The Mystery of Gamma-Ray Bursts

Black holes likely also reveal their presence by generating powerful bursts of gamma rays. Even more energetic than X rays, gamma rays are in fact the most energetic and penetrating kind of radiation in the electromagnetic spectrum. Like X rays, in high enough doses gamma rays can be lethal to living things; that is why doctors sometimes use small, controlled doses of gamma rays to destroy cancerous tumors.

Also like X rays, gamma rays captured the attention of astronomers in the 1960s. Detectors mounted on American rockets began to record strange, very powerful bursts of gamma rays coming from random parts of the sky. Typical bursts lasted a few seconds, although some flared up and then diminished in less than a second. Because nuclear explosions emit brief bursts of gamma rays, at first American scientists and security personnel worried that the Soviet Union might be conducting secret nuclear tests beyond Earth’s atmosphere.

By 1973, however, scientists had become convinced that the gamma-ray bursts (or GRBs) are a naturally occurring phenomenon originating deep in space. A new burst was detected at least once a week on average. Although the GRBs varied in intensity, some were truly enormous, in the range of a billion trillion times the luminosity of the Sun. Put another way, a big GRB releases as much energy in 10 seconds as the Sun does in 10 billion years. Astronomers were very hard pressed to explain what could be causing these huge cosmic outbursts. From 1973 to 1991, the prevailing theory was that GRBs were telltale signs of “star quakes” (similar to earthquakes) on neutron stars; some thought that perhaps a wayward planet crashing into a neutron star releases a sudden burst of gamma rays.

In recent years, however, astronomers have concluded that the physical attributes of gamma-ray bursts are better explained as by-products of the workings of large black holes. Several different scenarios have been suggested; any one of them, and indeed perhaps all of them, may be occurring at intervals not only in our own galaxy, the Milky Way, but also in the billions of other galaxies lying beyond it.

One recent hypothesis, advanced by MIT scientist Maurice Van Putten, suggests that in binary star systems containing a normal star and a black hole, the black hole eventually siphons off nearly all of the companion star’s material. Finally, all that is left of the normal star is a doughnut-shaped ring that becomes part of the hole’s spinning accretion disk. In the span of only a few seconds, the hole sucks in the remaining pieces of the companion, in the process creating an enormous outburst of gamma rays.

In another scenario, a large GRB is generated when a stellar black hole collides with either a neutron star or another black hole. In either case, the colliding bodies would merge as one, releasing an immense burst of energy. “The two stars spiral together,” Begelman and Rees speculate, slowly at first and then faster and faster, until they finally merge. The final coalescence, perhaps lasting only a millisecond, would certainly trigger a large enough release of energy. Such events are rare they would occur only once every 100,000 years in a typical galaxy. But there are at least a billion galaxies within [effective range of human detectors], so the rate at which bursts are detected [about one per day by the late 1990s] poses no real problem.

In still another model for black holes and GRBs, a large gamma-ray burst occurs when people on Earth witness the birth of a stellar black hole from a specific and favorable angle. In this view, the tremendous collapse of a giant star into a black hole produces more than a supernova and a superdense object. The implosion also generates an enormous outpouring of gamma rays from the collapsing object’s poles. Because the alignment of such objects in the universe is random, only rarely will one’s poles point directly at Earth. When they do not line up, humans see only the bright flash of the supernova; but when the object’s poles are aligned with Earth, human instruments record a giant outburst of gamma rays as well as the supernova.

When Black Holes Merge

Strong evidence that black holes do sometimes collide and merge was recently discovered by David Merritt, of Rutgers University, and Ronald Ekers, of the Australia Telescope National Facility. They studied patterns of radio waves given off by a group of distant galaxies. One strange feature that these galaxies have in common is a curious X-shaped structure at their centers. Merritt and Ekers believe that the four lobes making up such an X are high-speed jets of material emitted from a black hole that has recently merged with another black hole. In this view, the violence of the merger knocks the new black hole off its axis, and for a while it emits two sets of jets, one from its previous alignment, the other from its new alignment.

In an interview by Vanessa Thomas in the November 2002 issue of Astronomy, Merritt says, “Black holes are so large and massive, the only thing we can imagine that would have enough force to realign them is another black hole. . . . [Before this discovery] most astronomers were fairly sure that black holes coalesce, but we now regard the X-shaped galaxies as the first ‘smoking-gun’ evidence.”

The X-shaped structure in this gaseous cloud may be evidence of two black holes merging.