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ScienceWeek
2004 11 June B2 ASTROPHYSICS: ON BINARY RADIO PULSARS
The following points are made by Duncan Lorimer (Nature 2004 428:900):
1) An exciting discovery in astronomy in recent times was that of the binary system(1) J0737-3039, and its confirmation as a "double-pulsar" system earlier this year(2). Pulsars are rapidly spinning neutron stars that form during the supernova explosions of massive stars. Although their masses tend to be slightly larger than that of our Sun, their radii are only about 15 km. For the first time, both neutron stars in this binary have been identified as radio pulsars -- one that spins about its rotation axis every 22.7 milliseconds (referred to here as "A" and another ("B") that spins with a period of 2.77 seconds. The two stars hurtle around their common center of mass every 2.4 hours, at 0.1% of the speed of light.
2) This duo promise to surpass even the original Nobel-prizewinning pulsar in a binary system(3) as a testing ground for relativity, but they are also a laboratory for studying pulsar emission. The intense magnetic fields of pulsars accelerate charged particles around them, causing the emission of beams of radiation that sweep the sky like the rotating beams of a lighthouse. Already there are intriguing observations(2) of the emission from the double-pulsar system -- in particular that pulsar B seems to emit most strongly in two separate parts of its orbit.
3) The rotation periods of pulsars increase over time, reflecting the loss of rotational kinetic energy of the spinning neutron star as it emits a "wind" of electromagnetic radiation along its emission beams. The difference in spin properties of the neutron stars in the double-pulsar binary means that their winds carry away energy at significantly different rates: the rate of loss of energy from A is some 3000 times greater than that from B. This, and the compactness of the pulsar orbit, implies that the energy carried in the respective winds from A and B is actually balanced inside the emission region of B (2). As a result, the energetics of A can be expected to dominate the system.
4) Jenet and Ransom(4) postulate that the emission from B is somehow stimulated -- jump-started into action -- when the lighthouse beam of A sweeps through B's emission region. These authors make the reasonable assumption that A's beam is a wide, hollow cone1 whose size and opening angle can be determined directly. It is then a relatively straightforward geometrical exercise to show that pulsar B intercepts A's beam at precisely the points of the orbit where increased emission is observed(2). From current observations, the various angles in the system are constrained such that they fit two slightly different solutions of Jenet and Ransom's model.
5) As well as explaining observations, Jenet and Ransom's model makes testable predictions about the past and future visibility of the binary system. This is because the proposed geometry is strongly dependent on the relative orientation between A's emission beam and the line of sight from Earth. This angle varies with time through geodetic precession (a relativistic effect(5) that occurs when the spin axis of an orbiting body is misaligned with the angular momentum axis of the binary system). The perturbing effect of B on the space-time of A causes the spin axis of A to precess around the angular-momentum axis.
References (abridged):
1. Burgay, M. et al. Nature 426, 531-533 (2003)
2. Lyne, A. G. et al. Science 303, 1153-1157 (2004)
3. Taylor, J. H. Rev. Mod. Phys. 66, 711-719 (1994)
4. Jenet, F. A. & Ransom, S. M. Nature 428, 919-921 (2004)
5. Barker, B. M. & O'Connell, R. F. Astrophys. J. 199, L25-L26 (1975)
Nature http://www.nature.com/nature
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ASTRONOMY: A DOUBLE PULSAR
The following points are made by Edward P. van den Heuvel (Science 303:1143):
1) Pulsars, discovered in 1967 by Jocelyn Bell and Anthony Hewish, are rapidly spinning neutron stars whose lighthouse-like beams of radio waves sweep Earth, producing highly regular radio pulses. The steadiness of the pulses makes pulsars very accurate clocks, rivaling the best atomic clocks on Earth. At present, more than 1500 radio pulsars are known in our Galaxy, and a few have been found in nearby galaxies such as the two Magellanic Clouds. Lyne et al (1) recently described two pulsars orbiting each other every 2.4 hours, one of them even briefly eclipsing the radio waves from the other during each orbit.
2) Neutron stars and black holes are the most compact objects known in nature and have the strongest gravitational fields. They are formed by the collapse of the burned-out core of a massive star, the collapse accompanied by a supernova explosion in which the envelope of the star is violently ejected. With a mass some 400,000 times that of Earth and a diameter not larger than that of New York City, a neutron star is essentially a giant atomic nucleus, held together by gravity. The gravitational attraction at its surface is some 11 orders of magnitude greater than on the surface of Earth.
3) Finding an accurate pulsar "clock" orbiting another neutron star is a fantastic gift of nature that provides a unique laboratory for testing with high precision many of the strange predictions of Einstein's theory of general relativity. Among the predictions are that time slows down in a strong gravitational field, that the spacetime around a neutron star is curved, and that accelerated massive bodies emit gravitational waves. All these effects have been verified with high precision in the first binary pulsar system PSR B1913+16, discovered by Hulse and Taylor in 1974. For the measurement of the orbital shrinking of this system due to the emission of gravitational waves (exactly as predicted by Einstein's theory) and for the first time proving the existence of these waves, Hulse and Taylor were awarded the 1993 Nobel Prize in Physics (2). Similarly, the 900,000-km orbit of the new system is expected to be shrinking by about 7 mm per day as a result of the emission of gravitational waves. This effect is expected to be measurable within a few years.
4) In the Hulse-Taylor system as well as in the other half-dozen double neutron stars discovered in the past 30 years, only one of the neutron stars is a pulsar. The conclusion that the unseen other star in these systems is also a neutron star is derived from a variety of indirect arguments -- for example, from the fact that their orbits are elliptic in combination with the theory of binary stellar evolution (3-5). That the other star in the new system is a pulsar confirms these theoretical arguments.
References (abridged):
1. A. G. Lyne et al., Science 303, 1153 (2004)
2. J. H. Taylor, Rev. Mod. Phys. 66, 711 (1994)
3. B. P. Flannery, E. P. J. van den Heuvel, Astron. Astrophys. 39, 61 (1975)
4. L. L. Smarr, R. Blandford, Astrophys. J. 207, 574 (1976)
5. G. Srinivasan, E. P. J. van den Heuvel, Astron. Astrophys. 108, 143 (1982)
Science http://www.sciencemag.org
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ON MASSIVE STARS, NEUTRON STARS, AND PULSARS.
The following points are made by J.F. Hawley and K.A. Holcomb (citation below):
1) The fate of lower-mass stars is to cease fusion after using their available supply of helium, then to eject their outer layers quietly, with the cores left to cool slowly as white dwarfs. Stars more massive than a few solar masses experience more phases at the ends of their lives, going through one nuclear fuel after another to battle the crush of gravity. After the star's helium is exhausted, the core contracts and heats again, and the outer layers expand. In very massive stars, carbon may first ignite; for sufficiently massive stars, increasingly heavy elements are subsequently burned, fusing all the way to iron. The star becomes a gigantic cosmic onion, consisting of concentric shells in which increasingly heavier elements are fused. The final fusion product is iron...
2) A star with an iron core must seek an equilibrium with gravity that does not require further expenditure of energy. Smaller stars could find their final equilibrium in electron degeneracy. However, for any object with a mass greater than 1.4 times the solar mass, the pressure from even electron degeneracy is not sufficient to support the star against its own weight. In 1930, Subramaynyan Chandrasekhar realized that in order to provide the incredible pressures required to maintain more massive stars, the electrons supplying that support would have to move at greater than the speed of light, which was known from the special theory of relativity to be an impossibility. Thus special relativity demands an upper mass limit, today called the "Chandrasekhar limit". If the dying star fails to eject enough of its matter to allow its collapsing core to drop below this limit, the electrons cannot supply the necessary pressure. But if electron degeneracy pressure falls short, the star does not just slowly contract. It collapses catastrophically, sending a shock wave into its outer layers and blowing them off in a single cataclysmic explosion called a "supernova".
3) A supernova which arises from the collapse of a massive star is designated by astronomers as Type II. (This name suggests that there must be another type of supernova, the Type I supernova. The Type I supernova originates from the explosion of a white dwarf in a binary system.).
4) Although a great deal of the star is blown out into interstellar space by a Type II supernova, some fraction is probably left behind in a core remnant. If the mass of the remnant still exceeds the Chandrasekhar limit, what can the star do? It cannot settle down as a white dwarf star; so what remains? As the star collapses to greater and greater compaction, the electrons are squeezed into the atomic nuclei themselves, where they are forced to merge into the protons, forming neutrons. The neutrons, which are much more massive than electrons, can themselves exert a degeneracy pressure known as "neutron degeneracy pressure". The entire star is compressed essentially to the density of an atomic nucleus, but composed only of neutrons. This massive neutron nucleus is known as a "neutron star".
5) A neutron star is astonishingly compact; if an object with the mass of the Sun were to collapse completely to a neutron star, its radius would be only about 10 km, roughly the size of a typical large city on Earth. The neutron star is a remarkable object. Its existence was predicted as early as 1934 by Fritz Zwicky and Walter Baade, although their suggestion was ignored for decades; a neutron star seemed too bizarre to consider. This attitude changed in 1967 when the first pulsar was detected. A pulsar emits highly regular, energetic bursts of electromagnetic radiation, generally as radio waves.
6) The pulses from the first pulsar were so regular that the discoverers, Jocelyn Bell and Anthony Hewish at Cambridge University, first thought they had received signals from another civilization! No familiar astronomical process was known at the time that could produce electromagnetic bursts of such sharpness and regularity, at such a rapid rate. Ordinary oscillations would be inadequate to explain the signal. As more and more pulsars were observed, however, the mystery slowly yielded. Thomas Gold first suggested that pulsars might be associated with the exotic neutron star. Subsequent observations have borne this idea out; no other mechanism is remotely plausible to explain the properties of pulsars.
Adapted from: J.F. Hawley and K.A. Holcomb: Foundations of Modern Cosmology. Oxford University Press 1998, p.132. More information at: http://www.amazon.com/exec/obidos/ASIN/0195104978/scienceweek
ScienceWeek http://scienceweek.com
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