernova and gas clouds in this image) is about 170000 light years away from us. Since a light year is the distance light travels in one year, this means that we are seeing an event in this image that actually took place about 170000 years ago. This slide is a composite of three black-and-white photographs taken with different colour filters and then combined to make a colour image. The original photographs were taken using the I-metre Schmidt telescope at the European Southern Observatory at La Silla in Chile. Only about 20” away from the sky’s south pole, Supernova 1987A is too far south to be studied from the large observatories in the northern hemisphere. (Photograph courtesy of the European Southern Observatory.)
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The physics of the interior of a neut- Crab nebula. The nebula is believed to ron star differs considerably from that be the hot gas ejected in the explosion of an ordinary star like the Sun [2]. The and its speed of expansion shows that it sun is a ball of a hot ideal gas consisting was formed essentially at the time of the predominantly of ionized hydrogen and appearance of the ‘guest star’. In 1934 W. Baade and F. Zwicky helium in which its pressure, due to its high temperature, prevents the gas from made the startling suggestion that a collapsing under the pull of gravity. A supernova involves the transition of an neutron star on the other hand is com- ordinary star into a dense compact neutposed primarily of a ‘gas’ of neutrons so ron star. They speculated that the enerdense that the neutrons essentially gy required to power the explosion touch each other. In this case the gas is lO& J or 100 times the energy the Sun said to be degenerate and the pressure would put out in its entire lifetime of depends mainly on its density and only 10 000 million years - may have come very slightly on its temperature. The from the gravitational energy released pressure arises as a result of quantum in the collapse leading to the formation mechanical effects, more specifically the of the tiny neutron star. Subsequent Pauli exclusion principle, because it is studies of the evolution of stars show not possible to put more than two neut- that, most probably, stars with masses rons in the same state. Thus there is an which are greater than about 10 solar effective repulsion between them which masses will eventually become giant prevents them from coming together. stars containing a small dense core and Calculations of the structure of neutron a large dilute envelope of radius compastars made over the last half century rable to the earth-sun distance. In a show that they have massescomparable supernova the core collapses catasto that of the Sun (2 x 103’ Kg) but a trophically to form a neutron star while radius of only about 10 km, or that of an the shock waves produced when the average city. The density of a neutron core bounces eject the outer envelope, star is extraordinarily high, about lOi all in a few seconds. Attempts were times that of water on average, or simi- made in the 1960s to search for the lar to that of ordinary nuclei. Thus a X-rays emitted by the hot neutron star neutron star is like a gigantic nucleus in supernova remnants but the detectors containing about 10sr neutrons whereas were not sufficiently advanced at that the heaviest nuclei under normal condi- time to detect any neutron stars. tions contain only about 100 of them. Over 30 years after neutron stars Neutron stars are very similar to were predicted J. Bell, a research stuanother class of stars known as white dent working under the supervision of dwarfs, whose pressure is due to de- A. Hewish at Cambridge University, generate electrons. White dwarf stars made a serendipitous discovery - one of have massesalso comparable to that of the most important in astronomy in the the Sun but have radii comparable to last quarter century - of periodic radio that of the earth. The average density of signals from a point in the sky which a white dwarf is roughly a million times repeated every 1.3 seconds [4]. Such a that of water: this is extremely large but source is called a pulsar, or pulsating still 100 million times less than that of a radio source, and since then about 500 of them have been discovered. Now, neutron star. If neutron stars exist, where would the existence of stars whose light varied they exist in the universe and how periodically had been known for a long time and in many cases the light variawould one detect such astronomically small stars? The answer is found in tions were intrinsic to the star and not due to external causes such as a binary unexpected sources such as historical records of unusual astronomical events companion star. In the 17th century J. by ancient astronomers, the develop- D. Cassini and others proposed that the ments of modern technology, and luck. light variations arise because of the Thus, in 1054 AD Chinese and rotation of the star which was assumed Japanese astronomers noticed with the to emit light asymmetrically. However, naked eye the appearance of what they later studies showed that these stars called a ‘guest star’ which was visible in were vibrating (or pulsating) so that the the night sky for over a year [3]. It whole star expands and contracts radialshone so fiercely that for a few weeks it ly with a regular period. The typical was visible to the naked eye even in the periods for these variable stars are daytime. What they had observed is hours or days which is much longer than what was observed by Hewish and conow known as a supernova, a powerful explosion in which an ordinary star, workers. Since the pulsation period of a several times more massive than the star is inversely proportional to the Sun, explodes. In 1731 an amateur square root of the density it was clear astronomer J. Bevis, using the optical that a highly dense star such as a white telescope, detected a fuzzy patch or dwarf was necessary to explain the 1.3 nebula at the location where the second period observed for the first Chinese astronomers had seen a guest pulsar. It was not surprising therefore star. It was later found to look like a that some astrophysicists thought that crab, and so came to be known as the this pulsar was a pulsating white dwarf.
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On the other hand, T. Gold proposed in 1968, that pulsars are rotating neutron stars [5]. He argued that the neutron star would be highly magnetized and emit radio waves in a beam, like a lighthouse, and every time the beam passed through the observer, as the star turned, a radio pulse would be detected. This model, which was similar to Cassini’s incorrect model for variable stars, soon became accepted with the discovery of a pulsar in the Crab nebula, since its period of 0.033 seconds was much shorter than the few seconds period of a vibrating or rotating white dwarf. The corresponding value is of order 10m4seconds for a typical neutron star [2]. The use of the word pulsar, which stands for pulsating radio source, is therefore somewhat of a misnomer because the word pulsating had been used earlier in astronomy to signify a star which is vibrating. The detection of the pulsar in the Crab nebula and one with a period of 0.089 secondsin the remnant of another supernova remnant in the constellation Vela, seemed to support the view that supernovae give rise to the formation of pulsars. However, more recent observational and theoretical studies show that not all supernovae give birth to neutron stars as it is possible that the entire star is dispersed without a stellar remnant being left behind or because a black hole is formed instead. By the year 1982 a considerable amount of data had been accumulated about pulsars [6]. It was found that the periods of pulsars, which ranged from 0.033 seconds to about 5 seconds, slowly increase, in agreement with the predictions of Gold that they would slowly spin down as they lose energy. Furthermore it was believed that pulsars are powerful magnets (dipoles) which are inclined to their rotation axes. From the rotation period and rate of change of period it was possible to deduce that they have huge magnetic fields which range between lo’-lo9 Tesla. Note that by comparison the dipolar component of the magnetic fields of the Sun and earth are of order only 10e4 T or 1 gauss, while the highest steady field that can be created in the laboratory is of order only 100 T. Higher steady fields which are desirable in fusion reactors cannot be easily created because the magnetic pressure exceeds that of steel. Thus the pulsars provide a unique opportunity of studying the behaviour of matter in strong magnetic fields which cannot be attained in the laboratory. Statistical analyses of pulsar observations led to the suggestion by J. E. Gunn and J. P. Ostriker in 1970, that as a pulsar ages the intensity of its radio emission weakens because its magnetic field decays exponentially on a time scale of about a few million years [6].
On the other hand, some theorists such as C. F. Michel of Rice University and W. Kundt [7] argue that the evidence is not convincing that magnetic fields of pulsars decay, while Y. Sang and G. Chanmugam have shown in 1987 that there are serious difficulties with all theoretical explanations of field decay [8]. The magnetic field of the X-ray pulsar Her X-l has been determined from its X-ray spectrum and shows that even though the system, which is a binary, is probably over 100 million years old its magnetic field is over 10’ T, implying that its field has not decayed. Millisecond pulsars This was the situation in 1982 when a remarkable discovery [9] was made by D. C. Backer and co-workers, at the University of California at Berkeley, who detected a pulsar (PSR 1937 + 214) with a period of only 1.6 milliseconds in the constellation Vulpecula. The pulsar’ is rotating with a frequency of 642 I-Ix (1 Hz = 1 revolution per second), which corresponds to the musical note E-flat, just over one octave higher in frequency than middle C. It is spinning so fast that its surface is moving at one-tenth of the speed of light. Further observations showed that it was spinning down much more slowly than other pulsars. This implied that its magnetic field is only about lo4 T, a factor of 10 000 less than that of a typical pulsar. If the pulsar had a much higher field, it would radiate energy more quickly and spin down more quickly than observed. Why does this pulsar spin so quickly and how was it formed? According to the widely, but not unequivocally, accepted view pulsars are not born spinning as fast as they possibly can without breaking up, with a period of about 1 millisecond, but more slowly with a period of about 100 milliseconds to 1 second and then slow down gradually over long timescales. This may be seen by noting that if all pulsars were born spinning as rapidly as possible, then because they slow down one would expect to see many pulsars with periods in the range of 10 to 100 milliseconds, which is not the case. Hence, Radhakrishnan and Srinivasanm and M. A. Alpar and co-workers [8] proposed in 1982 that the millisecond pulsar belonged to a different class of pulsars. They suggested it was not born spinning fast, at a rate close to the break-up rate for neutron stars, but instead was born like ordinary pulsars with a relatively long period of perhaps 0.1 to 1 seconds in a binary star system. A neutron star in a binary may under certain conditions - which depend on the separation between the two stars, the masses of the two stars, and the evolutionary status of the companion attract and accrete matter from the companion star. The matter first swirls
around the neutron star in an accretion disk which_&.@~dis&ne&%ztyfrom it by the magnetic field of the neutron star. The matter is then channelled by the magnetic field on to the magnetic poles of the neutron star where it is heated to high temperatures of over 100 million degrees Kelvin and X-rays. These X-rays are emits observed as pulses with a period corresponding to the rotation period of the neutron star. Such a system is known as an X-ray pulsar and does not emit any detectable radio waves because any radio emission by the neutron star will be absorbed by the gas flowing in the system. Several such systems have been seen. For a steady rate of matter being transferred from the companion the neutron star adjusts its spin rate and reaches a steady rotation period known as the equilibrium period for disk accretion [lo]. This equilibrium period is smaller - that is, the pulsar spins faster if the matter transfer rate is higher or if the magnetic field is weaker. If the magnetic field decays, the inner boundary of the accretion disk moves closer to the neutron star and the equilibrium period decreases, and like an ice skater who draws his or her hands in, the neutron star spins faster and faster. Calculations show that when the magnetic field has decayed to a value of about lo4 T, the equilibrium period of rotation of the neutron star approaches a few milliseconds, provided that the accretion takes place at the maximum allowable rate known as the Eddington rate: if the rate were any higher, the X-radiation from the neutron star would be so high that the matter would be prevented from falling on to it. If the accretion from the companion stops, the radio waves are no longer absorbed and the neutron star will become a millisecond radio pulsar. If the companion can somehow be made to disappear then an isolated millisecond pulsar would be left behind. Some theorists argue that high energy radiation and particles emitted by the fast pulsar may do the job of evaporating the companion. This model for the formation of millisecond pulsars is known as the ‘resurrection’ model since the pulsar first dies and then is spun up and made to come alive (figure 2). On the other hand, in 1978, even before the millisecond pulsar was detected, Brecher and Chanmugam raised the question as to why neutron stars rotate so slowly in the sense that the then known pulsars spun much more slowly than their break up rate. Furthermore, they had much less angular momentum compared to many ordinary stars. They argued that if the core of the progenitor giant star from which a neutron star was formed had a strong magnetic field, it was more likely to be spinning slowly, because the magnetic
Accretion
’ 9
Disk
P-1 ms ;r x-roys
i P-1 ms q
9 rodio woves
Figure 2 A schematic example of the ‘resurrection’ model (not to scale). Initially, a pulsar is assumed to be formed.in a supernova explosion in a binary star system with a period P of about 0.1-l seconds and a magnetic field of 10s T. When the system evolves so that the neutron star can accrete matter from the companion it ceases to be a pulsar because the’radio emission is absorbed by the accreting gas and instead becomes an X-ray pulsar. It then adjusts its period to a value of about 10 seconds. Note that the accretion disk does not reach the surface of the neutron star, but is pushed out at some distance from the star by the neutron star’s magnetic field. After about 10 to 100 million years the field decays, and the accretion disk comes nearer the neutron star which is spun up to a period of a few milliseconds. When the accretion stops the neutron star becomes a binary millisecond pulsar. High energy particles and radiation produced by it evaporates the companion leaving behind a single millisecond pulsar. In the ‘original spin’ model the neutron star is born spinning fast as in the final stage above.
field would have enhanced transfer of angular momentum from the core to the envelope during -earlier evolutionary phases. Hence if the magnetic field in the core was relatively weak it would be more likely to spin fast and yield a weak-field neutron star which is spinning fast. When the millisecond pulsar PSR 1937 + 214 was discovered they [ll] and independently J. Arons and F.
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a neutron star was formed. Neutrinos are believed to be massless, or nearly massless, particles, which interact extremely weakly with matter. Scientists working on underground neutrino experiments in Japan and the United States examined their data for any neutrino bursts that might have been associated with the supernova explosion. Two independent research groups, one known as the IMB collaboration involving scientists from the University of California at Irvine, the University of Michigan, and Brookhaven National Laboratory in the United States, and another in Kamiokande in Japan both reported detecting a pulse of neutrinos at 7.35 am on 23 February 1987 about 24 hours before Shelton’s sighting of the supernova. Of course, the actual explosion must have taken place about 155000 years earlier since neutrinos travel at essentially the speed of light. Although the detection was consistent with the formation of a neutron star it does not conclusively prove this because as W. Hillebrandt, of the Max-Planck Institut fur Astrophysik , in Garching, Supernova 19am The important astronomical discovery West Germany, has remarked it is not of SN 1987A was made by Shelton who possible easily to overrule the possibilwas observing at the Las Campanas ity that a black hole might have been Observatory in Chile. He had photo- formed in the event. Astronomers therefore made valiant graphed the LMC and noticed a very bright star-like image on the plate. searches to detect either the X-rays Being surprised by its brightness he emitted by the hot neutron star that went outside to look at the LMC and may have been formed or the pulsed observed it with the naked eye, con- emission at various wavelengths of the firming his suspicion that he had dis- electromagnetic spectrum in case a pulcovered a supernova. The discovery was sar was formed. The principal difficulty important because the LMC is a nearby in making such a detection is that it is galaxy only 155000 light years away. likely that the radiation would be Astronomers estimate that a super- absorbed by the expanding debris in the nova goes off roughly each second on remnant. Nevertheless, in January average somewhere in the universe, and 1989, an international team of 15 astromany have been discovered. But most nomers claimed to have made an excitof these are in other galaxies and so far ing discovery using the 4-metre teleaway that it would be difficult to discov- scope at Cerro To1010 Inter-American er a newborn pulsar in them. Furth- Observatory in Chile. They reported ermore, no supernova as bright as SN detecting a pulsar flashing visible light 1987A had been seen since Kepler’s with a frequency of 1969Hz, or a period discovery of one in 1604. When news of of about 0.5 milliseconds, at the locathe discovery of SN 1987A was re- tion of SN 1987A in the LMC [l]. The ported, an amateur astronomer observations were made over -a seven McNaught found that he had taken a hour period with the use of highly sophphotograph 20 hours before Shelton isticated power spectral analyses and which showed the bright supernova. supercomputers, since it is difficult to Studies by N. Walbom and collabor- pick out the signal from the pulsar ators, who had observed the LMC be- against the background light coming fore and after the supernova had gone from the supernova. The statistical off, showed that the progenitor star was significance of the spectral power the blue supergiant star Sanduleak-69 appeared to be extremely convincing. 202, which is no longer visible [12]. After the observations of the supernova How such a star can undergo a super- remnant was completed the telescope nova explosion has been discussed by S. was pointed at another object in the sky and no periodic signal was detected, Woosley and T. Weaver [13]. The time of the supernova event was strengthening their claim that the redetermined more accurately than by the ported pulsations from SN 1987A were optical detection in another way. Stu- real and not due to an instrumental dies by many theoretical astrophysicists artefact. Regrettably, they did not go had shown that the collapse of the core back and look at the remnant afterof a giant star would result in the emis- wards, as detection of the same signal sion of bursts of neutrinos, especially if would have further supported their Pacini proposed in 1983 that a millisecond pulsar can be born spinning fast if its magnetic field was weak. Such a model does not require ‘resurrection’ of a dead pulsar and the pulsar is said to be born in ‘original spin’ [lo]. Since that time a total of 9 millisecond pulsars with periods less than 10 milliseconds have been discovered. Of these five are found to be in binary star systems, so that the ‘resurrection’ model became more widely accepted despite difficulties in understanding how the companion can be got rid of. The binary pulsar PSR 1957 + 209, recently discovered by A. Fruchter and coworkers in 1988, contains a neutron star and a very light companion star which shows evidence of matter being ejected from it. Although this seems to support the suggestion of Kluzniak and coworkers that the companion can be evaporated [8], in this particular case the energy loss rate from the pulsar is probably insufficient to evaporate it completely.
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case. No significant pulsations were seen in subsequent observations made on a number of occasions with the lOOinch telescope at Las Campanas observatory in Chile and the 2.3-metre telescope at Siding Springs in Australia. They suggested, therefore, that perhaps the pulsar may have been obscured by moving debris in the supernova remnant. The failure to confirm the pulsar raised doubt as to whether the original observations were correct. The expertise of the group who reported the discovery and the high quality of the data, argued against such a possibility, but as J. Middleditch, a member of the team that made the discovery, pointed out it was possible that a strange quirk of nature had contrived to give spurious results. In any case, this did not prevent a flurry of activity by theorists on interpreting the reported results. Discussion
How does one interpret the basic observation of optical pulsations, if correct, with a period of 0.5 milliseconds. If the period is due to the rotation the neutron star would be spinning three times as fast as the fastest pulsar PSR 1937 + 214 previously known and it is of great interest to know whether neutron stars can rotate that fast without breaking up. In order to determine how fast a neutron star can rotate it is necessary to know what its structure, mass, and radius are. To do so one needs to know the equation of state, or the relationship between pressure and density, of neutron star matter. Unfortunately, because laboratory experiments are not possible at densities very much above nuclear densities, we do not know with any certainty what the equation of state is at such densities, although several equations of state have been proposed. Given an equation of state it is relatively straightforward to determine the mass and radius of a non-rotating neutron star which depends on the assumed value for the density at the centre of the star [2]. The massesof neutron stars, for several such equations of state, have been calculated for non-rotating stars by numerous authors [2]. An equation of state is said to be relatively ‘stiff or ‘soft’ according to whether slight increases in .the density lead to relatively large or small increases of the pressure. Models constructed with stiff equations of state generally lead to non-rotating neutron star models with larger maximum massesand with correspondingly larger radii. The mass of a neutron star can be determined observationally only if it is in a binary system. An analysis of one such binary, PSR 1913 + 16, has led to the precise determination of the mass of the pulsar to be 1.45 solar masses. Such a high mass in this binary cannot be explained by some of the very soft
equations of state which have been proposed. By studying the stability of rotating neutron stars, J. Friedman and coworkers have determined [14], for a given equation of state, how fast a neutron star can spin without breaking up. They find that neutron star models constructed with stiff equations of state are unstable if they rotate with a period of 0.5 millisecond: stable models can be constructed if the period is 1 millisecond. Instead, it appears that a softer equation of state than widely used is required. They therefore concluded that only a narrow range of equations of state can explain both a 0.5 millisecond pulsar and the required mass for PSR 1913 + 16. Hence even more exotic interiors for neutron stars involving strange-quark matter have been proposed [15]. This shows how fast pulsars can be used by scientists to probe the behaviour of matter at high densities. An alternative model for the 0.5 millisecond period optical pulsations was also proposed [ 161where the period was not due to rotation, but to radial vibrations of the neutron star which were excited during its violent birth in the supernova event; the period of 0.5 millisecond is typical for most reasonable masses and radii of neutron stars with preferred equations of state which have been proposed. Unfortunately, at a meeting of the American Association for the Advancement of Science held in February 1990, Middleditch announced a fatal error in the reported discovery of the pulsar. He had found precisely the same signals as those reported in [l] when the telescope was pointing at the Crab pulsar. A careful analysis showed that the false signal was due to a television camera used for image transmission, and that it had not been used when the telescope was off target in the original observations which led to the reported discovery. This was because dawn was break-
ing when that observation was made and the highiy light-sensitive camera may have been damaged [17]. Thus the controls in the experiment were not perfect. This disappointing but courageous retraction raises the question whether all the efforts and newsprint over the pulsar in the past year were in vain. This is not necessarily so, since significant advances in making detections of rapid optical pulsations from sources with a high unpulsed background flux have been made. In addition, it shows that it is not necessary, at least for now, to invoke exotic matter at ultra-high densities such as strange-quark matter in the interiors of neutron stars. What are the important scientific questions which can be resolved by future observations of SN 1987A? First and foremost, it is important to confirm that a neutron star was formed in the explosion. There are several ways in which this can be established. If a pulsar is formed it should, as in the case of the Crab nebula, drive the expansion of the supernova remnant. Hence the overall decline of the luminosity of the supernova remnant should flatten out when the energy input to the remnant from the pulsar becomes dominant. Already there are signs that this is beginning to happen but it is important to establish that this is not due to the energy input from the radioactivity of some of the nuclei formed in the explosion. Another way would be to detect the neutron star from its X-ray emission, which is likely since the surface of the neutron star will be hot. The most exciting prospect would be to detect a radio, optical, or gamma-ray pulsar. This is only possible if the beam of radiation from the pulsar does not miss us as it sweeps around when the neutron star turns. If the pulsar is spinning relatively slowly it would support the resurrection model for millisecond pulsars, while if it was born spinning fast it would show that at least some pulsars can be born in ‘ori-
ginal spin’. In addition, it would clearly be of importance to determine its rate of change of period, as this would enable one to estimate the magnetic field of the neutron star. In particular, it would enable us to decide whether neutron stars are born with strong magnetic fields, as proposed independently by V. Ginzburg and L. Woltjer in 1964 before pulsars were discovered, or whether their magnetic fields are created after they are born, as proposed by Urpin and Yakovlev in 1980 and Blandford, Applegate, and Hernquist in 1983 due to thermoelectric effects [8]. In the meantime, searches for the pulsar continue at wavelengths across the electromagnetic spectrum. References
[l] Kristian, J. etal. Nature, 385,234,1989. [2] Shapiro, S. L. and Teukolsky, S. A. ‘Black Holes, White Dwarfs and Neutron Stars’, Wiley, New York, 1983. [3] Clark, D. H. and Stephenson,F. R. ‘Historical Supernovae’, Pergamon Press,Oxford, 1977. [4] Hewish, A., Bell, S. J., Pilkington, J. D., Scott, P. F. and Collins, R. A. Nature, 217, 709, 1968.
[5] Gold, T. Nature, 218, 731, 1968. [6] Taylor, J. H. and Stinebring, D. R. Ann. Rev. Astr. Ap., 24, 285, 1986. [7] Kundt, W. Comments Ap., 12, 1988. (81 Srinivasan,G. Astr. Ap. Review, 1,209, 1989. [9] Backer, D. C. et al. Nature, 300, 615, 1982. [lo] Ruderman, M., and Shaham,J. Comments Ap., 10, 15, 1983.
[ll] Brecher K. and Chanmugam,G. Nuture, 302, 124, 1983. [12] Walborn, N., Lasker, B. M., Laidler, V. G. and Chu, Y-H. Ap J., 321, 421, 1987. [13] Woosley,S. and Weaver,T. Sci. Am., p. 32, August 1989.
[14] Friedman, J. L., Ipser, J. R., and Parker, L. Phys. Rev. Let., 62,3015, 1989. [15] Glendenning, N. K. Phys. Rev. Let., 63, 2629, 1989.
[16] Wang,Q. et al. Nature, 338, 319, 1989. [17] Waldrop,M. M. Science, 247,910,1990.
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The discovery of a supernova visible to the naked eye in the Large Magellanic Cloud (LMC), a nearby galaxy, by the Canadian astronomer I. Shelton in 1987 was an extraordinarily fortuitous astronomical event (figure l), since such a bright supernova had not been seen since Kepler’s discovery of one in our own Milky Way galaxy nearly four centuries ago. The detection of neutrinos, from SN 1987A, established the precise time of the explosion and implied that a neutron star was probably formed in the event. Astronomers therefore subsequently searched at various wavelengths of the electromagnetic spectrum for the pulsar, or spinning neutron star, such an explosion was expected to produce. The reported discovery [l] in 1989 of a pulsar emitting visible light with a period of 0.5 milliseconds in the remnant of the supernova stimulated a flurry of theoretical activity in order to understand its rapid spin. However, these observations were recently found to be an instrumental effect due to a television camera used in the observations. Thus the search for the pulsar continues in earnest. Here, after a review of the properties of neutron stars, pulsars, and supernovae we discuss the implications of what the discovery of a pulsar in SN 1987A would have for theories of pulsars and the physics of neutron stars. Neutron
understanding of the basic constituents of matter. Prior to that time only three elementary particles were known, the negatively charged electron, the positively charged proton, and the neutral and massless carrier of electromagnetic radiation the photon. When news of the discovery of the neutron reached the Niels Bohr Institute in Copenhagen, several of its members met to discuss its
impact on physics. During the same evening one of them, the Soviet physicist L. D. Landau, conceived of the idea that there may exist in the universe stars composed predominantly of neutrons. The theory of such stars was examined in more detail by J. R. Oppenheimer and G. M. Volkoff in 1939 who showed how such stars, held together by gravity, could exist [2].
stars and pulsars
J. Chadwick’s discovery of the neutron in 1932 added considerably to our Ganesar Chanmugam, B.A., BSc., Ph.D. Was born’in Colombo in 1939 and has BSc. and B.A. degrees from the University of Ceylon and Cambridge University respectively, and a Ph.D. in Physics from Brandeis University. He is Professor of Physics and Astronomy at Louisiana State University, Baton Rouge. His research, which is particularly concerned with white dwarfs and pulsars, is supported by the National Science Foundation.
EURO-ARTICLE. (see page ii) This article is published in association with Bild der Wissenschat?, Stuttgart. Endowour. New Series, Volume 14, No. 2.1990. 0160-9327190$3.00 + 0.00. Pergamon Press pk. Printed in Great Britain.
Figure 1 This beautiful colour image shows Supernova 1987A and the regions of the Large Magellanic Cloud surrounding it on 27 February 1987 -just three days after the supernova was first detected. The supernova is the bright spot of light with ‘spikes’ radiating from it above and to the right of centre. (The spikes are not real; they are ‘diffraction effects’ caused by the telescope when such a very bright point of light is photographed.) The reddish clouds in the image are large regions of star formation in which the gas glows, powered by ultraviolet radiation from hot, massive stars within them. The largest one in the photograph -the diamondshaped cloud at upper-left - is called the ‘Tarantula Nebula’ (because of its shape) or the ‘30 Doradus Nebula’ (after a hot, bright star within it). It is the largest such cloud in the Local Group - the cluster of about two dozen galaxies that includes our Milky Way and the Large Magellanic Cloud. The Large Magellanic Cloud (including the supernova and gas clouds in this image) is about 170000 light years away from us. Since a light year is the distance light travels in one year, this means that we are seeing an event in this image that actually took place about 170000 years ago. This slide is a composite of three black-and-white photographs taken with different colour filters and then combined to make a colour image. The original photographs were taken using the I-metre Schmidt telescope at the European Southern Observatory at La Silla in Chile. Only about 20” away from the sky’s south pole, Supernova 1987A is too far south to be studied from the large observatories in the northern hemisphere. (Photograph courtesy of the European Southern Observatory.)
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The physics of the interior of a neut- Crab nebula. The nebula is believed to ron star differs considerably from that be the hot gas ejected in the explosion of an ordinary star like the Sun [2]. The and its speed of expansion shows that it sun is a ball of a hot ideal gas consisting was formed essentially at the time of the predominantly of ionized hydrogen and appearance of the ‘guest star’. In 1934 W. Baade and F. Zwicky helium in which its pressure, due to its high temperature, prevents the gas from made the startling suggestion that a collapsing under the pull of gravity. A supernova involves the transition of an neutron star on the other hand is com- ordinary star into a dense compact neutposed primarily of a ‘gas’ of neutrons so ron star. They speculated that the enerdense that the neutrons essentially gy required to power the explosion touch each other. In this case the gas is lO& J or 100 times the energy the Sun said to be degenerate and the pressure would put out in its entire lifetime of depends mainly on its density and only 10 000 million years - may have come very slightly on its temperature. The from the gravitational energy released pressure arises as a result of quantum in the collapse leading to the formation mechanical effects, more specifically the of the tiny neutron star. Subsequent Pauli exclusion principle, because it is studies of the evolution of stars show not possible to put more than two neut- that, most probably, stars with masses rons in the same state. Thus there is an which are greater than about 10 solar effective repulsion between them which masses will eventually become giant prevents them from coming together. stars containing a small dense core and Calculations of the structure of neutron a large dilute envelope of radius compastars made over the last half century rable to the earth-sun distance. In a show that they have massescomparable supernova the core collapses catasto that of the Sun (2 x 103’ Kg) but a trophically to form a neutron star while radius of only about 10 km, or that of an the shock waves produced when the average city. The density of a neutron core bounces eject the outer envelope, star is extraordinarily high, about lOi all in a few seconds. Attempts were times that of water on average, or simi- made in the 1960s to search for the lar to that of ordinary nuclei. Thus a X-rays emitted by the hot neutron star neutron star is like a gigantic nucleus in supernova remnants but the detectors containing about 10sr neutrons whereas were not sufficiently advanced at that the heaviest nuclei under normal condi- time to detect any neutron stars. tions contain only about 100 of them. Over 30 years after neutron stars Neutron stars are very similar to were predicted J. Bell, a research stuanother class of stars known as white dent working under the supervision of dwarfs, whose pressure is due to de- A. Hewish at Cambridge University, generate electrons. White dwarf stars made a serendipitous discovery - one of have massesalso comparable to that of the most important in astronomy in the the Sun but have radii comparable to last quarter century - of periodic radio that of the earth. The average density of signals from a point in the sky which a white dwarf is roughly a million times repeated every 1.3 seconds [4]. Such a that of water: this is extremely large but source is called a pulsar, or pulsating still 100 million times less than that of a radio source, and since then about 500 of them have been discovered. Now, neutron star. If neutron stars exist, where would the existence of stars whose light varied they exist in the universe and how periodically had been known for a long time and in many cases the light variawould one detect such astronomically small stars? The answer is found in tions were intrinsic to the star and not due to external causes such as a binary unexpected sources such as historical records of unusual astronomical events companion star. In the 17th century J. by ancient astronomers, the develop- D. Cassini and others proposed that the ments of modern technology, and luck. light variations arise because of the Thus, in 1054 AD Chinese and rotation of the star which was assumed Japanese astronomers noticed with the to emit light asymmetrically. However, naked eye the appearance of what they later studies showed that these stars called a ‘guest star’ which was visible in were vibrating (or pulsating) so that the the night sky for over a year [3]. It whole star expands and contracts radialshone so fiercely that for a few weeks it ly with a regular period. The typical was visible to the naked eye even in the periods for these variable stars are daytime. What they had observed is hours or days which is much longer than what was observed by Hewish and conow known as a supernova, a powerful explosion in which an ordinary star, workers. Since the pulsation period of a several times more massive than the star is inversely proportional to the Sun, explodes. In 1731 an amateur square root of the density it was clear astronomer J. Bevis, using the optical that a highly dense star such as a white telescope, detected a fuzzy patch or dwarf was necessary to explain the 1.3 nebula at the location where the second period observed for the first Chinese astronomers had seen a guest pulsar. It was not surprising therefore star. It was later found to look like a that some astrophysicists thought that crab, and so came to be known as the this pulsar was a pulsating white dwarf.
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On the other hand, T. Gold proposed in 1968, that pulsars are rotating neutron stars [5]. He argued that the neutron star would be highly magnetized and emit radio waves in a beam, like a lighthouse, and every time the beam passed through the observer, as the star turned, a radio pulse would be detected. This model, which was similar to Cassini’s incorrect model for variable stars, soon became accepted with the discovery of a pulsar in the Crab nebula, since its period of 0.033 seconds was much shorter than the few seconds period of a vibrating or rotating white dwarf. The corresponding value is of order 10m4seconds for a typical neutron star [2]. The use of the word pulsar, which stands for pulsating radio source, is therefore somewhat of a misnomer because the word pulsating had been used earlier in astronomy to signify a star which is vibrating. The detection of the pulsar in the Crab nebula and one with a period of 0.089 secondsin the remnant of another supernova remnant in the constellation Vela, seemed to support the view that supernovae give rise to the formation of pulsars. However, more recent observational and theoretical studies show that not all supernovae give birth to neutron stars as it is possible that the entire star is dispersed without a stellar remnant being left behind or because a black hole is formed instead. By the year 1982 a considerable amount of data had been accumulated about pulsars [6]. It was found that the periods of pulsars, which ranged from 0.033 seconds to about 5 seconds, slowly increase, in agreement with the predictions of Gold that they would slowly spin down as they lose energy. Furthermore it was believed that pulsars are powerful magnets (dipoles) which are inclined to their rotation axes. From the rotation period and rate of change of period it was possible to deduce that they have huge magnetic fields which range between lo’-lo9 Tesla. Note that by comparison the dipolar component of the magnetic fields of the Sun and earth are of order only 10e4 T or 1 gauss, while the highest steady field that can be created in the laboratory is of order only 100 T. Higher steady fields which are desirable in fusion reactors cannot be easily created because the magnetic pressure exceeds that of steel. Thus the pulsars provide a unique opportunity of studying the behaviour of matter in strong magnetic fields which cannot be attained in the laboratory. Statistical analyses of pulsar observations led to the suggestion by J. E. Gunn and J. P. Ostriker in 1970, that as a pulsar ages the intensity of its radio emission weakens because its magnetic field decays exponentially on a time scale of about a few million years [6].
On the other hand, some theorists such as C. F. Michel of Rice University and W. Kundt [7] argue that the evidence is not convincing that magnetic fields of pulsars decay, while Y. Sang and G. Chanmugam have shown in 1987 that there are serious difficulties with all theoretical explanations of field decay [8]. The magnetic field of the X-ray pulsar Her X-l has been determined from its X-ray spectrum and shows that even though the system, which is a binary, is probably over 100 million years old its magnetic field is over 10’ T, implying that its field has not decayed. Millisecond pulsars This was the situation in 1982 when a remarkable discovery [9] was made by D. C. Backer and co-workers, at the University of California at Berkeley, who detected a pulsar (PSR 1937 + 214) with a period of only 1.6 milliseconds in the constellation Vulpecula. The pulsar’ is rotating with a frequency of 642 I-Ix (1 Hz = 1 revolution per second), which corresponds to the musical note E-flat, just over one octave higher in frequency than middle C. It is spinning so fast that its surface is moving at one-tenth of the speed of light. Further observations showed that it was spinning down much more slowly than other pulsars. This implied that its magnetic field is only about lo4 T, a factor of 10 000 less than that of a typical pulsar. If the pulsar had a much higher field, it would radiate energy more quickly and spin down more quickly than observed. Why does this pulsar spin so quickly and how was it formed? According to the widely, but not unequivocally, accepted view pulsars are not born spinning as fast as they possibly can without breaking up, with a period of about 1 millisecond, but more slowly with a period of about 100 milliseconds to 1 second and then slow down gradually over long timescales. This may be seen by noting that if all pulsars were born spinning as rapidly as possible, then because they slow down one would expect to see many pulsars with periods in the range of 10 to 100 milliseconds, which is not the case. Hence, Radhakrishnan and Srinivasanm and M. A. Alpar and co-workers [8] proposed in 1982 that the millisecond pulsar belonged to a different class of pulsars. They suggested it was not born spinning fast, at a rate close to the break-up rate for neutron stars, but instead was born like ordinary pulsars with a relatively long period of perhaps 0.1 to 1 seconds in a binary star system. A neutron star in a binary may under certain conditions - which depend on the separation between the two stars, the masses of the two stars, and the evolutionary status of the companion attract and accrete matter from the companion star. The matter first swirls
around the neutron star in an accretion disk which_&.@~dis&ne&%ztyfrom it by the magnetic field of the neutron star. The matter is then channelled by the magnetic field on to the magnetic poles of the neutron star where it is heated to high temperatures of over 100 million degrees Kelvin and X-rays. These X-rays are emits observed as pulses with a period corresponding to the rotation period of the neutron star. Such a system is known as an X-ray pulsar and does not emit any detectable radio waves because any radio emission by the neutron star will be absorbed by the gas flowing in the system. Several such systems have been seen. For a steady rate of matter being transferred from the companion the neutron star adjusts its spin rate and reaches a steady rotation period known as the equilibrium period for disk accretion [lo]. This equilibrium period is smaller - that is, the pulsar spins faster if the matter transfer rate is higher or if the magnetic field is weaker. If the magnetic field decays, the inner boundary of the accretion disk moves closer to the neutron star and the equilibrium period decreases, and like an ice skater who draws his or her hands in, the neutron star spins faster and faster. Calculations show that when the magnetic field has decayed to a value of about lo4 T, the equilibrium period of rotation of the neutron star approaches a few milliseconds, provided that the accretion takes place at the maximum allowable rate known as the Eddington rate: if the rate were any higher, the X-radiation from the neutron star would be so high that the matter would be prevented from falling on to it. If the accretion from the companion stops, the radio waves are no longer absorbed and the neutron star will become a millisecond radio pulsar. If the companion can somehow be made to disappear then an isolated millisecond pulsar would be left behind. Some theorists argue that high energy radiation and particles emitted by the fast pulsar may do the job of evaporating the companion. This model for the formation of millisecond pulsars is known as the ‘resurrection’ model since the pulsar first dies and then is spun up and made to come alive (figure 2). On the other hand, in 1978, even before the millisecond pulsar was detected, Brecher and Chanmugam raised the question as to why neutron stars rotate so slowly in the sense that the then known pulsars spun much more slowly than their break up rate. Furthermore, they had much less angular momentum compared to many ordinary stars. They argued that if the core of the progenitor giant star from which a neutron star was formed had a strong magnetic field, it was more likely to be spinning slowly, because the magnetic
Accretion
’ 9
Disk
P-1 ms ;r x-roys
i P-1 ms q
9 rodio woves
Figure 2 A schematic example of the ‘resurrection’ model (not to scale). Initially, a pulsar is assumed to be formed.in a supernova explosion in a binary star system with a period P of about 0.1-l seconds and a magnetic field of 10s T. When the system evolves so that the neutron star can accrete matter from the companion it ceases to be a pulsar because the’radio emission is absorbed by the accreting gas and instead becomes an X-ray pulsar. It then adjusts its period to a value of about 10 seconds. Note that the accretion disk does not reach the surface of the neutron star, but is pushed out at some distance from the star by the neutron star’s magnetic field. After about 10 to 100 million years the field decays, and the accretion disk comes nearer the neutron star which is spun up to a period of a few milliseconds. When the accretion stops the neutron star becomes a binary millisecond pulsar. High energy particles and radiation produced by it evaporates the companion leaving behind a single millisecond pulsar. In the ‘original spin’ model the neutron star is born spinning fast as in the final stage above.
field would have enhanced transfer of angular momentum from the core to the envelope during -earlier evolutionary phases. Hence if the magnetic field in the core was relatively weak it would be more likely to spin fast and yield a weak-field neutron star which is spinning fast. When the millisecond pulsar PSR 1937 + 214 was discovered they [ll] and independently J. Arons and F.
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a neutron star was formed. Neutrinos are believed to be massless, or nearly massless, particles, which interact extremely weakly with matter. Scientists working on underground neutrino experiments in Japan and the United States examined their data for any neutrino bursts that might have been associated with the supernova explosion. Two independent research groups, one known as the IMB collaboration involving scientists from the University of California at Irvine, the University of Michigan, and Brookhaven National Laboratory in the United States, and another in Kamiokande in Japan both reported detecting a pulse of neutrinos at 7.35 am on 23 February 1987 about 24 hours before Shelton’s sighting of the supernova. Of course, the actual explosion must have taken place about 155000 years earlier since neutrinos travel at essentially the speed of light. Although the detection was consistent with the formation of a neutron star it does not conclusively prove this because as W. Hillebrandt, of the Max-Planck Institut fur Astrophysik , in Garching, Supernova 19am The important astronomical discovery West Germany, has remarked it is not of SN 1987A was made by Shelton who possible easily to overrule the possibilwas observing at the Las Campanas ity that a black hole might have been Observatory in Chile. He had photo- formed in the event. Astronomers therefore made valiant graphed the LMC and noticed a very bright star-like image on the plate. searches to detect either the X-rays Being surprised by its brightness he emitted by the hot neutron star that went outside to look at the LMC and may have been formed or the pulsed observed it with the naked eye, con- emission at various wavelengths of the firming his suspicion that he had dis- electromagnetic spectrum in case a pulcovered a supernova. The discovery was sar was formed. The principal difficulty important because the LMC is a nearby in making such a detection is that it is galaxy only 155000 light years away. likely that the radiation would be Astronomers estimate that a super- absorbed by the expanding debris in the nova goes off roughly each second on remnant. Nevertheless, in January average somewhere in the universe, and 1989, an international team of 15 astromany have been discovered. But most nomers claimed to have made an excitof these are in other galaxies and so far ing discovery using the 4-metre teleaway that it would be difficult to discov- scope at Cerro To1010 Inter-American er a newborn pulsar in them. Furth- Observatory in Chile. They reported ermore, no supernova as bright as SN detecting a pulsar flashing visible light 1987A had been seen since Kepler’s with a frequency of 1969Hz, or a period discovery of one in 1604. When news of of about 0.5 milliseconds, at the locathe discovery of SN 1987A was re- tion of SN 1987A in the LMC [l]. The ported, an amateur astronomer observations were made over -a seven McNaught found that he had taken a hour period with the use of highly sophphotograph 20 hours before Shelton isticated power spectral analyses and which showed the bright supernova. supercomputers, since it is difficult to Studies by N. Walbom and collabor- pick out the signal from the pulsar ators, who had observed the LMC be- against the background light coming fore and after the supernova had gone from the supernova. The statistical off, showed that the progenitor star was significance of the spectral power the blue supergiant star Sanduleak-69 appeared to be extremely convincing. 202, which is no longer visible [12]. After the observations of the supernova How such a star can undergo a super- remnant was completed the telescope nova explosion has been discussed by S. was pointed at another object in the sky and no periodic signal was detected, Woosley and T. Weaver [13]. The time of the supernova event was strengthening their claim that the redetermined more accurately than by the ported pulsations from SN 1987A were optical detection in another way. Stu- real and not due to an instrumental dies by many theoretical astrophysicists artefact. Regrettably, they did not go had shown that the collapse of the core back and look at the remnant afterof a giant star would result in the emis- wards, as detection of the same signal sion of bursts of neutrinos, especially if would have further supported their Pacini proposed in 1983 that a millisecond pulsar can be born spinning fast if its magnetic field was weak. Such a model does not require ‘resurrection’ of a dead pulsar and the pulsar is said to be born in ‘original spin’ [lo]. Since that time a total of 9 millisecond pulsars with periods less than 10 milliseconds have been discovered. Of these five are found to be in binary star systems, so that the ‘resurrection’ model became more widely accepted despite difficulties in understanding how the companion can be got rid of. The binary pulsar PSR 1957 + 209, recently discovered by A. Fruchter and coworkers in 1988, contains a neutron star and a very light companion star which shows evidence of matter being ejected from it. Although this seems to support the suggestion of Kluzniak and coworkers that the companion can be evaporated [8], in this particular case the energy loss rate from the pulsar is probably insufficient to evaporate it completely.
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case. No significant pulsations were seen in subsequent observations made on a number of occasions with the lOOinch telescope at Las Campanas observatory in Chile and the 2.3-metre telescope at Siding Springs in Australia. They suggested, therefore, that perhaps the pulsar may have been obscured by moving debris in the supernova remnant. The failure to confirm the pulsar raised doubt as to whether the original observations were correct. The expertise of the group who reported the discovery and the high quality of the data, argued against such a possibility, but as J. Middleditch, a member of the team that made the discovery, pointed out it was possible that a strange quirk of nature had contrived to give spurious results. In any case, this did not prevent a flurry of activity by theorists on interpreting the reported results. Discussion
How does one interpret the basic observation of optical pulsations, if correct, with a period of 0.5 milliseconds. If the period is due to the rotation the neutron star would be spinning three times as fast as the fastest pulsar PSR 1937 + 214 previously known and it is of great interest to know whether neutron stars can rotate that fast without breaking up. In order to determine how fast a neutron star can rotate it is necessary to know what its structure, mass, and radius are. To do so one needs to know the equation of state, or the relationship between pressure and density, of neutron star matter. Unfortunately, because laboratory experiments are not possible at densities very much above nuclear densities, we do not know with any certainty what the equation of state is at such densities, although several equations of state have been proposed. Given an equation of state it is relatively straightforward to determine the mass and radius of a non-rotating neutron star which depends on the assumed value for the density at the centre of the star [2]. The massesof neutron stars, for several such equations of state, have been calculated for non-rotating stars by numerous authors [2]. An equation of state is said to be relatively ‘stiff or ‘soft’ according to whether slight increases in .the density lead to relatively large or small increases of the pressure. Models constructed with stiff equations of state generally lead to non-rotating neutron star models with larger maximum massesand with correspondingly larger radii. The mass of a neutron star can be determined observationally only if it is in a binary system. An analysis of one such binary, PSR 1913 + 16, has led to the precise determination of the mass of the pulsar to be 1.45 solar masses. Such a high mass in this binary cannot be explained by some of the very soft
equations of state which have been proposed. By studying the stability of rotating neutron stars, J. Friedman and coworkers have determined [14], for a given equation of state, how fast a neutron star can spin without breaking up. They find that neutron star models constructed with stiff equations of state are unstable if they rotate with a period of 0.5 millisecond: stable models can be constructed if the period is 1 millisecond. Instead, it appears that a softer equation of state than widely used is required. They therefore concluded that only a narrow range of equations of state can explain both a 0.5 millisecond pulsar and the required mass for PSR 1913 + 16. Hence even more exotic interiors for neutron stars involving strange-quark matter have been proposed [15]. This shows how fast pulsars can be used by scientists to probe the behaviour of matter at high densities. An alternative model for the 0.5 millisecond period optical pulsations was also proposed [ 161where the period was not due to rotation, but to radial vibrations of the neutron star which were excited during its violent birth in the supernova event; the period of 0.5 millisecond is typical for most reasonable masses and radii of neutron stars with preferred equations of state which have been proposed. Unfortunately, at a meeting of the American Association for the Advancement of Science held in February 1990, Middleditch announced a fatal error in the reported discovery of the pulsar. He had found precisely the same signals as those reported in [l] when the telescope was pointing at the Crab pulsar. A careful analysis showed that the false signal was due to a television camera used for image transmission, and that it had not been used when the telescope was off target in the original observations which led to the reported discovery. This was because dawn was break-
ing when that observation was made and the highiy light-sensitive camera may have been damaged [17]. Thus the controls in the experiment were not perfect. This disappointing but courageous retraction raises the question whether all the efforts and newsprint over the pulsar in the past year were in vain. This is not necessarily so, since significant advances in making detections of rapid optical pulsations from sources with a high unpulsed background flux have been made. In addition, it shows that it is not necessary, at least for now, to invoke exotic matter at ultra-high densities such as strange-quark matter in the interiors of neutron stars. What are the important scientific questions which can be resolved by future observations of SN 1987A? First and foremost, it is important to confirm that a neutron star was formed in the explosion. There are several ways in which this can be established. If a pulsar is formed it should, as in the case of the Crab nebula, drive the expansion of the supernova remnant. Hence the overall decline of the luminosity of the supernova remnant should flatten out when the energy input to the remnant from the pulsar becomes dominant. Already there are signs that this is beginning to happen but it is important to establish that this is not due to the energy input from the radioactivity of some of the nuclei formed in the explosion. Another way would be to detect the neutron star from its X-ray emission, which is likely since the surface of the neutron star will be hot. The most exciting prospect would be to detect a radio, optical, or gamma-ray pulsar. This is only possible if the beam of radiation from the pulsar does not miss us as it sweeps around when the neutron star turns. If the pulsar is spinning relatively slowly it would support the resurrection model for millisecond pulsars, while if it was born spinning fast it would show that at least some pulsars can be born in ‘ori-
ginal spin’. In addition, it would clearly be of importance to determine its rate of change of period, as this would enable one to estimate the magnetic field of the neutron star. In particular, it would enable us to decide whether neutron stars are born with strong magnetic fields, as proposed independently by V. Ginzburg and L. Woltjer in 1964 before pulsars were discovered, or whether their magnetic fields are created after they are born, as proposed by Urpin and Yakovlev in 1980 and Blandford, Applegate, and Hernquist in 1983 due to thermoelectric effects [8]. In the meantime, searches for the pulsar continue at wavelengths across the electromagnetic spectrum. References
[l] Kristian, J. etal. Nature, 385,234,1989. [2] Shapiro, S. L. and Teukolsky, S. A. ‘Black Holes, White Dwarfs and Neutron Stars’, Wiley, New York, 1983. [3] Clark, D. H. and Stephenson,F. R. ‘Historical Supernovae’, Pergamon Press,Oxford, 1977. [4] Hewish, A., Bell, S. J., Pilkington, J. D., Scott, P. F. and Collins, R. A. Nature, 217, 709, 1968.
[5] Gold, T. Nature, 218, 731, 1968. [6] Taylor, J. H. and Stinebring, D. R. Ann. Rev. Astr. Ap., 24, 285, 1986. [7] Kundt, W. Comments Ap., 12, 1988. (81 Srinivasan,G. Astr. Ap. Review, 1,209, 1989. [9] Backer, D. C. et al. Nature, 300, 615, 1982. [lo] Ruderman, M., and Shaham,J. Comments Ap., 10, 15, 1983.
[ll] Brecher K. and Chanmugam,G. Nuture, 302, 124, 1983. [12] Walborn, N., Lasker, B. M., Laidler, V. G. and Chu, Y-H. Ap J., 321, 421, 1987. [13] Woosley,S. and Weaver,T. Sci. Am., p. 32, August 1989.
[14] Friedman, J. L., Ipser, J. R., and Parker, L. Phys. Rev. Let., 62,3015, 1989. [15] Glendenning, N. K. Phys. Rev. Let., 63, 2629, 1989.
[16] Wang,Q. et al. Nature, 338, 319, 1989. [17] Waldrop,M. M. Science, 247,910,1990.
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