Gamma Ray Bursts

In 1967, a satellite designed to spot Soviet nuclear explosions detected gigantic energy blasts from deep space — and astronomy was confronted with another mystery. Gamma-ray bursts.

This is for Tuesday, July 17 — with a landmark in 20th century science.

In 1963 the United States signed a treaty with the Soviet Union that banned nuclear explosions in space. To verify treaty compliance, the Defense Department launched the Vela series of satellites to detect gamma-ray flashes from nuclear explosions.

Ray Klebesadel was a scientist at Los Alamos National Laboratory responsible for building the Vela gamma-ray detectors — and for sifting through the data they collected. He and his team noticed that the satellite was detecting powerful gamma-ray bursts lasting seconds or minutes — beginning in July, 1967. The bursts didn’t come from human-made nuclear explosions — they originated somewhere in outer space.

Klebesadel and his colleagues announced their discovery in 1973 — and since then gamma-ray bursts have been one of astronomy’s most compelling mysteries. Technological advancement has produced better detectors — so that astronomers now can say for certain that the bursts originate billions of light-years away. One theory says that gamma-ray bursts occur when ultradense neutron stars collide. Or they may be due to the collapse of a kind of supermassive star common in the early universe — but rare today.

Gamma rays are part of the electromagnetic spectrum — the same spectrum that includes radio waves, heat, visible light, and X-rays. Gamma rays have very short wavelengths. They can destroy DNA in living cells. Fortunately for life on Earth, the atmosphere blocks most gamma radiation. Unfortunately for astronomy, this means that gamma-rays can only be studied using detectors on high-altitude balloons, special aircraft, and spacecraft.

Until recently, gamma-ray detectors could tell us very little about the sources of gamma-ray bursts. Ray Klebesadel started filing away unusual gamma-ray detections after Vela 3 reached orbit in 1965. Vela 4a and Vela 4b detected the first recognizable burst on July 2, 1967, but from their data very little could be learned about the burst or the direction from which it came. In fact, it was determined that this was the earliest gamma-ray burst detected only in 1976. Soon, however, Vela satellites with better detectors reached orbit. These allowed Klebesadel and his colleagues to determine one or two possible burst directions and to locate the burst source within about 10 degrees of sky. In 1972, Klebesadel, Ian Strong, and Roy Olsen sifted through results from the Vela 5a, Vela 5b, Vela 6a, and Vela 6b satellites, and determined for the first time that the bursts originated from beyond the Solar System. After they announced their findings in 1973, astronomers using the IMP-6 and OSO-7 satellites confirmed the Vela data.

The 1990s saw a revolution in gamma-ray astronomy, beginning with the launch of the Compton Gamma-Ray Observatory GRO into Earth orbit in 1991. Compton GRO logged new bursts every day and plotted their positions all around the sky. The Burst and Transient Source BATSE experiment on Compton GRO found that gamma-ray bursts do not concentrate in the plane of the Milky Way Galaxy, where most of the Milky Way’s stars are located. This pointed to an origin beyond the Milky Way, and pointed up what astronomers already suspected – that gamma-ray bursts are created in an enormously energetic process. A typical burst puts out as much energy in a few minutes as will the Sun over its entire 10 billion-year lifetime. In fact, gamma-ray bursts are the most powerful explosions in the universe, surpassed only by the Big Bang itself.

In 1996 a joint Italian-Dutch satellite called BeppoSAX revolutionized gamma-ray astronomy. Using an X-ray camera triggered by its burst detector, it pinpointed bursts with enough accuracy to allow optical and radio telescopes to home in on the burst sources for the first time. It now appears that they originate in galaxies several billion light-years across the universe.

When we see something several billion light-years away, we see it as it was several billion years in the past. Seven billion years ago the universe was about half its present age. Scientists now believe that most perhaps all gamma-ray bursts date from that time.

There are now two leading theories for the origin of gamma-ray bursts. Seven billion years ago, astronomers believe, galaxies underwent violent star formation. Perhaps many massive stars were born, including some much more massive than any seen today.

Massive stars typically rush through their lives in under a million years. The energy they produce keeps them inflated against the pull of their own gravity. But when they exhaust their fuel they collapse under their own weight. This yields a huge explosion called a supernova. The collapsed core of the star forms an ultradense, rapidly spinning neutron star or in rare cases a black hole.

Astronomers believe that a really massive, fast-spinning star could collapse so quickly into a black hole that it would not have time to become a supernova. All the energy put out by a supernova in months would then be blasted into space in seconds or minutes. Astronomers call this theoretical super-explosion a hypernova, a name right out of a video game. As the star went hypernova, it would form a short-lived disk around the equator of the new black hole, and two huge beams of gamma-rays and other forms of radiation would blast out from the stars north and south poles perpendicular to the disk. If Earth was in line-of-sight of one of the beams, our gamma-ray detectors would see a powerful gamma-ray burst lasting seconds or minutes.

It’s awe-inspiring to think that, since we would see only one of the two beams, we would see only half the energy the hypernova produced. Yet even half a hypernova would appear to us as one of the most powerful explosions in the universe.

It also gives one pause to think about what this says of the frequency of gamma-ray bursts. BeppoSAX and Compton GRO see gamma-ray bursts every day. Going by the hypernova theory, however, they see only the bursts in which one beam is pointed directly at Earth. That means that, for every burst they detect, perhaps hundreds go undetected.

The other theory for gamma-ray burst origins also involves massive stars and black holes. If the early universe saw the birth of many massive, short-lived stars, then it must have seen many supernovae and the birth of many neutron stars. Then as now, many stars form as twins, triplets, quadruplets or even rarely larger groupings. Say, for example, that twin stars went supernova. That would yield two neutron stars orbiting about each other. The neutron stars would gradually fall together and collide, producing a black hole and a huge burst of energy which we might detect as a gamma-ray burst.

In fact, it could be that both theories are right. As our gamma-ray detectors have improved, we’ve seen that there seem to be two slightly different classes of burst. Or, possibly, neither theory is correct. Astronomers are still trying to explain these titantic explosions.

One last note – gamma-ray bursts appear to have occurred more frequently in the universe seven billion years ago, but that doesn’t rule out their occurring in the Milky Way Galaxy today. Though stars massive enough to create hypernovae appear rare or even non-existent in the Milky Way Galaxy, it’s possible one lurks out of sight – for example, behind the dust lanes obscuring the galactic core. Also, there are almost certainly many twin neutron stars in the galaxy. If two neutron stars collided within a few thousand light-years of Earth, the flash of gamma-rays could sterilize our planet. In their more speculative moments astronomers muse over whether extinction events in Earth’s past might be attributed to nearby gamma-ray bursts, or whether gamma-ray bursts might have killed off all the other young civilizations in the Milky Way Galaxy, leaving no one for us to find when we search the sky with our radio telescopes.

References:

  • Compton Gamma Ray Observatory Science Support Center – http://heasarc.gsfc.nasa.gov/docs/cgro/cossc/
  • BeppoSAX – http://heasarc.gsfc.nasa.gov/docs/sax/saxgof.html
  • A Brief History of the Discovery of Gamma-ray Bursts – http://apod.nasa.gov/htmltest/jbonnell/www/grbhist.html
  • Gamma-Ray Bursts: Overview – http://www2.astro.psu.edu/users/nnp/grbgen.html
  • Gamma Ray Astrophysics at NASA Marshall Space Flight Center – http://www.batse.msfc.nasa.gov/
  • Cosmic Discovery: The Search, Scope, and Heritage of Astronomy, Martin Harwit, MIT Press, 1984, pp. 146-147.
  • Gamma Ray Astronomy, Rodney Hillier, Oxford University Press, 1984, pp. 163-167.
  • Gamma-ray Astronomy, Ramana Murthy, Cambridge University Press, 1986.
  • Gamma-ray Astronomy: Nuclear Transition Region, E. Chupp, D. Reidel Publishing, 1976, pp. 189-191.
  • Gamma-Ray Bursts Light the Way to the Early Universe, NASA Headquarters Press Release 99-139, November 23, 1999.

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