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Mercury, September/October 2003 Table of Contents

Image courtesy of David Aguilar (Harvard-Smithsonian Center for Astrophysics).

by Dan Reichart

After 30 years of bewilderment, astronomers finally know that collapsing massive stars generate most gamma-ray bursts.

Gamma-ray bursts (GRBs) were first detected in the late 1960s by American military satellites designed to monitor Soviet compliance with the Nuclear Test Ban Treaty. The United States thought the Soviets might violate the treaty by testing nuclear weapons behind the Moon. The satellites detected no clandestine nuclear explosions, but they discovered something far more interesting: powerful bursts of gamma rays emanating from random directions in space. Gamma rays are like X rays, but even more energetic — the highest energy form of light — meaning these bursts were exploding with unimaginable violence. By 1973, 14 GRBs had been discovered, and when Los Alamos scientist Ray Klebesadel announced their existence to the world, the race began to solve one of astronomy’s greatest mysteries.

Almost immediately, astronomers conceived more theories to explain GRBs than the number of detected GRBs — a situation that did not reverse itself until 1992, a year after NASA launched the Compton Gamma-Ray Observatory. Compton’s BATSE instrument, which detected GRBs at a remarkable rate of about one per day, showed that GRBs occur in all directions with equal probability. This ruled out a large number of theories in which GRBs were thought to be explosions associated with objects in the disk of our galaxy, such as neutron stars. But the statistics were not good enough to rule out models in which the GRBs were caused by neutron stars in an extended halo around our galaxy.

BATSE also allowed us to identify two distinct classes of GRBs: long- and short-duration GRBs. Long-duration GRBs, which are more common, emit gamma rays for about 2 seconds to several minutes, and their gamma rays tend to be lower in energy. Much has been learned about these GRBs over the past 6 years. Short-duration GRBs last from tens of milliseconds to about 2 seconds, and their gamma rays tend to be of higher energy. They remain a mystery to this day.

By 1997, the Italian/Dutch BeppoSAX satellite allowed astronomers to pinpoint several long-duration GRBs on the same day they occurred, leading to a revolutionary discovery: GRBs continue to shine from X-rays to radio waves for days, sometimes even months, after the brief gamma-ray phase has ended. Afterglow spectra showed that GRBs occur in galaxies billions of light-years away. For the gamma-ray phase even to be detectable at such great distances, GRBs have to be mind-bogglingly explosive, beating supernovae by six to nine orders of magnitude. Overnight, GRBs claimed the title of "Biggest Bangs Since the Big Bang Itself."