On January 23 at 4:47 a.m. EST, astronomers discovered that something big had exploded in the distant reaches of the cosmos. In less than a few tens of seconds, the blast emitted energy that might correspond to 10,000 times the energy released by our sun over the past five billion years. This so-called gamma-ray burst, now known officially as GRB 990123, was one of the most energetic and the most closely observed ever. But the cause of these random and elusive flashes remains a mystery.
Like the flashbulbs of paparazzi, gamma-ray bursts pop off all the time in the universe, about one a day. They are marked by a fleeting, powerful pulse of gamma rays. If astronomers act quickly, they can turn other instruments toward the point of origin and record a rapidly fading afterglow of x-rays, visible light and radio waves. In the case of GRB 990123, astronomers were able for the first time to obtain a record of the event from a variety of instruments as its radiation reached Earth.
"The burst appeared to be more luminous than the whole rest of the universe, and that would be very hard to explain by most current theories," says California Institute of Technology professor of astronomy Shrinivas Kulkarni, who was part of a team that investigated GRB 990123. Their initial results on the discovery and its afterglow were published in the March 26 issue of Science and the April 1 issue of Nature; additional details about the host galaxy will appear in a forthcoming issue of the Astrophysical Journal Letters.
These mysterious flashes of energy were discovered in the 1960s by U.S. military Vela satellites. In 1991 NASA launched the Compton Gamma-Ray Observatory satellite, which carried the Burst And Transient Source Experiment (BATSE). It has detected several thousand bursts so far. But the key to studying them is locating them quickly enough to point other instruments at their sources before the afterglow fades. That problem was finally solved in early 1997, when an Italian/Dutch satellite named BeppoSAX was launched. Its mission: to detect bursts and alert astronomers worldwide.
When GRB 990123 was spotted by BeppoSAX on January 23, investigators immediately scrambled into action. Within three hours, a Caltech team, including senior postdoctoral scholar in astronomy Stephen Odewahn and graduate student Joshua Bloom, had pointed Palomar Observatory's 60-inch telescope at the site to discover a rapidly fading visible-light afterglow (animation below). "This adventure began at 5 a.m. with a wake-up call from our Italian friends," says Bloom.
Several hours later, a team of astronomers known as the ROTSE (Robotic Optical Transient Search Experiment) collaboration, led by Carl Akerlof of the University of Michigan, reported that the visible-light counterpart of the burst was also seen in the images taken with a small, robotic telescope operated by their team, starting only 22 seconds after the burst. This was the first time that such rapid measurement of a burst afterglow was made.
Meanwhile, a new radio source at the same location as the visible-light afterglow discovered at Palomar was found at the National Radio Astronomy Observatory's Very Large Array radio telescope, near Socorro, N.M., by Kulkarni and his colleagues. As soon as they were alerted, astronomers at the W. M. Keck Observatory's 10-meter Keck-II telescope at Mauna Kea, Hawaii, recorded a spectrum of the burst afterglow that would permit astronomers to calculate the distance of the burst from Earth.
Analysis of the spectrum confirmed earlier findings that the bursts come from distant space, not inside our galaxy. Michael Andersen and his colleagues at the Nordic Optical Telescope in the Canary Islands that the blast took place nine billion light-years away. "We were stunned," says S. George Djorgovski of Caltech, one of the principal investigators on the team. "This was much further than we expected, and together with the observed brightness of the burst it implied an incredible luminosity."
As GRB 990123 faded, astronomers at Keck Observatory also observed a faint galaxy adjacent to it in infrared image that they believe is where the burst originated. To confirm that finding, the Hubble Space Telescope obtained visible-light images of this galaxy and the burst's afterglow (image, top). The host galaxy is not the classic spiral or elliptical shape. Rather it consists of fingerlike filaments extending above the bright blob of the fireball.
Astronomers speculate that the galaxy might have been distorted by a collision with another galaxy. This would induce rapid starbirth as gas clouds were heated and compressed, precipitating millions of newborn stars. Both the Hubble and Keck images indicate that the galaxy is exceptionally blue, indicating the presence of a large number of blue newborn stars. These observations strengthen the idea that these powerful explosions take place in regions where new stars are being formed.
The new data also raise the possibility that these enormous explosions are not quite as cataclysmic as they seem. Calculations of the energy released assume it is emitted in all directions. But there are indications that the telescopes could have observed an enormous jet or beam aimed toward Earth. Follow-up study by the Caltech investigators revealed a change in the brightness of the afterglow that might be a sign of a jet of energy. "This was the first time that such behavior was seen in a gamma-ray burst," says Kulkarni, an author of a report in Nature, "and it may help explain in part its enormous apparent brightness."
Although the results of GRB 990123 observations add a few pieces to the puzzle, the cause of these explosive flashes remains a mystery. Nothing in the new data seems to narrow the possibilities, which include the collapse of a massive star into a black hole, collisions between neutron stars, or a neutron star colliding with a black hole. So for now, gamma-ray bursts remain the most mysterious phenomenon in the cosmos.