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ScienceWeek
ASTROPHYSICS: ON OBSERVED PULSARS
The following points are made by R.N. Manchester (Science 2004 304:542):
1) Pulsars are naturally occurring celestial objects whose defining characteristic is that the observed emission is a highly periodic pulse train. For known pulsars, the pulse period lies between 1.5 ms and 11 s. These pulsations probably originate as beamed emission from rotating neutron stars -- tiny stars, composed predominantly of neutrons, that are formed in the supernova explosions that mark the end-point of the evolution of massive stars (1).
2) The large mass and small radius of a neutron star allows rotation at speeds approaching 1000 revolutions per second and also accounts for the extraordinary stability of the periodicity. Pulsars are also characterized by extremely strong magnetic fields, up to 10^(15) G (10^(11) T) in some cases. The combination of rapid rotation and a strong magnetic field means that a pulsar is an efficient dynamo, generating electric fields of 10^(12) V/cm or more near its surface. Charged particles are accelerated to ultrarelativistic energies in these large fields, leading to an electron-positron pair production avalanche and ultimately to the generation of a radiation beam.
3) The electrodynamics of the pulsar magnetosphere are complicated [2), and neither these nor the mechanism responsible for the beamed emission are well understood. Nonetheless, a model in which the radiation is beamed outward from field lines emanating from the magnetic polar caps explains many of the observed properties (3).
4) Although pulsar periods are very stable, they are not constant. All pulsars lose energy, either to magnetic dipole radiation (electromagnetic radiation with a frequency equal to the spin frequency of the neutron star) or to charged particle winds, resulting in a gradual increase in spin period. If the magnetic fields have a dipolar form, then the rate of period increase, or spin-down rate, can be used to estimate the pulsar age and the magnetic field strength. Pulsars with typical periods (~ 1 s) and period time derivatives (~ 10^(-15)) have ages of 10^(6) to 10^(7) years and field strengths at the neutron star surface of 10^(12) G. Pulsars with periods less than 20 ms are known as millisecond pulsars (MSPs). MSPs are also characterized by spin-down rates four to six orders of magnitude less than those of normal pulsars, implying ages of 10^(9) to 10^(10) years and magnetic fields of 10^(8) to 10^(9) G. About two-thirds of all MSPs are in binary systems, whereas fewer than 1% of normal pulsars are binary (4). MSPs probably acquire their short periods through a recycling process in which mass and angular momentum are transferred to an old and slowly rotating pulsar from a binary companion (5).
5) In summary: Pulsars are remarkable clocklike celestial sources that are believed to be rotating neutron stars formed in supernova explosions. They are valuable tools for investigations into topics such as neutron star interiors, globular cluster dynamics, the structure of the interstellar medium, and gravitational physics. Searches at radio and x-ray wavelengths over the past 5 years have resulted in a large increase in the number of known pulsars and the discovery of new populations of pulsars, posing challenges to theories of binary and stellar evolution. Recent images at radio, optical, and x-ray wavelengths have revealed structures resulting from the interaction of pulsar winds with the surrounding interstellar medium, giving new insights into the physics of pulsars. About 1500 pulsars are known. Almost all of these are located within the Milky Way Galaxy, most within the galactic disk.
References (abridged):
1. J. M. Lattimer, M. Prakash, Science 304, 536 (2004)
2. A. Spitkovsky, in IAU Symposium 218, ASP Conference Proceedings, F. Camilo, B. M. Gaensler, Eds., in press http://arxiv.org/abs/astro-ph/0310731
3. V. Radhakrishnan, D. J. Cooke, Astrophys. Lett. 3, 225 (1969)
4. I. H. Stairs, Science 304, 547 (2004)
5. D. Bhattacharya, E. P. J. van den Heuvel, Phys. Rep. 203, 1 (1991)
Science http://www.sciencemag.org
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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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