Features

Only one year into its mission, NASA’s High Energy Transport Explorer satellite just earned its keep. 

Scientists have finished analyzing the data from a Sept. 21 gamma ray burst that HETE detected, and found a rare thing. Thanks to the satellite’s tip-off, several observers were able to see the visible-light afterglow of the burst. 

Gamma-ray bursts have been one of the hottest mysteries in astronomy for 20 years now. They are flashes of extremely high-energy radiation that come speeding at us from the farthest reaches of the universe. One burst may emit, in less than a minute, more energy than our sun will during its entire life. They pop up randomly, like flashbulbs in a distant galaxy, then disappear in a matter of seconds and never come from the same spot again. Although they happen every day, no one knows how, why or even really where these mysterious beacons occur. 

Each burst also produces an afterglow, a longer burst of lower-energy radiation – such as visible or infrared light or radio waves. The visible light may be seen for up to six days after the gamma-ray burst, and radio signals may persist as long as a month. 

But catching the afterglow may be more a matter of luck than skill. Bursts have been observed for over 20 years, but only about 30 afterglows have been seen. Most of those were in the past two years, using new satellites and technology. If it’s foggy, rainy, daylight, or just a bad time at your telescope, or if you don’t look in exactly the right spot, you’re out of luck. 

Even if conditions are right at your telescope, the bursts come from so far away the visible light could be blocked by dust in space before it ever gets here, or may be shifted down the spectrum into the infrared where astronomers may not look. 

The HETE satellite was launched in October 2000 specifically to study gamma-ray bursts. When HETE detects a burst, which means it was pointed in the right direction at the right time to see one go off, it transmits the data to a ground station. The station then sends out an automated alert to astronomers around the world. Anyone can subscribe to the alerts and try to observe the afterglow. 

After the burst on September 21st, several observers found the visible and radio afterglows. LOTIS, a robotic telescope operated by the Lawrence Livermore National Laboratory, found it within a day, followed by a telescope in Arizona, the Mt. Palomar observatory near Los Angeles, and the Very Large Array radio telescope in New Mexico. 

This burst came from about five billion light-years away, in the constellation Lacerta. That large region of space is densely populated with stars, so it was hard to pick the visible burst out of its bright background. 

“I don’t think anybody had a lot of hope that they were actually going to find anything, because the region was so large,” said Dr. Kevin Hurley of UC Berkeley Space Sciences Laboratory, a HETE collaborator.  

The challenging conditions made the researchers that much more pleased with their luck, and with HETE’s performance. 

There are two major competing theories to explain gamma-ray bursts. One is that they are caused by the death of a super-massive star, producing an explosion up to ten times larger than average supernovas. The other theory is they mark the collision of two ultra-dense neutron stars, or maybe a neutron star and a black hole. Two objects so massive smashing together would produce incredible amounts of energy. 

Detecting the afterglow is an important step in understanding what causes the bursts. Scientists are interested in bursts because they come from very young galaxies that seem normal in every other way. Learning more about them may help us understand what our own galaxy was like when it was young.  

With more data, scientists may even be able to tell how old the universe was when galaxies first began to form. 

Studying gamma-ray bursts and their afterglows also gives astronomers exciting glimpses into the insides of distant galaxies.  

“You can think of it as shining a flashlight through the galaxy to detect what the galaxy is made of, what elements it contains, the density around the exploding object,” Hurley said.