Swift Gammas: One Minute of Fame, Everyday

Imagine an explosion that could emit a million times more energy than the combined output of all the stars in the Milky Way. If you could hear this burst, it would deafen you. If you could see one, it would blind you. Any life within a radius of many solar systems would be annihilated.

swift_burst
SWIFT instrument, amidst an artist’s concept of a burst
Credit: GSFC

But just such an event happens a thousand times a day, lasts for about a minute, and then fades. Called gamma-ray bursts, these violent, distant explosions represent the greatest release of energy the universe has seen since the Big Bang. Satellites detect at least one of these bursts a day, and scientists do not know what causes them. Soon, however, a new satellite called SWIFT could offer more information about gamma-ray bursts (GRBs)

In May 2004, when SWIFT launches, astronomers might finally be able to unravel some of the mysteries about gamma-ray bursts. SWIFT will find hundreds of these powerful flashes of energy each year.

Unlike visible light, gamma-rays are non-thermal, meaning that they are not produced in hot celestial bodies like the sun. Gamma rays occur in exceptional circumstances, such as in the aftermath of a stellar explosion, in the vicinity of black holes, or at the core of active galaxies.

"The sun emits gamma-rays regularly but only as part of intense solar flares", said John Horack, who led the assembly, testing and calibration program for the gamma-ray burst experiment on NASA’s Compton Gamma-Ray Observatory, "and we see many objects in the galaxy that are emitters of gamma-rays. Even terrestrial thunderstorms can produce gamma-rays out of the top, detectable from low-Earth orbit. All of these transients, however, are markedly different than the classical gamma-ray burst in many ways."

Most charged particles in our solar system come from two sources: solar flares, which produce a rain of dangerous protons, and distant supernova explosions, which accelerate atomic nuclei –called "cosmic rays"– to nearly light speed. Fortunately for life on Earth, a gamma particle from the universe does not penetrate to the Earth’s surface.

Explaining What We Can’t See?

Although this year marks the thirtieth anniversary of the discovery of gamma-ray bursts, much about them – such as why they happen – still remains mysterious. The first gamma-ray burst detection by the Air Force’s Vela satellite dates to July 1967; however, because of security concerns, it wasn’t until 1973 that Ray Klebesadel, Ian Strong and Roy Olson of Los Alamos National first announced the discovery of sixteen gamma-ray bursts.

In 1991, the Burst and Transient Source Experiment (BATSE) aboard NASA’s Compton Gamma-Ray Observatory discovered that this gamma-ray burst radiation was "isotropic," or uniformly coming from everywhere. BATSE could monitor nearly the entire sky for gamma-ray transient sources (i.e., sources that suddenly gave off a large amount of gamma-rays and then fade). In addition, BATSE could localize where in the sky the burst was coming from.

"We’ve had to create an explanation to fit what we’re seeing, rather than understanding what gamma-ray bursts are by predicting them," said Ed Fenimore, a Los Alamos astrophysicist. "What we know is that something has to accelerate a huge mass – close to the mass of the sun – to close to the speed of light in a very short period of time. We just don’t know why that’s happening."

Predicting Randomness?

"The thing that would strike you about them is how intense they are," Fenimore said. "The problem is we can’t predict these things. Some happen so fast we miss them entirely. It takes about 60 seconds for us to get everything in place, and by then many of them have already faded."

herschel_mission
The changing intensity of a gamma-ray burst. On the left is an image of the gamma-ray sky showing the burst becoming the brightest object. On the right is a plot of the changing brightness with time. The first gamma-ray burst was seen in the year 1967 (although it was not reported to the world until 1973) by satellite-borne detectors intended to look for violations of the Nuclear Test Ban Treaty. Credit: BATSE

"Prediction", said Horack, "is more like asking the question ‘where should I look next, and when, to detect the next GRB?’"

SWIFT will help scientists see the phenomenon from outside the Earth’s atmosphere, but it may not track them any faster than the current High Energy Transient Experiment satellite [HETE]. HETE tries to locate a gamma-ray burst quickly and send tracking data to ground-based telescopes so they can catch the end of the GRB event before it disappears.

One striking feature that has surprised gamma-ray astronomers was that the signals don’t seem to come from a known stage of a star’s life cycle (unlike old stars that supernova). Instead, they come from all directions, and not necessarily just from where the stars are most concentrated.

"Unlike galactic objects", said Horack, "where you’d say ‘concentrate on the galactic plane’ or where the object that is ‘about to pop’, the distribution of bursts on the sky is extremely uniform, to an amazing degree of randomness. It’s more isotropic than most computer simulations of a few hundred objects pasted on the sky at random. Telescopes, in the sense that we commonly think of them, have limited fields of view and are typically pointed in a certain direction, so they’re unlikely to be the right direction for the next burst. The short time span of the burst emission can also make it challenging to ‘slew’ your telescope quickly enough to catch the event."

The prevailing explanation for this intense burst of radiation is that a collapsing star has burnt out its mass of fusionable energy, and thus announces its inevitable collapse into a black hole with a last, large explosion.

"A supermassive star collapses, a traditional supernova as we had come to know them does not occur, but something in the physics of the environment causes a different result, with equal or greater catastrophic energy release — a hypernova and a gamma-ray burst," said Horack. "Whatever it is, it pumps out a huge amount of energy in an short period of time, at extremely high wavelengths."

About 1,000 GRBs occur daily, but Earth-based telescopes only are able to see perhaps one a day Because SWIFT will be outside the Earth’s atmosphere, it will see the GRB events even if the sky is cloudy or it’s the middle of the day.

"Hopefully, as we catch more of them we’ll be able to catch one as it goes through most of its life cycle," Fenimore said.

What’s Next

Although the history of astronomy dates back centuries, observations in the gamma spectrum are a new area in celestial research.

The high-energy light is swallowed by the Earth’s atmosphere, and gamma-rays cannot be captured with conventional lenses or mirrors. A number of instruments are planned that will make additional contributions to unraveling the scientific mystery of these intense explosions.

Allen Telescope Array
Allen Telescope Array (ATA)

For instance, when the Allen Telescope Array turns on in 2005, it will be capable of searching to the farthest of 17,000 nearby habitable stars, just beyond 300 parsecs. For those search distances, an electromagnetic signal, if detected, would have begun broadcasting around a millennium ago, just about 1000 AD on a terrestrial calendar. A by-product of the Allen Telescope Array will be the opportunity to catch gamma-ray bursts.

"For me," wrote Dr. Jill Tarter, Director of SETI Research, SETI Institute, "the most compelling use of the Allen Telescope Array (after SETI, of course!) may be its exploration of the very early universe, the search for primordial dark matter concentrations, and investigations of transient events such as supernova explosions and gamma-ray bursts. As a scientist who finds great power and beauty in evolution as an organizing theme that ties together so much of astrobiology, including SETI, it is very satisfying to know that the array will be probing emergent cosmic evolution and the evolution of technology, two extremes on a continuum that stretches from primal matter to minds that contemplate their origins."

Ultimately the array will consist of 350 individual 20-foot antennas, providing a larger total collecting area than the Green Bank Telescope in West Virginia and higher resolution than the Arecibo dish in Puerto Rico.

The notion of ‘seeing the universe’ to an astronomer is often about seeing a particular wavelength, or spectrum of electromagnetic radiation, so each large instrument must be tuned for its particular task. For instance, the "Great Observatories" that NASA first proposed in the 1970s each examine the heavens in a different electromagnetic spectrum. Because they are space borne telescopes, orbiting above the distorting atmosphere of the Earth, they are able to gain unprecedented views of the universe. The most famous is the Hubble Space Telescope, the visible light telescope, and it is expected to operate until 2010. The recently launched SIRTF will study the infrared portion of the electromagnetic spectrum. The Compton Observatory, launched in 1991, examined gamma-rays until its mission ended in 1999. The Chandra Observatory, launched in 1999, examines X-rays and is scheduled to operate through 2004.


Related Web Pages

SWIFT: Goddard Spaceflight Center (NASA)
Allen Telescope Array Capabilities
HabStars: Speeding Up in the Zone
The Use of Gamma-ray Bursts as Direction and Time Markers in SETI Strategies: Corbet