Gamma-Ray Bursts: Flashes in the Sky
Gamma-ray bursts—flashes of intense radiation in space that are often just seconds long—were accidentally discovered in the 1960s by satellites built to monitor nuclear bomb explosions. They’ve been one of the leading astrophysical mysteries ever since. This Astro Bulletin introduces you to the scientists and instruments working to unravel the origins of gamma-ray bursts. It highlights Swift, NASA’s burst-detecting satellite, and PAIRITEL, one of a fleet of ground-based telescopes that point toward a gamma-ray burst in response to Swift’s alert to capture the afterglow before it fades. Astrophysicists at Penn State and other institutions are analyzing these afterglows to understand what causes the most powerful explosions known.
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Capturing Phantoms: Gamma-Ray Bursts
The human eye detects light only in the visible part of the electromagnetic spectrum, but that's just a fraction of the energy that cosmic phenomena produce. If your eyes were gamma-ray sensitive, you'd see a very different sky. Mostly, it'd be black. Occasionally the Sun would flare with a wisp of light. But about once a day, a sudden flash would overwhelm the sky, outshining the Sun itself by a million trillion times. The flash would grow from a pinpoint in a matter of seconds, nearly blind you, and then fade just as quickly.
These ephemeral events are called gamma-ray bursts. Humans can't see gamma-rays because our eyes had no reason to evolve that way — Earth's atmosphere blocks gamma-rays from reaching the ground. Be happy that it does. "Gamma-ray bursts are the most intense explosions that you can imagine in the universe," says Peter Mészáros, a theoretical physicist at Penn State University. "That's in terms of how much energy they pack into how short a time, and into what kind of wavelength range they pack it."
The wavelength range of gamma-ray bursts is high on the electromagnetic spectrum—beyond even x-ray—sowing to the extremely energetic cosmic events that create them. These flashes in the sky are thought to be the dying roar of ancient stars, the screeching collisions of neutron stars, the energetic spew of voracious black holes.
Despite their powerful presence, figuring out what causes gamma-ray bursts hasn't been easy, as these events have been notoriously hard to catch. Scientists didn't know they existed at all until the 1970s. And while some bursts can be 30 seconds long, some last only 10 milliseconds. Only the latest technology has managed to capture the quickest of these bright phantoms.
Gamma-ray bursts were first detected by accident. Gamma rays are also produced by nuclear explosions, so in 1963, the U.S. military launched satellites to scout for detonations, which the Nuclear Test Ban Treaty prohibited. Researchers soon realized that they were detecting gamma-ray emissions from outer space, not Earth.
The observations were confusing. "There was a worry these might be some alien civilization making nuclear explosions," says Mészáros, "but that idea didn't hold water." Scientists then proposed that the bursts were coming from our own galaxy, but this notion didn't stick either. The events occurred at a regular rate, and they were well distributed in the sky, much like galaxies. So experts concluded that the strange explosions were naturally occurring cosmological phenomena.
Gamma-ray bursts were publicly announced in 1973 to the immense excitement of scientists. Hundreds of theories were formulated about what produced them. But it wasn't until 1991, when the Compton Gamma-Ray Observatory was launched into space, that technology was sufficiently advanced to analyze their basic properties before they faded. "Gamma-ray bursts never repeat in the same place in the sky," says David Burrows, an astrophysicist at Penn State. "By the time you move a telescope to look at one, it's gone."
A Swift Technological Turn
The latest instrument, the Swift spacecraft, is agile enough to detect gamma-ray bursts as brief as 50 milliseconds. It orbits 650 km (400 miles) above Earth's surface and can monitor about a sixth of the sky at once. One of the few NASA satellites without an acronym, Swift was named for its ability to position itself in the direction of a new burst, rapidly and automatically. It can slew, or repoint, in under a minute, an unheard-of speed for a space telescope.
When Swift detects a burst, it's a race to gather as much information as possible before the event fades. Swift immediately sends a message to ground control at Penn State, even if it's 3 am or in the middle of the Super Bowl (both have happened.) The message gets distributed via cell phone, pager, or e-mail to nearly 900 astronomers worldwide. Some of the messages are sent to robotic ground telescopes, which immediately point toward the burst. Why bother, if gamma-rays can't be detected on Earth? In 1997, scientists realized that gamma-ray bursts have dying embers called afterglows. The afterglows, which can last for hours or even days, cascade down the electromagnetic spectrum – through x-rays, ultraviolet, visible, and infrared, all the way to the lowest-energy wavelengths, radio waves. Traditional ground telescopes are sensitive to wavelengths below x-rays. Their analysis of the afterglow can reveal crucial details about the source.
After the Afterglow
The afterglow of a gamma-ray burst enables scientists to identify its host galaxy, the source's distance from Earth, and many other properties. Afterglows have divulged that gamma-ray bursts are among the most distant cosmic phenomena known – up to 13 billion light-years away – yet so powerful we can see them from our own galaxy.
Thanks to the growing observational record, just two out of the hundreds of theories posited about gamma-ray bursts in the 1970's remain most viable today. Scientists now know that the vast majority of gamma-ray bursts are "long bursts"longer than 2 seconds. They are thought to occur when a massive star runs out of fuel. Without a force to resist the crush of its own gravity, the core of the star collapses into a black hole. The material falls into the hole at near light speed and bears a tremendous amount of energy. That energy spews into space as two jets of gamma-rays shooting in opposite directions. (Only when a jet is directed toward Earth can instruments detect the burst.) The jets also explain why gamma-ray bursts are so intense – the energy is concentrated in just two directions and not diluted in all directions.
Short gamma-ray bursts, which are under 2 seconds long, were an utter mystery before Swift came along. In 2005, Swift orchestrated the first observations of a short burst's afterglow: a 50 millisecond burst with a 2-hour ember. "What we learned was fascinating," says Neil Gehrels, Swift's lead scientist. "They're very different from long bursts, not just in their duration, but in the objects that create them. They come from different regions of space, places where you don't see stars being formed." Gehrels and his colleagues think that short gamma-ray bursts are formed from merging neutron stars. These small rapidly spinning, late-stage stars are so dense that a teaspoon of their material would tip the scales at a billion tons. Neutron stars often pair in binaries, circling one another until their orbits decay. Eventually they coalesce violently, releasing their energy as a short, explosive gamma-ray burst.
What Gamma-Ray Bursts Tell Us
In astronomy, distance is time. Since it takes light so long to travel across cosmic distances, the gamma-ray bursts that Gehrels, Burrows, and Mészáros study actually happened 7, 10, 13 billion years ago. The bursts' light is just now reaching Earth. Therefore, these are one of the few cosmic events that enable scientists to study the early universe.
One way gamma-ray bursts do this is by acting as a beacon, a bulb of background light that allows the study of the gas between Earth and the burst source. Analysis of the gas can reveal the chemical elements that were present in the burst's host galaxy in its earliest stages. Because gamma-ray bursts originate from massive stars, they also lend insight into star formation rates in the early universe. Indeed, scientists see the most gamma-ray bursts occurring at distances of around 7 billion light-years, which coincides with the time when most massive stars were formed.
"Gamma-ray bursts are now being used to study some of the most distant regions that we've ever been able to look at," says Burrows. "We hope that in the next few years we will find a gamma-ray burst that is farther away than anything else we have ever seen. That will allow us to step even a little bit farther into the unknown." Would the universe's most mysterious phantoms offer a step in any other direction?