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| Pulsar |
This material is crushed together so tightly that gravity overcomes the repulsive force between negatively charged electrons and positively charged protons. The resulting structure of the star is complex, with a solid crystalline crust about half a mile (one kilometer) thick encasing a core of superfluid neutrons and superconducting protons. Above the crust exists both an ocean and atmosphere of much less dense material.
From our earthly vantage point, pulsars appear to pulse with light with each rotation. Their light, like a lighthouse beam, sweeps across the Earth. Some pulsars emit visible light, X-rays, and even gamma rays. All pulsars are neutron stars, but (so far as we know) not all neutron stars are pulsars, because not all neutron stars radiate light (such as radio waves or X-rays) with such steady pulses.
Pulsars are divided into two main categories, isolated pulsars, and binary pulsars. Isolated pulsars (which include most radio pulsars) produce radiation primarily through their rotation, as they gradually slow down and cool off. Their light is generated by electrons caught in the pulsar’s strong magnetic field, concentrated and emitted near the magnetic poles. Thus, much of an isolated pulsar’s visible energy is funneled from the rotating magnetic pole region. The precise location of the beam is debated.
Binary pulsars are those in orbit with a companion star, most often hydrogen-burning stars like our Sun. Such a pulsar can pull over, or accrete, matter from its stellar companion as their orbits bring these objects in close contact with each other. The violent accretion process can heat the gas being transfer and produce X-ray light. The X-ray light from this matter, under the influence of magnetic fields, can also appear to pulsate at the pulsar’s rotation rate. Thus, binary pulsars are often called Xray binaries. The flow of matter from the stellar companion, called the accretion disk, also glows in X-ray light owing to its high temperature. While some X-ray binaries are steady X-ray sources, others are bright only for a few weeks or months at a time, lying dormant for years between outbursts. The X-ray sky is thus highly variable, unlike the visible sky that we see at night.
The process of accretion can speed up the spin of a binary pulsar, since the high-velocity accreting material hits the pulsar at a grazing angle, constantly spinning it faster like a child’s toy top. Rotation rates for such accretion-powered pulsars can reach hundreds of revolutions per second, or nearly once per millisecond. The transfer of material onto the neutron star also slowly cannibalizes the stellar companion so that, over millions of years, enough matter is accreted to completely whittle away a once healthy star.
There are many known isolated millisecond pulsars detected in the radio-wave regime. The fastest known is the pulsar B1937+21, spinning at an astonishing 640 revolutions per second. Scientists believe that these isolated millisecond radio pulsars were once X-ray binary millisecond pulsars that accreted material, spun up, and cannibalized their companions.
The Galaxy is thought to contain about 100,000 pulsars, yet fewer than a thousand are known. Only a small fraction of these are millisecond pulsars. Isolated pulsars are hard to find because they are dim. X-ray binary millisecond pulsars are discovered when they flare up in a rare, sporadic accretion event that lasts only a few weeks.
This explanation can eventually be tested through the direct detection of gravitational waves from these pulsars. Because the effects of gravitational waves are very subtle, however, distorting spacetime to alter the distance between the Earth and the Moon by less than the width of a single atom, they are very difficult to measure. Indeed, gravitational waves have never been directly detected, although there is considerable indirect evidence for their existence. Several experiments are now underway to directly detect gravitational waves. One of these is LIGO, the Laser Interferometer GravitationalWave Observatory, with detectors four kilometers in length located in Hanford, Washington, and Livingston, Louisiana.
The best indirect evidence for the existence of gravitational waves comes from analysis of the orbital decay of a binary radio pulsar by Dr. Russell Hulse and Prof. Joseph Taylor of Princeton University. Hulse and Taylor were awarded the 1993 Nobel Prize in Physics for this work.
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