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Extra resources for [Magazine] Scientific American. Vol. 269. No 6

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On a typical observing run, Compton remains pointed in the same direction for two weeks. During that time, COMPTEL and EGRET collect data on one region in the sky, while BATSE continuously monitors gamma rays coming from all directions. In its orbit 400 kilometers above the surface of the earth, Compton completes one circuit every 92 minutes; the earth blocks any given part of the sky for about one half of each orbit. Because OSSE points independently of the other instruments, it can switch to a second target while its Þrst one is obscured by the earth.

The relation between the pulsarÕs period and the rate of deceleration of its rotation can reveal approximately how long ago the pulsar was born in a supernova explosion. To calculate the gamma-ray eÛciency of a pulsar, astronomers divide the objectÕs energy output in gamma rays by the total energy loss of the system as inferred by its changing period. Amazingly, Geminga and PSR 1055-52 seem to emit almost all their energy in the form of gamma rays. How they accomplish this feat is still unknown, but from our gamma-ray point of view, pulsars are objects that improve with age.

Gamma rays cannot be reßected and focused like light, so the lenses and mirrors of conventional telescopes are useless for Compton. Instead the satelliteÕs four instruments rely on technologies borrowed directly from the world of high-energy particle physics. BATSE and OSSE contain detectors composed of sodium iodide. When a gamma ray enters the sodium iodide, it excites the molecules and induces them to emit a ßash of visible light, which is then recorded electronically. COMPTEL senses higher-energy gamma rays by using a layer of liquid gamma-ray-detecting material located above a layer of sodium iodide crystal.

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[Magazine] Scientific American. Vol. 269. No 6


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