Feature
Fast-Slinging Satellite Catches the Biggest Bursts in the Universe
Artist's concept of HETE
HETE, short for the High-Energy Transient Explorer, detects gamma-ray bursts. These mysterious bursts are, by far, the most powerful explosions in the Universe. They are as bright as a million trillion suns.
In the past few months, HETE has led to breakthroughs in our understanding of gamma-ray bursts. Scientists are now confident that these bursts announce the birth of a black hole.
"HETE has taken us into the delivery room," said Dr. Derek Fox of the California Institute of Technology. He used information from the HETE satellite to find a star explosion that produced both a newly born black hole and its gamma-ray burst.
A gamma-ray burst is an intense flood of gamma-ray light, the most energetic form of light. Gamma rays are millions of times more powerful than the visible light that our eyes can detect. Special telescopes in space are needed to detect gamma-ray bursts.
For HETE (pronounced "hetty"), speed is crucial. Gamma-ray bursts don't hang around for long. They last no longer than a minute. Some are as short as a millisecond, one thousandth of a second. They are random, too, and never occur in the same place twice. HETE must be always on guard for a burst. When one occurs -- and they do at a rate of about one per day -- HETE turns to snap a picture.
Astronomers are very excited about two recent gamma-ray bursts that HETE detected, one on October 4, 2002, and another on March 31, 2003. Hundreds of scientists worldwide have turned toward these bursts, making them the most watched cosmic events in recent times.
Scientists essentially piece together the explosion that caused the gamma-ray burst by studying the burst afterglow. Here's how it works: HETE detects gamma rays and relays a location to the Gamma-Ray Burst Coordinates Network. This network, called the GCN, then sends a message to the astronomy community. Some astronomers get email alerts; others get beeper or phone calls.
Next, scientists race to turn their telescopes to the location that HETE provides. Even amateur astronomers join the hunt, because the location is publicly available at NASA's GCN web site. The sooner they can view the afterglow, the better. A gamma-ray burst afterglow will disappear too, but fortunately not as quickly as the burst itself. The burst afterglow can linger in X-ray, ultraviolet and visible light for hours to days.
What can the afterglow tell us? In the October 4 burst, called GRB 021004 (named for the date it was detected), Dr. Fox and other scientists saw energy continuously pumped into the afterglow. The energy source, Dr. Fox said, might very well be the newly formed, spinning black hole in the heart of the explosion, flinging material into space.
In the March 31 burst, called GRB 030331, Dr. Thomas Matheson of the Harvard-Smithsonian Center for Astrophysics detected heavy atoms, such as iron and silicon. Scientists know that these kinds of atoms are created in star explosions. With this detection, Dr. Matheson and his colleagues found direct evidence that gamma-ray bursts are from star explosions called supernovae (the plural of supernova).
This was an open-shut case, he said. HETE detected the gamma-ray burst. Dr. Matheson's colleagues turned a ground-based optical telescope toward the region of the burst. Within minutes, they were at the scene of the event. Then they observed the afterglow for eight nights in a row. The afterglow faded just like well-studied supernovae have been seen to fade.
Much like American western frontier people, HETE is a pioneer. HETE is the first satellite to provide and distribute accurate burst locations within seconds. In December 2003, NASA will launch the Swift satellite, which will have an even greater capability to detect and locate bursts, as well as onboard optical, ultraviolet and X-ray telescopes for immediate follow-up observations.
HETE has taken us into the delivery room, and Swift may allow us to see the actual birth of a black hole.