pulsar

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pulsar,

in astronomy, a neutron starneutron star,
extremely small, extremely dense star, with as much as double the sun's mass but only a few miles in radius, in the final stage of stellar evolution. Astronomers Baade and Zwicky predicted the existence of neutron stars in 1933.
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 that emits brief, sharp pulses of energy instead of the steady radiation associated with other natural sources. The study of pulsars began when Antony Hewish and his students at Cambridge built a primitive radio telescope to study a scintillation effect on radio sources caused by clouds of electrons in the solar wind. Because this telescope was specially designed to record rapid variations in signals, in 1967 it readily recorded a signal from a totally unexpected source. Jocelyn Bell Burnell noticed a strong scintillation effect opposite the sun, where the effect should have been weak. After an improved recorder was installed, the signals were received again as a series of sharp pulses with intervals of about a second. By the end of 1968 it was clear that the team had discovered a rapidly spinning neutron starneutron star,
extremely small, extremely dense star, with as much as double the sun's mass but only a few miles in radius, in the final stage of stellar evolution. Astronomers Baade and Zwicky predicted the existence of neutron stars in 1933.
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, a remnant of a supernovasupernova,
a massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold. Supernovas are the principal distributors of heavy elements throughout the universe; all elements heavier than
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.

In 1974 the first binary pulsar—two stars, at least one of which is a neutron star, that orbit each other—was discovered by Russell A. Hulse and Joseph H. Taylor, for which they shared the 1993 Nobel Prize in Physics. Using this binary system, they observed indirect evidence of gravitational waves and also tested the general theory of relativity. Several dozen binary pulsars are now known. In 1995 the orbiting Compton Gamma Ray Observatory detected the first object that bursts and pulses at the same time. This bursting pulsar, another class of pulsars, is currently the strongest source of X rays and gamma rays in the sky. Fewer than a dozen bursting pulsars are known to exist.

The intense magnetic field and plasmaplasma,
in physics, fully ionized gas of low density, containing approximately equal numbers of positive and negative ions (see electron and ion). It is electrically conductive and is affected by magnetic fields.
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 that are believed to surround a neutron star provide an effective source of radio waves. The high-energy electrons of the plasma spiral around the magnetic field and emit radio waves and other forms of electromagnetic radiation. This synchrotron radiationsynchrotron radiation,
in physics, electromagnetic radiation emitted by high-speed electrons spiraling along the lines of force of a magnetic field (see magnetism). Depending on the electron's energy and the strength of the magnetic field, the maximum intensity will occur as
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 is highly directional, like a flashlight beam. If the neutron star is rotating, it will act like a revolving beacon and produce the observed pulses. The pulses recur at precise intervals, but successive pulses differ considerably in strength. Since 1968 more than 700 pulsars have been observed, with pulse rates from 4 seconds to 1.5 milliseconds; the very rapid ones are called millisecond pulsars. The interval between pulses decreases ever so slightly with the passage of time, and it is believed that the slower pulsers are the older stars while the rapid pulsers are the younger. Pulsars in the Crab NebulaCrab Nebula,
diffuse gaseous nebula in the constellation Taurus; cataloged as NGC 1952 and M1, the first object recorded in Charles Messier's catalog of nonstellar objects (see Messier catalog).
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 and at the site of the Vela supernova can be detected optically as well as at X-ray and gamma-ray frequencies.

pulsar

(pul -sar) A very regularly ‘pulsing’ source of radiation, which almost certainly originates from a rotating neutron star. Pulsars were originally discovered at radio wavelengths, and a few of these have been detected at optical and gamma-ray wavelengths; pulsars can be powerful gamma-ray emitters (see gamma-ray pulsars) and there is also a class of X-ray pulsators or pulsars.

The first radio pulsar, PSR 1919+21, was detected by Antony Hewish and Jocelyn Bell during a pioneering study of interplanetary scintillation (see scattering). More than 500 radio pulsars are now known. The periods range from 1.56 milliseconds to 4 seconds and can be measured to accuracies of, typically, one part in 1010. The width of the pulse is usually a few per cent of the pulsar's period but can reach 50%. Pulsars with periods shorter than about 0.01 seconds constitute the distinct class of millisecond pulsars. Most pulsars are single, but binary pulsars are now known, about half of which are millisecond pulsars.

The total number of pulsars in the Galaxy is estimated at about 100 000. Their rate of formation is now calculated as maybe 5 per century. Pulsars, together with a smaller number of black holes, are currently thought to originate in supernova explosions. The estimated supernova production rate in the Galaxy is only 2–3 per century. The discrepancy is less than was thought a few years ago, but it may still be necessary to invoke the formation of some pulsars by another means (perhaps the accretion of gas on to a white dwarf).

The received pulses occur when a beam of radio waves, emitted by a rotating neutron star, sweeps past the Earth in an identical manner to the flashes produced by a lighthouse lamp. This beam of radiation is comprised of synchrotron emission; it arises from electrons moving in the neutron star's strong magnetic field (about 108 tesla), whose direction differs from that of the pulsar's rotation axis. There is still dispute over the emission site of the beam: it may be near the star, at the magnetic poles, so that radio waves are beamed down the magnetic axis; alternatively it may be farther out near the speed of light cylinder, where the magnetic field is rotating at almost the speed of light.

Despite their impressive regularity, the periods of all radio pulsars are very gradually increasing as the neutron star loses rotational energy. The central star of the Crab nebula – the youngest known pulsar – is slowing at the rate of one part in a million per day. The slow-down rate of both this pulsar and the young Vela pulsar are occasionally interrupted by glitches (or spin-ups): temporary changes in rotation rate. These are thought to be due to rearrangements of the crust or core of the neutron star. The Crab and Vela pulsars are both optical pulsars. They are also among the brightest gamma-ray sources in the sky. Both optical and gamma-ray luminosity seem to decrease rapidly with a pulsar's age. The Crab pulsar is the fastest of the young pulsars (having a period of 33 milliseconds) but the millisecond pulsars represent a much faster and much older class of pulsar, probably ‘spun-up’ by mass transfer.

X-ray pulsars occur in close binary systems when gas from a companion is channeled on to the neutron star at its magnetic poles: the X-rays appear to pulse on and off as these hot gas patches are exposed by the star's rotation. The gas flow affects the neutron star's spin and as a result all X-ray pulsars are gradually speeding up. Although some have typical pulsar periods (up to a few seconds) there are many ‘slow’ X-ray pulsars (with periods of several minutes), whose rotation must somehow have been strongly ‘braked’.

Pulsar

 

a weak source of cosmic radio emissions, bursts of which repeat with a period that changes extremely slowly. The first pulsar was discovered in 1967 by British astronomers; by 1975 approximately 100 such objects had been detected. Pulsars differ in their radio emission from all previously known cosmic sources, which are characterized either by constant intensity, as are galaxies or radio galaxies, or by irregular bursts, as are the sun and certain flare stars.

The periods of known pulsars, that is, the intervals of time between successive bursts of radiation, range from 0.033 to 3.75 sec. The first observations of pulsars indicated that their periods were constant to an extraordinarily high degree. Subsequent observations, however, established that pulsars have periods that increase extremely slowly. The time required for the periods of most pulsars to double is of the same order of magnitude as the age of the pulsar, amounting to millions and tens of millions of years. However, there are two pulsars whose periods double in substantially less time: the pulsar in the Crab Nebula—a remnant of the Supernova of 1054—whose period doubles every 2,400 years, and the pulsar in the supernova in the constellation Vela, whose period doubles every 24,000 years. These are the youngest pulsars and have the shortest periods. The fact that these pulsars have envelopes characteristic of supernovas favors the hypothesis that they were formed as a result of the explosion of supernovas. Other, older, pulsars lack such envelopes, apparently because the envelopes were able to disperse into space. An interesting feature of young pulsars is the sudden and irregular shortening of the period as a result of violent internal processes. Nearly all pulsars are observed only in the radio band of electromagnetic radiation; the only exception is the pulsar in the Crab Nebula, which can also be detected in the optical, X-ray, and gamma-ray bands.

Studies of the radio emission from pulsars in the radio-wavelength band between 10 cm and 10 m have established that the maximum emission is usually in the meter-wavelength band. It has also been determined that the same pulse is not simultaneously detected in observations at different wavelengths; that is, emissions at the shorter wavelengths reach the earth before emissions at longer wavelengths. This separation of a radio burst occurs because radio emissions with shorter wavelengths propagate through the plasma of interstellar space with a velocity close to that of light in a vacuum, while those with longer wavelengths travel noticeably more slowly. Thus, the time lag of a pulse observed at two distinct wavelengths is proportional to the distance from the observer to the pulsar and the average line-of-sight concentration of electrons. Since this concentration is known, it is possible to calculate the distance to the pulsar and to estimate the strength of the radio emission by measuring the radio emission flux in the direction of the earth and determining the lag time. The distances to currently known pulsars have been found to vary between tens of parsecs and several thousand parsecs; the radio-emission strength of a pulsar is millions of times greater than that of the sun, even during periods of violent solar activity.

The most likely explanation of pulsars is provided by the “lighthouse” theory. According to this theory, a pulsar is a rotating star that emits a narrow beam of radio waves. An observer in the path of the beam detects periodically repeating radio pulses. The theory holds that the pulse period of a pulsar is equal to the rotational period, which explains why the pulse periods are very nearly constant. The lighthouse model also explains many other observed data; in particular, it suggests that the slow increase in the pulse period is a consequence of a slowdown in the rotation of the star.

However, serious difficulties arise in selecting the class of stars that could be responsible for the observed phenomena. The very high angular velocity of rotation characteristic of pulsars is possible only for extremely dense and small stars. White and red dwarfs (dense stars) cannot have such angular velocities of rotation because centrifugal forces would rapidly break them up. Neutron stars, whose existence is postulated only on the grounds of theoretical research, represent the only possible relevant class of stars. Observations of pulsars have thus provided confirmation of the existence of neutron stars.

Neutron stars are characterized by extremely small dimensions; the diameter of a neutron star with a mass approximately that of the sun is only several tens of kilometers. The density of matter within such a star reaches 10l4–1015 g/cm3, that is, approximately the density of matter within atomic nuclei. A neutron star is apparently an immense atomic nucleus consisting chiefly of neutrons. Its kinetic rotational energy is assumed to be the source of the energy emitted by a pulsar. The mechanism by which a pulsar emits energy is associated with strong magnetic fields on the surface of the pulsar, with strengths reaching thousands of billions of oersteds. The transformation of the kinetic rotational energy of a star into emissions is apparently the result of the fact that a rotating magnetic star induces an electric field about itself, which accelerates particles of the surrounding plasma to high energy levels. These accelerated particles produce the observed emissions.

In the 1970’s, pulsars that emit chiefly in the X-ray band were discovered. These pulsars are apparently neutron stars that are part of a binary system; the second component of such systems is a normal star. Gas from the envelope of the normal star flows toward the neutron star, twists around it, and finally falls onto its surface following magnetic lines of force of the neutron star’s field. As a result, directional X-radiation is generated, which is responsible for the pulsations detected by an observer in the path of the directional-radiation beam.

REFERENCE

Dyson, F. and D. Ter Haar. Neitronnye zvezdy i pulsary. Moscow, 1973. (Translated from English.)

V. V. USOV

pulsar

[′pəl‚sär]
(astrophysics)
A celestial radio source, emitting intense short bursts of radio emission; the periods of known pulsars range between 33 milliseconds and 3.75 seconds, and pulse durations range from 2 to about 150 milliseconds with longer-period pulsars generally having a longer pulse duration.

pulsar

any of a number of very small extremely dense objects first observed in 1967, which rotate very rapidly and emit very regular pulses of polarized radiation, esp radio waves. They are thought to be neutron stars formed following supernova explosions
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