Blasts from the Past

Astronomers begin to go the distance with gamma-ray bursts

Just before 10 p.m. EDT, last Sept. 3, Dan Reichart’s cell phone started playing “The Stars and Stripes Forever.” A fitting tune, since it was heralding a call from the heavens. Reichart’s phone was signaling that a detector on NASA’s Swift satellite had registered a gamma-ray burst, the most powerful type of explosion in the universe. Such bursts—none of which lasts longer than a few minutes—typically mark the violent death of a massive star as it collapses to become a black hole.

BIG BOOMS. Information about the first stars (illustrated here) may come from gamma-ray bursts and their afterglows, which are so bright that telescopes can see some that occurred in the first stellar generation, less than a billion years after the birth of the universe. D. Aguilar/CFA
GOING THE DISTANCE. Using the near-infrared SOAR telescope in Chile, astronomers discovered the afterglow of the most distant gamma-ray burst yet found, some 12.8 billion light-years from Earth. V.L. Krabbendam, © SOAR

Since its launch in late 2004, the Swift satellite has recorded more than 100 gamma-ray bursts. About 20 other detections have turned out to be spurious. Reichart, an astronomer based at the University of North Carolina at Chapel Hill, didn’t want to miss an opportunity to find the new burst’s afterglow. With most of his team at a seminar on a remote island in Greece, Reichart immediately contacted his only available student, undergraduate Josh Haislip. They needed to take control of several mountaintop telescopes in Chile within 3 hours. That’s when the afterglow, deep in the constellation Pisces, would lie directly overhead in the southern night sky, and detectors would have their best view.

Although gamma-ray bursts are absorbed by Earth’s atmosphere, their afterglows shine at wavelengths, ranging from visible light to radio, that can be recorded at ground level. What’s more, an afterglow can last for hours or days, providing critical information on the collapsing star’s location, the nature of the galaxy from which the burst arose, and the composition of the interstellar material through which the radiation passed on its long journey to Earth.

Nearly all the afterglows that astronomers have detected come from gamma-ray bursts that arose in galaxies that lie 2 billion to 8 billion light-years away. But a few recently detected afterglows hail from much more-remote depths of space (SN: 9/17/05, p. 179: Farthest Bang: A burst that goes the distance). Since peering deeper into space is the same as looking farther back in time, finding such distant flashes of light “will likely drive a new era in the study of the early universe, using the bursts as probes,” Reichart says.

With recently launched spacecraft such as Swift and the High Energy Transient Explorer pinning down the location of a multitude of bursts, Reichart has made a bold prediction. Within the next 2 years, he says, astronomers will document gamma-ray bursts and their afterglows at distances more remote than those of the galaxies and quasars that now are the most-distant light-emitting objects known.

Gamma-ray bursts are about to open a new frontier in cosmology, says theorist Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Then came light

Gamma-ray bursts offer great promise for studying the era when the first stars formed and flooded the universe with light, Loeb says. Astronomers still have only the vaguest of notions about how and when the universe emerged from darkness.

The cosmic Dark Ages began a few hundred thousand years after the Big Bang, when the radiation left over from the birth of the universe had faded and the cosmos had cooled enough for electrons and protons to combine into neutral atoms of hydrogen. Soon, some of the hydrogen atoms began gathering into the clouds that produced the first generation of stars.

But the universe as a whole stayed dim. Hydrogen atoms readily absorbed much of the light generated by new, massive stars. It was only after these stars generated enough ultraviolet radiation to break apart, or ionize, the hydrogen atoms that the light could shine through and illuminate the cosmos.

That illumination was a gradual process, as Loeb and his colleague Volker Bromm of the University of Texas at Austin envision it. At first, the ultraviolet light from the young stars created small bubbles of ionized gas in their immediate surroundings. Only when the bubbles from individual stars or groups of stars began overlapping could the light from this first stellar generation shine through.

Gamma-ray bursts are the ideal tools to study the transition from a dark universe filled with neutral hydrogen atoms to a shining cosmos containing an abundance of ionized atoms, says Loeb. For starters, these explosions were likely to have been common in the early universe, he notes. Computer modeling suggests that the first stars were extremely massive—so heavy that most of them died out rapidly and violently, collapsing into black holes several times as heavy as the sun while generating gamma-ray bursts.

Hydrogen from the Dark Ages left its fingerprints on a variety of bright beacons from that early era, including the afterglow of gamma-ray bursts. A neutral hydrogen atom absorbs a specific wavelength of light passing through it. If light from the afterglow of a gamma-ray burst travels through a region of such atoms, it exhibits a gap in its spectrum.

By analyzing the spectrum of light from a distant afterglow, astronomers are attempting to determine the ionization state of gases in the early universe. Even if only 1 in 100,000 atoms of hydrogen in a patch of space were neutral, all the light at this wavelength would be blocked, and observers examining the spectrum would see the gap.

Burst versus quasar

Researchers have already recorded the telltale gap caused by atomic hydrogen in the spectrum of another type of bright beacon, known as a quasar (SN: 8/11/01, p. 84: Light’s Debut: Good Morning, Starshine!). For more than 3 decades, astronomers have used quasars to study the composition of the intergalactic material that these beacons pass through en route to Earth. Each chemical element, depending on its location, creates a different bump or wiggle in the quasar spectrum. The spectra of distant quasars have revealed, for example, that carbon and other metals had formed by the time the universe was less than one-fifth its current age.

Whereas afterglows of gamma-ray bursts last only hours or days, quasars shine for millions of years. However, the much shorter duration of an afterglow from a gamma-ray burst may offer an advantage, note Reichart and cosmologist Don Lamb of the University of Chicago. Quasars last so long that they can disturb their immediate surroundings and create clouds of ionized hydrogen gas within a region that would otherwise be neutral. So, instead of acting as a passive probe of hydrogen at the end of the Dark Ages, as a gamma-ray burst would, a distant quasar may primarily be revealing its own capacity to break apart hydrogen atoms.

Moreover, the typical gamma-ray afterglow is about 100,000 times as bright as a quasar, providing astronomers with a more penetrating beacon that can, in theory, be seen farther back in time.

Distant bursts may also be more plentiful than distant quasars. Gamma-ray bursts require only a black hole roughly the mass of a single star, Lamb notes, and the early universe was apparently chockfull of such small black holes. In contrast, quasars require millions to billions of stars combining to create a supermassive black hole. Few if any of these black holes are likely to have existed when the universe was only a few hundred million years old.

Furthermore, he notes, bursts—unlike quasars—tend to come from small, run-of-the-mill galaxies. Gamma-ray bursts, therefore, shine a light on what may be the Joe Average galaxies in the early universe, from which most of the stars were formed, rather than on the rarer beasts that form quasars.

There’s one other property that makes distant gamma-ray bursts better tools than quasars for looking at the early cosmos, says Loeb. For the steady light of a quasar, the greater the distance and the farther back in time that the light was emitted, the fainter it appears. But gamma-ray afterglows aren’t steady; they’re brightest immediately after a burst and then they fade rapidly.

In a phenomenon consistent with Einstein’s theory of general relativity, the afterglow from a distant gamma-ray burst takes longer to fizzle out than does the afterglow of a nearby burst. Therefore, astronomers have more time to record the brightest afterglow of a distant gamma-ray burst.

All in all, “this is a great time to study distant gamma-ray bursts,” said Loeb at a meeting on gamma-ray bursts in Washington, D.C., last December. “There are a lot of question marks about what really happened [in the early universe], and bursts may be a great tool to answer them.”

A chase pays off

Back at his laboratory in Chapel Hill, 3 hours had elapsed since Reichart’s cell phone had announced a gamma-ray burst. Via the Internet, he had taken the controls of a pair of small telescopes that his team had built at the Cerro-Tololo Inter-American Observatory in Chile, pointing them at the patch of sky from which the burst had emanated. Each telescope views the sky at a different wavelength of visible light.

Reichart and Haislip had also arranged for a larger, infrared telescope on the adjacent Chilean mountaintop Cerro Pachon to hunt for the same afterglow. By chance, Haislip had just finished learning how to analyze data from the instrument, known as SOAR (Southern Observatory for Astrophysical Research).

The researchers immediately found something odd. The afterglow appeared bright in the infrared, but the visible-light telescopes didn’t detect it. A larger telescope at Palomar Observatory near Escondido, Calif., also failed to detect visible light.

That pattern of brightness in the infrared and of darkness in visible light had two possible explanations, Reichart knew. The more likely one was that the light came from a nearby but dusty galaxy, in which the dust had absorbed visible light and reemitted it in the infrared.

But there was a more intriguing possibility. The burst might have come from one of the most distant galaxies in the universe, providing a glimpse of the cosmos as it emerged from its Dark Ages.

Further observations indicated that the visible-light cutoff was abrupt and that the glow remained bright over a range of infrared wavelengths, properties that only a remote galaxy could reproduce.

Without a spectrograph, Reichart’s team could only estimate that the afterglow that they had found had been emitted from a galaxy less than a billion years after the Big Bang. The astronomers posted their finding on an Internet site devoted to the study of new bursts.

“We were a little nervous because this was a big deal, and we didn’t want to put out something out we’d have to retract,” says Reichart.

Three nights later, Japanese astronomers using the large, near-infrared Subaru Telescope on Mauna Kea in Hawaii finally obtained a spectrum of the ember. The spectrum indicated the redshift of the light, a measure of the extent to which the expansion of the universe had shifted the radiation to longer, or redder wavelengths. The more distant an object, the greater its redshift.

The Japanese team found that the afterglow had an unusually high redshift of 6.29, indicating that the burst had erupted in a galaxy 12.8 billion light-years away and dated to less than 900 million years after the Big Bang. The spectra confirmed that Reichart’s team had broken the record, finding the most distant gamma-ray burst ever discovered. The previous confirmed record holder, discovered on Jan. 31, 2000, resided 500 million light-years closer to Earth. Only a few other known objects—a handful of remote galaxies and one quasar—are more remote.

After reading the e-mail announcement from the Subaru team, Reichart grabbed a yardstick from his laboratory and ran into the hallway of the physics building. Pointing the yardstick at anyone who passed by, he shouted, “6.29, 6.29!”

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