The Berkeley Science Review: Articles

Blinded by the Light
A distant star signals its demise (view PDF)
by Linda Strubbe

The record-breaking stream of energetic photons, having traveled for half the age of the universe, fi nally reached Earth while Daniel Perley was playing the video game Smash Brothers. Perley, a Berkeley graduate student in the astronomy department working with Professor Joshua Bloom, received an automated message on his cell phone at 11:12:49 pm, initiated by a telescope-laden satellite called Swift. On board Swift, the Gamma Ray Burst Alert Telescope determined the sky coordinates of the photon source and sent another message. Next, Swift's X-ray Ultra- Violet and Optical Telescopes detected the burst and refi ned their coordinates. Another message. Perley quickly checked his email and read that the event was already hundreds of times brighter than a typical burst. "I thought, ‘Wow, okay, this is a big deal,' " he says. " ‘I'd better get to work!' "

Swift had detected one of the universe's most extreme explosions: a gamma ray burst, or GRB. Gamma rays are photons so energetic that they can break apart atomic nuclei, and a GRB is a fl ash of these photons from outer space lasting as long as a minute. The fl ash is followed by an "afterglow," a surge of lower-energy photons that fades in brightness over several hours. Gamma rays are absorbed by our planet's atmosphere, which is fortunate for the atoms in our bodies, but which means that GRBs can only be discovered from space, by satellites like Swift. The X-rays and ultraviolet light of the afterglow are absorbed by our atmosphere as well, but visible and infrared light can be observed from the ground.

Most GRBs are thought to occur in very distant galaxies, an extreme version of the violent death of a star. When a very massive star exhausts its fuel supply, it succumbs to its own gravity and collapses. The collapse, which can produce a black hole at the star's center, leads to a powerful explosion called a supernova that shines as brightly as a galaxy for several weeks. If the original star was spinning very rapidly, the supernova may be accompanied by a GRB millions of times brighter than the supernova. Since the rotation prevents the collapsing gas from falling straight to the center, it spirals inward instead, forming a disk of mind-blowingly hot, fast-moving, magnetized matter pouring into the black hole.

These extreme conditions cause a double- ended jet of material to shoot out along the disk's axis at speeds exceptionally close to that of light. The jet focuses the GRB's power into a narrow beam only a few degrees across, like a flashlight, rather than emitting light in all directions like a light bulb. Most locations in the universe don't get to see a given GRB because its fl ashlight beam doesn't point at them, but for the lucky ones, it's intense. Violent collisions of matter inside the jet itself release the initial pulse of gamma rays, which the Swift satellite can pick up. The jet then crashes into the ambient gas that fi lls the galaxy's interstellar space; this collision releases the longer-lived, lower-energy afterglow.

About ten times each month, Swift detects a gamma ray burst and sends astronomers around the world rushing to follow the fading afterglow with telescopes on the ground. Berkeley astronomers use two robotic telescopes, KAIT (the Katzman Automatic Imaging Telescope) at Lick Observatory near San Jose, and Pairitel (the Peters Automated Infrared Imaging Telescope) at Whipple Observatory in Arizona. On March 19, 2008, these telescopes automatically downloaded the coordinates of GRB 080319B (so named as the second GRB discovered that day), just as Perley's cell phone did. They pointed toward it and began measuring the burst's brightness at optical and infrared wavelengths many times each minute. Because the GRB was close in the sky to the day's fi rst discovered GRB, Pairitel managed to start taking data almost immediately.

At the same time, a team in Chile led by Paul Vreeswijk measured the spectrum of the event using the Very Large Telescope. The spectrum tells us how long ago the GRB occurred by assessing how much the wavelengths of light have been stretched as they travel from the GRB to Earth through the expanding universe. This spectrum showed that the burst occurred 7.5 billion years ago, long before the formation of our solar system, when the universe was less than half its present age—and the burst happened so far away that its light only just reached us. When astronomers analyzed the night's data, they found that the peak optical brightness of the afterglow had shattered records. An explosion halfway across the universe was so powerful that for thirty seconds it could have been seen with the naked eye, had the bright full moon not been close by. Bloom points out the amazing implication, "Our ancestors could have witnessed events from the other side of the observable universe with their own eyes."

The Berkeley team acquired exquisite data of the exceptional event. The Pairitel and KAIT telescopes provided thousands of brightness measurements, spanning a range of optical and infrared wavelengths. Combining their in-house data with public ultraviolet and X-ray data taken by Swift, the team was able to construct the brightness history of the event in detail. "Our decision was, ‘We are going to try to get our data out as fast as possible so that other people can use it,' " says Perley. Conveniently, Bloom's group had been planning a retreat to Lake Tahoe the following weekend. While ten feet of snow glistened invitingly in the sunshine outside their cabin, the team stayed in and worked through their data. Just six days after the burst, they had submitted and posted their paper. "In some ways, it's like playing poker where everyone can see everyone else's cards," says Bloom about studying a publicly announced event. "To make a significant impact, you either have to be quick or have the best cards." This time, UC Berkeley had both. Their beautiful measurements are helping astronomers refine their models of the physics inside GRBs. One theory proposes that GRB 080319B was off the charts because Earth lay directly along its jet beam's centerline, rather than in more typical spot closer to the beam's edge.

Researchers like Bloom are also starting to think about how to use GRBs as tools for understanding how galaxies form. Since GRBs are so bright, astronomers can observe them even in very distant galaxies too faint to detect otherwise. Such galaxies appear to us as they looked long ago, when they were first forming, because of the time light takes to travel to us. The light from a GRB passes through matter within its home galaxy, leaving an imprint that contains clues about the inner workings of that galaxy. Together, these attributes mean that GRBs may make excellent probes of galaxies as they form. And seeing very bright GRBs, like 080319B, "gives us a extra little hope that indeed current and upcoming instrumentation are going to be able to detect these things beyond where any galaxies have been found," Bloom says. He adds, "Berkeley is now one of the world centers of gamma ray burst research. There's a nice confluence of people and resources for the time that is really making this one of the hot beds. I'm very happy to be part of it."

Linda Strubbe is a graduate student in astronomy.

Want to know more? Check out:
Bloom, J. et al. (2008), "Observations of the Naked-Eye GRB 080319B: Implications of Nature's Brightest Explosion" arxiv.org/abs/0803.3215

Bloom lab webpage: astro.berkeley.edu/~jbloom/

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