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'If a gamma-ray burst were a crime, then we now have strong evidence that a supernova explosion was at the scene.' -- Nathaniel Butler, MIT astrophysicist





black hole storm This animation illustrates the activity surrounding a black hole. While the matter that has passed the black hole's "event horizon" can't be seen, material swirling outside this threshold is accelerated to millions of degrees and radiates in X-rays. At the end of the animation, the black hole is shown shrouded in a cloud of gas and dust, obscuring it from most angles at wavelengths other than the X-rays picked up by the Chandra X-ray Observatory.

Courtesy Chandra X-Ray Observatory; Animation: CXC/A.Hobart

  Do wasted stars make gamma-ray bursts?
The steady march of technology -- providing the world with, among other things, fire-breathing computers, hair-trigger sensors and optics, and materials with useful and sometimes weird properties -- has helped scientists solve many a conundrum of nature.

Occasionally, however, new technology presents science with a challenge, an unexpected phenomenon that begs for an answer. Such was the case in the 1960s, when, at the height of the Cold War, the government launched satellites to spy on the Soviets to make sure they were keeping their test ban treaty promises. These satellites, designed to look for the telltale signs of nuclear explosions in Central Asia, however, stumbled upon something much different: powerful pulses of radiation -- X-rays and gamma rays -- coming from deep space.

"These are very energetic bursts of radiation that seem to occur about once a day," says Nathaniel Butler, an astrophysicist at MIT's Center for Space Research. "There is a crazy, long history with them."

Astrophysics poser?
Indeed. Gamma-ray bursts and where they come from have puzzled scientists since their accidental discovery nearly 40 years ago. Scientists do know that gamma-ray bursts come from distant, supremely violent events. "By and large, they are cosmologically distant, about 5-10 billion light years away," says Butler.

Gamma Ray Burst Illustration (no labels)

In terms of energy production, the events that spawn gamma-ray bursts are extreme, cranking out in a few seconds an amount of energy equivalent to all of the energy stored in the sun. That the high-energy particles that make up gamma-ray bursts can travel such great distances is testimony to the monstrous events that spawned them.

But what are those events? What creates these shockingly powerful bursts of energy, some of which have coursed through space since nearly the beginning of time?

According to Butler, there are three competing theories, all implicating massive stars in their death throes.

Suspect number one is the finale of a cosmic waltz between a neutron star -- an aged, supredense star -- and a black hole. As the neutron star and black hole merge and coalesce over time, the result is a massive gamma-ray burst-generating explosion.
The second suspect in the lineup is a "hypernova." In this scenario, a massive star in the last stages of life attempts to go supernova in too much of a hurry. Instead of a nice, neat explosion, the collapsing core of the star is retained in its shell creating a cosmic mess -- and, presumably, a burst of gamma-ray energy detected billions of years later by humans in a distant galaxy called the Milky Way.
The last gamma-ray burst event suspect is the "supra-nova" model. The picture here is of a massive star whose outer layers are expelled in an explosion, but the core of the star remains as a dense neutron star. The neutron star lasts about a year and eventually collapses on itself creating a black hole with a jet, tightly focused plumes of energy emanating from opposite poles of the black hole.

Narrowing the list of suspects
It is this latter theory that is supported by a new observation of a recent gamma-ray burst made by Butler using the Chandra X-ray Observatory. "If a gamma-ray burst were a crime, then we now have strong evidence that a supernova explosion was at the scene," Butler says.

The evidence was gathered over nearly a day-long observation made by Chandra after the gamma-ray burst was first detected by a satellite called the High-Energy Transient Explorer which serves as sort of an early-warning system for astrophysicists, alerting them to gamma-ray bursts whose galaxies of origin can then be quickly pinpointed by ground-based telescopes.

The observation made by Chandra for the August, 2002 event was unusually long, roughly 21 hours. What it saw was the afterglow of the burst that emanated from a galaxy roughly 8 billion light years from Earth. Using a Chandra spectrometer, a device that breaks light down into its constituent wavelengths, Butler detected silicon and sulfur ions, chemical elements typically blasted into space when stars explode.

Chandra X-ray Spectrum of GRB 020813 The Chandra data showed that the ions, atoms stripped of most of their electrons, were moving away from the site of the gamma-ray burst at a tenth of the speed of light, and probably are from the shell of matter blasted into space by the original supernova explosion. The peaks in the spectra obtained by Chandra show that only a small part of the shell was illuminated by the gamma-ray burst, suggesting a beam of gamma-rays as would come from a jet or narrow cone of energy produced by the newly formed black hole.

"An extremely massive star likely exploded less than two months prior to the gamma-ray burst, and the radiation from the gamma-ray burst was beamed into a narrow cone" and illuminated the ionic debris of the shell like the beam of a flashlight.

"We were able to observe for almost a whole day," Butler told The Why Files. "This is really the best (gamma-ray burst) observation to date."

The lines of evidence detected by Butler's group using Chandra, while not necessarily a smoking gun, lend to the idea of the two-step explosion notion embodied by the supra-nova model.

While the new findings do support one of the favored gamma-ray burst theories, us Why Filers have little doubt future observations and new technologies will permit further refinements and new ideas about the origins of enigmatic gamma-ray bursts.

-- Terry Devitt



The X-ray Afterglows of GRB 020813 and GRB 021004 with Chandra HETGS: Evidence for a Supernova Prior to GRB 020813, Nathaniel Butler et al, High-Energy Division of the American Astronomical Society, March 2003.

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