September 22, 1997
Gamma-ray bursts not only light up the universe, they might also illuminate the history of the universe by showing dim ancient galaxies. (An image of a field of dim galaxies, taken by the Hubble Space Telescope, is shown at left.)
"These could be extremely useful tools to study the very young universe," said Dr. Peter Meszaros of Pennsylvania State University. "They're like Chinese lanterns. With them, we get to see the outline of very faint objects, of protogalaxies as they are forming."
The analogy to a fragile lamp is an odd contrast with the violence of the favored model for gamma-ray bursts, a relativistic fireball from the collisions of two neutron stars. Meszaros said this would happen once in every galaxy every million to 100 million years. Given the vastness of the universe, that would nicely account for the current discovery rate of one per day.
Meszaros said that the fireball is needed because it's the only way that gamma-ray bursts can be observed at such great distances and intensities.
To be fair, the question is not absolutely settled. Several discussions last week (September 15-20) at the Fourth Huntsville Gamma-Ray Burst Symposium did suggest a few alternative theories. In the past, when the mystery was deepest, theories were as fanciful as intergalactic wars.
The collision model, though, is no less violent or impressive. Neutron stars are the collapsed remains of supermassive stars - perhaps 10 times the mass of our sun - which expend the last of their energies in supernova explosions (which yield gamma rays but are not the source of gamma-ray bursts).
Left are a beautiful expanding shell of gas (like the Crab Nebula) and a rapidly rotating star consisting entirely of neutrons and an intense magnetic field (also at the core of the Crab). If the star is massive enough, the explosion all but squeezes the core out of existence and leaves a black hole detectable only by its gravity and the energy released when it swallows more matter. (Shown at right is the nebula and neutron star of Puppis-A, imaged by the ROSAT X-ray satellite - the neutron star is in the box. Click on the picture for a larger view. More information on this image is maintained by Goddard Space Flight Center.)
Because most stars are found in binary or multiple systems, it is common for neutron stars and black holes to be found orbiting with visible companions that themselves fade into dwarfs or neutron stars. Eventually, tidal forces wear these systems down, and the two bodies collide.
"The questions is, after you have this collision, how do you produce the fast fireball?" Meszaros asked.
Neutron stars are incredibly compact, packing about 1.4 to 3 times the mass of the sun into a ball only about 20 kilometers wide. Anything falling onto the surface will be torn apart - even atoms are ripped apart - and will be travelling incredibly fast at impact. Two neutron stars running into each other will produce more than a galactic fender-bender. The fluid dynamics are still not understood, but we do know that the collision releases incredible amounts of energy.
The collision rarely will be head-on, but probably is a rapid spiral like "the last waltz" described by William Lee, a graduate student at the University of Wisconsin at Madison. He and Wlodzimierz Kluzniak simulated neutron star-black hole collisions with hydrodynamic equations. Computer animations show the two bodies spinning around each other as up to half the mass of the neutron star streams into the black hole. The neutron star loops outward, then crashes into the black hole, all in the matter of a few milliseconds.
At this point, Meszaros said, the two stars produce a massive cloud of neutrinos and anti-neutrinos, nearly massless particles that literally stop at nothing as they carry away energy from nuclear reactions. (Massive neutrino detectors deep underground detect only a few particles a year as countless numbers pass through the Earth.)
Yet so dense is this cloud that the neutrinos collide with each other to make electrons and positrons (anti-electrons) which, in turn, annihilate each other to form gamma rays.
Further, the energy is so intense that it accelerates the material in the fireball to more than 99.9999 percent the speed of light where time and space are changed by the effects of relativity, hence the name: relativistic fireball. Initially it is too dense for light within to escape. The blast wave trails just behind the gamma radiation it emits. As it expands the fireball thins and more photons escape, giving the sudden pulse of radiation followed by a second pulse as energy emerges from within the fireball which, at the same time, crashes through comparatively slow interstellar gas.
"This produces synchrotron radiation and inverse Compton radiation which produce the gamma-ray spectra that we observe," Meszaros said. It is followed by a additional flashes as radiation escapes from shock waves and other activities inside the fireball.
Shouldn't the accelerated matter come blasting past the Earth as well? It might. Collisions with interstellar gas slow most of the shock wave to respectable speeds. Some materials may speed onward, arriving eventually as cosmic rays. But that's another theory and another story.
The collision model is not the only one.
"The calculations are inconclusive," Meszaros said. Another proposal is the "failed supernova" or hypernova, a star that doesn't quite blow up. An intense magnetic field - about 10 billion billion gauss (Earth's magnetic field is about 0.5 gauss) - reaches from the neutron star into a torus of neutrons surrounding the center. It's possible that both models could be at work, but for now, scientists cannot distinguish between the two.
"An interesting point is that with hypernovae and a magnetically driven fireball, you achieve very high luminosities," Meszaros continued. "You can see gamma-ray bursts at much higher distances. And it appears that the faintest gamma-ray bursts we see are very much more distant than what we suspected."
Because these explosions are not linked to the
cores of galaxies (yet another theory), they could illuminate the structures
of dwarf galaxies and other large bodies that formed early in the universe.
As they go off, gas throughout the galaxy reflects light for all the universe
to see, including telescopes at Earth. This candle does not consume the
lantern, but it flickers so briefly that we have to look right away, or
wait a few million years for the next one.
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Curator: Linda Porter
NASA Official: Gregory S. Wilson