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ScienceWeek
ASTROPHYSICS: ON MILLISECOND PULSARS
The following points are made by Duncan R. Lorimer (Science 2005 307:855):
1) When a massive star explodes in a supernova, it leaves behind a compact object called a neutron star. The spin of the neutron star and its enormous magnetic field generate a rotating radiation beam. Rather like viewing a lighthouse from a distance, observers on Earth receive pulses of radiation each time the neutron star's beam crosses the line of sight of a radio telescope. The clocklike rotational stability of these objects --referred to as pulsars -- has provided a wealth of insights into general relativity, galactic astronomy, planetary physics, and even cosmology.
2) Recent technological improvements and new instruments are enabling astronomers to find many new pulsars and to probe a rich variety of astrophysical settings. The latest breakthrough is the discovery of 21-millisecond pulsars in the globular cluster Terzan 5 by Ransom et al [1]. The study provides new opportunities for studying the extreme environments in globular clusters and the formation mechanisms for millisecond pulsars.
3) With rotation rates as fast as 642 Hz, millisecond pulsars act as cosmic flywheels that can sustain their rapid rotation on time scales of up to 10 billion years. In the most likely formation mechanism, a neutron star with a spin rate of a few hertz is spun up through the transfer of mass from a binary companion. The substantial heating of the material as it spirals into the strong gravitational field of the neutron star produces x-rays. Such x-ray binaries exist in our own Galaxy, but they are 10 times more common in globular clusters (dense conglomerations of 100,000 or more stars that are packed into a radius of about 10 light-years; they are about 200 times denser than the Milky Way). Globular clusters are excellent breeding grounds for millisecond pulsars, because the extremely high stellar density increases the probability of encounters between cluster members. For example, in a so-called exchange interaction [2], a neutron star or black hole can collide with a binary system and capture the more massive star to form a new binary.
4) Before the new discoveries reported in (1), the record number of pulsars known in a single cluster was 22 in 47 Tucanae [3]. Studies of the pulsars in 47 Tucanae, which have spin periods of 2 to 8 ms, have provided a wealth of information, including the first definite evidence for the existence of gas in a globular cluster (4). As one of the densest and most massive clusters, Terzan 5 has long been thought to harbor many more pulsars than the three found in earlier searches [5]. The main indication for this was the excess radio emission from the core of Terzan 5. Assuming that the radio emission of the putative pulsar population is similar to that known from studies of other pulsars [3], this excess can be explained [8] by the combined emission of up to several hundred pulsars.
References (abridged):
1. S. M. Ransom et al., Science 307, 892 (2005)
2. S. Sigurdsson, E. S. Phinney, Astrophys. J. Suppl. 99, 609 (1995)
3. F. Camilo et al., Astrophys. J. 535, 975 (2000)
4. P. C. C. Freire et al., Astrophys. J. 557, L105 (2001)
5. A. G. Lyne et al., Nature 347, 650 (1990)
Science http://www.sciencemag.org
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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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PULSARS AND WOMEN IN ASTRONOMY
Jocelyn Bell, now known as S. Jocelyn Bell Burnell, was the de facto discoverer of the first pulsar in 1967. She was a graduate student at that time, and although her supervisor Antony Hewish later received a Nobel Prize for the discovery, Jocelyn Bell was not included.
The following points are made by S. Jocelyn Bell Burnell (Science 2004 304:489):
1) Pulsars are phenomenal objects: rapidly rotating neutron stars that send out beams of radio waves which, like lighthouse beams, sweep around the sky as the star rotates. They are amazingly precise timing devices that can be used as clocks for testing relativity theory and may be used for timekeeping and navigation. With a diameter of only about 15 kilometers and a density comparable to that of the nucleus of an atom, they also provide a laboratory for some extreme physics.
2) In the fall of 1967, the author was conducting a routine mapping project studying the radio scintillation of quasars for her doctoral thesis at Cambridge University, under the direction of her adviser, Antony Hewish. Investigation of a puzzling weak signal showed it to be a string of pulses, 1.33 seconds apart. The author and her advisor spent a month trying to find out what was wrong, so unexpected was the signal; and they nicknamed it "Little Green Men" (LGM). At the end of that month, the author found a second pulsar, killing the LGM hypothesis and indicating a new kind of astronomical source.
3) Being a research student, the author had time to understand the instrument, recognize real and spurious signals, and investigate the anomalous or unexpected. Arguably, her student status and perhaps her gender were also her downfall with respect to the Nobel Prize, which was awarded to Professor Antony Hewish and Professor Martin Ryle. At that time, science was still perceived as being carried out by distinguished men leading teams of unrecognized minions who did their bidding and did not themselves contribute other than as instructed. Although the author was not included, she celebrated that first award in 1974 of the Physics Prize for an astronomical discovery. Now she celebrates the fact that we have a better understanding of the teamwork necessary for scientific progress.
4) It took a relatively long time to recognize the first pulsar. However, once that happened, pulsar research rapidly advanced, although often in unexpected ways and in sudden spurts. Following the developments in pulsar research over the past 36 years has given the author immense pleasure. More disappointing have been the developments in the recognition and advancement of women in astronomy. In December 2003, the International Astronomical Union (IAU) published an analysis of its membership. With only 10% of their membership female, the UK and the US fall well below the international average. The only thing that has changed since a similar survey about 5 years ago is that the proportion in most countries has gone up a few percent. Admittedly, it tends to be the more senior astronomers who are IAU members, and there tend to be more women in the junior ranks, but at this rate it will take 50 years until 50% of senior astronomers are female.
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