Analog SFF, October 2010 by Dell Magazine Authors

Author:Dell Magazine Authors
Language: eng
Format: epub
Publisher: Dell Magazines

Reader's Department: THE ALTERNATE VIEW: PULSAR TIMING AND GRAVITY WAVE DETECTION by John G. Cramer

The first pulsar was observed on November 28, 1967 by Antony Hewish and his graduate student Jocelyn Bell Burnell of the Cavendish Laboratory at Cambridge University in the UK. The object they detected with their radio telescope emitted surprisingly regular bursts of radio waves. They initially called their discovery “LGM-1.” The LGM designation stood for “little green men,” an inside joke based on the speculation that they might be receiving radio signals from an alien civilization. It was subsequently concluded that the radio pulses were the result of a “lighthouse effect” made by radio waves beamed from the magnetic poles of a spinning neutron star, so that the radio beam swept across the detector making regular pulses as the neutron star rotated.

A neutron star is an ultra-dense stellar remnant left behind by a low-mass supernova. (High-mass supernovas leave behind black holes.) A typical neutron star has a mass about 1.4 times that of our sun, but it has a radius of only about 10 kilometers. The large mass and small size give it a density comparable to that of atomic nuclei. Essentially, a neutron star is a giant nucleus consisting almost entirely of neutrons held together by the force of gravity. Neutron stars have about the same angular momentum that their parent star had before the supernova and typically spin very rapidly, about one rotation every 0.0014 seconds or 42,900 RPM. They have polar magnetic fields between 104 and 1010 tesla, much larger than any magnetic field that we are able to produce in our puny Earth-bound laboratories. This huge field, combined with the high spin rate, causes many neutron stars to beam radio pulses (and also visible light and x-ray pulses) as they rotate.

Perhaps the most remarkable thing about neutron-star pulsars, aside from the fact that they exist at all, is that the pulses are so regular in time. They are not perfectly regular, however, because neutron stars spin down, due mainly to the emission of electromagnetic radiation, and lose spin energy and angular momentum as they age. Young neutron stars, created within the last 100,000 years, have the highest spin-down rates and the most rotational “noise” due to internal star-quakes. Older systems, with ages up to 100 million years, have rotational periods that are more stable and reliable.

In 1974 Joseph H. Taylor of Princeton University and his graduate student Russell A. Hulse detected radio pulses from one of a pair of neutron stars that were closely orbiting each other. The timing and Doppler shift of the pulses allowed them to precisely determine the rotation period of the binary system (about eight hours). They made repeated measurements over several years and found that the rotation period of the binary system was increasing because the two stars were slowly “spinning down,” losing a tiny fraction of their rotational energy as they orbited. They showed that this loss of energy was precisely the amount predicted by Einstein's

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