Michelle Thaller
There Be Monsters: Gamma-Ray Bursters
When I look up into the cold, starry night sky, I can understand why people throughout history have thought the heavens to be perfect and eternal. Everything looks so peaceful and distant; it's easy to imagine the stars being hung from vast, icy spheres of crystal.
Modern astronomy gives us a much different picture. We know now that the universe is a swirl of creation and destruction. In our own galaxy, we chart the locations of giant clouds of gas and dust that are birthing hundreds of stars at a time. We watch stars rip themselves apart, sometimes exploding so violently that a single dying star outshines an entire galaxy of a trillion stars. Farther out into the universe, we see quasars, young galaxies being consumed by titanic central black holes, spewing matter and energy into jets that we can see a billion light years away. We know there are monsters out there.
But recently, we found something else. I like using the analogy of exploring the ocean, because underneath the water lies another realm of mysteries. It's almost certain that there are giant creatures, perhaps the basis of the sea-monster legends, we haven't found yet. What's happened to astronomers is this: in terms of the ocean, we've been cataloging sharks and barracudas. The tentacle of a 90-foot giant squid just tapped us on the shoulder. In the last few years, astronomers have gotten a glimpse of the biggest monster yet found: the gamma-ray burster.
At first glance, gamma-ray bursters don't seem very impressive. Their name says it all: just bursts of gamma-rays from some random direction in the sky, lasting only a few seconds. Like so many of the most dramatic discoveries, they were detected by complete accident.
In the late 1960s a series of satellites were launched to detect nuclear weapons tests in other countries. The Vela Satellites were looking for gamma rays, a super-high-energy kind of light created by nuclear explosions, specifically hydrogen bombs. Gamma rays are very rare in nature, because in order to give off this kind of light, an object has to have a temperature over a billion degrees. As the Vela satellites scanned the Earth for these high temperature explosions, they detected about 2-3 gamma ray "pops" a day that couldn't have come from nuclear tests.
Why not? The reason was simple: these gamma rays were coming from the wrong direction, from above the satellites, up in space. No one gave this much thought (and thankfully, no one seemed to be worried about nuclear tests being conducted by aliens) for some time, until some astronomers decided to take a closer look at these events.
A burst of gamma rays doesn't tell you much. True, whatever is creating the radiation has to be pretty energetic to get up to temperatures of billions of degrees, but we know of objects that can do that, like a supernova explosion. But unlike supernovae, gamma-ray bursters didn't seem to create any other form of radiation; no visible light, no heat, nothing but gamma-rays.
One thing astronomers noticed right away was that the gamma-ray bursts were distributed completely randomly around the sky. That seemed telling. Our galaxy is shaped like a disk, the edge of which you can see as the faint, glowing line across our sky we call the Milky Way. This dim shine comes from the combined light of the billions of stars that make up the body of our galaxy. If the gamma ray bursts were coming from stars in the disk of the Milky Way, they should be concentrated in that area of the sky.
Almost all the stars in our galaxy exist in the disk of the Milky Way, but there are exceptions. There are some dim, ancient stars that swing around the center of our galaxy in large, randomly oriented orbits. It's thought that these stars formed before our galaxy had really organized itself, before the disk stars had been born. Taken together, these older stars make up a sparse, dim sphere centered around our galaxy, called the "halo" of the Milky Way.
Maybe the gamma-ray bursters come from these stars. When fairly massive stars die, they can become neutron stars, hot compact balls of neutrons that have been known to produce little pops of gamma rays now and again. Since halo stars are so old, surely some of them have collapsed into neutron stars. Gamma-ray bursts never lasted longer than a few minutes, the peak lasting only a fraction of a second. Perhaps these events were some kind of cracking or faulting on the surface of a neutron star? It seemed like a good first guess, and without more clues, nothing much else was said about gamma-ray bursters for some time.
Finally, astronomers got a break in the case. An ingenious idea was hatched to probe the mystery of gamma-ray bursters. Two orbiting observatories, the Compton Gamma-Ray Observatory and Beppo-Sax were both scanning the sky for a good, strong gamma-ray burster. Once either satellite found one, it would relay the information down to Earth in a few seconds, allowing astronomers to turn their biggest ground-based telescope to the same area of the sky. Maybe there was something to be seen in the few seconds or minutes after the burst that would help close the book on these events.
On January 23, 1999, both satellites sent out the call. An automated array of small telescopes in New Mexico called ROTSE-1 was able to observe the burster just 25 seconds after it had been detected. Within four hours the 200-inch telescope at Mount Palomar had picked it up, followed by the largest optical telescope in the world, the Keck in Hawaii.
And finally, something other than just a burst of gamma-rays was seen. For just a few seconds, there was a visible glow strong enough to be seen with a good pair of binoculars. And for a day afterward, a dim afterglow remained that the giant telescopes were able to follow and analyze. With actual visible light to work with, astronomers were able to take a spectrum of the glowing region, and measure its redshift.
The redshift is an incredibly powerful tool for astronomers, as it's the only way to estimate distances once you get out into the realm of faraway galaxies (see my column "The Cosmic Distance Scale for more). It's basically a measurement of the distance light has traveled through the expanding universe. The expansion of space causes light to lose energy, which is called "reddening" in astrophysical terms. A wave of light may start out as an X-ray (very high energy light) in a distant galaxy, but by the time it reaches us, it's been "reddened" into a less energetic kind of light, like ultraviolet or even visible light.
When astronomers finally measured the redshift of the gamma-ray burster afterglow, I know for a fact they scratched their heads and went back to their computers, thinking they'd made a mistake. The implications were simply unbelievable.
Their measurement suggested that the afterglow was ten billion light years away, almost at the edge of the known universe. Instead of tiny neutron star pops from the outer confines of our own galaxy, these gamma rays had originated in colossal explosions bright enough to be seen from ten billion light years away. And don't forget the redshift!
If this light was arriving at the Earth as gamma rays after ten billion years of redshift, what kind of light was it when it started? Instead of billions of degrees, it must have been quadrillions or quintillions of degrees (or maybe even a number we don't have a name for).
How bright is an object that you can physically see explode from ten billion light-years away? In astronomical terms, when whatever produced the gamma-ray burster exploded, it shone with the light of 1,000,000,000,000,000,000,000 Suns (sorry, I just have to do that sometimes), or the combined light of ten billion galaxies. All at once, in the space of just a few seconds. In a real sense, that object out-shone the combined light and energy from the entire rest of the universe at the point of explosion, and produced conditions unseen since the Big Bang.
And we detect one of these beasts two or three time a day! Now, in true scientific fashion, people are scrambling to come up with an explanation of what sort of a monster could possibly produce that much energy (and if there's any chance of it happening close to our own neighborhood). It's important to emphasize that we don't have any idea what gamma-ray bursters are, we're just trying to do the physics and guess what could possibly create an explosion that intense.
Numbers suggest the only event that could pack that much punch is the creation of a massive black hole, either by two neutron stars colliding, or the collapse of a very massive, rapidly rotating star (recently termed a collapsar). I've started to hear astronomers use words like "hypernova", as compared to a supernova, which had held the previous title of the most violent explosion in the universe.
But the real truth is we don't know what creates these titanic explosions. We've just seen the giant tentacle slide back into the water. What lies connected to it underneath the surface has yet to be seen. There are monsters out there, and the biggest and scariest are yet to be found.