A key element of their theory is the discovery that gamma-ray bursts occurring early in the history of the universe are brighter and therefore just as easy to see as modern bursts. The discovery means that scientists can use the ancient bursts to identify the moment when the first stars formed and address other important cosmological questions.
Combining data from NASA’s High Energy Transient Explorer 2 (HETE-2) satellite with computer simulations, a team led by Chicago’s Don Lamb Jr. has for the first time firmly linked three types of cosmic
explosions, including gamma-ray bursts, to the collapse of massive stars. Lamb, the Louis Block Professor in Astronomy & Astrophysics and HETE-2 mission scientist, and his team presented the challenge in a series of papers this week at the 2003 GRB Conference in Sante Fe, New Mexico, in September.
Dale Frail, a scientist at the National Radio Astronomy Observatory (NRAO) in Socorro, N.M., said that Lamb has fit together several pieces of a large puzzle. “His explanation for the diversity that we see in cosmic explosions is so simple and elegant, you want it to be true,” Frail said. “I would caution that it is not the only way to put those pieces of the puzzle together, so time will tell if he is right.”
Since their discovery in 1973, gamma-ray bursts have ranked among the greatest mysteries of astronomy. Only recently have scientists produced evidence linking the longest of these bursts to supernovae, which result from the collapse of massive stars. Lasting anywhere from fractions of a second to many minutes and packing the power of as many as 1,000 supernovae, these unpredictable bursts occur almost daily and come from every direction in the sky. The bursts are followed by afterglows that are visible for a few days at X-ray and optical wavelengths. HETE-2 was specifically designed to make it possible for astronomers to observe these afterglows.
Related to the mystery of gamma-ray bursts proper are the fainter, X-ray-rich gamma-ray bursts and the fainter-still X-ray flashes. Lamb and his associates, Carlo Graziani and Tim Donaghy, propose that massive, collapsing stars account for all three types of cosmic explosions. “X-ray flashes, X-ray-rich gamma-ray bursts, and gamma-ray bursts have a continuum of properties and are almost certainly the same phenomenon,” Lamb said.
The type of emissions HETE-2 detects depends on the angle at which they are viewed from Earth. “The gamma-ray burst is like a laser pointed at you. It’s incredibly bright because the jet is so narrow, like a pencil beam,” Lamb said. X-ray-rich gamma-ray bursts emit jets at a wider angle, one of approximately 10 degrees. X-ray flashes, meanwhile, emit energy in almost every direction.
The finding builds on the work of the NRAO’s Frail, who suggested that every gamma-ray burst releases the same amount of energy, but the opening angles of their narrow jets vary somewhat. The Chicago team extended that idea to X-ray flashes and X-ray-rich gamma-ray bursts.
Lamb’s team also has extended the work of Italian astrophysicist Lorenzo Amati. Using data from the Italian Space Agency’s BeppoSAX X-ray satellite, Amati found a tight relationship between the peak energy and total energy emitted by a gamma-ray burst. He found that bright bursts correlated with high-energy peaks, and faint bursts correlated with low-energy peaks. Lamb’s team has shown that the same relationship pertains to X-ray flashes and X-ray-rich gamma-ray bursts.
The relationship is more surprising than it sounds, said Graziani, a senior scientist at Chicago.
“In principle, bursts that have high-peak energies could be very faint, or bursts at low-peak energies could be very bright,” Graziani said. “There’s absolutely no reason for those two things to be correlated in the source of a gamma-ray burst.”
According to the dominant theory of gamma-ray bursts, every such burst produces exactly the same type of jet. This jet is intense at the core, but its brightness fades rapidly when viewed at angles away from the core.
“If you try to extend that picture to incorporate the X-ray-rich gamma-ray bursts and X-ray flashes, it fails utterly,” Lamb said. If the dominant theory were correct, HETE-2 would see many more X-ray flashes and X-ray-rich gamma-ray bursts, because they would be viewed at wider angles than gamma-ray bursts proper.
“That’s not what HETE sees at all,” Lamb said. “In fact, HETE sees about the same numbers of gamma-ray bursts, X-ray-rich gamma-ray bursts and X-ray flashes.”
If Lamb and his associates are correct, it means that gamma-ray bursts are approximately 100 times more numerous than previously suspected. “The jets’ openings are so small that we see almost none of them,” Lamb said. “They’re almost all pointed in other directions.”
Key components of the Chicago team’s new theory is their finding on the evolution of gamma-ray bursts and their computer simulations.
The Chicago scientists found that bursts of massive stars occurring when the universe was only 1 billion years old were 1,000 times brighter than those occurring today, 13 billion years later. The team based its finding on data taken on 20 bursts whose ages are well-established. HETE detected 11 of the bursts. The Italian Space Agency’s BeppoSAX contributed nine more. The cause of this evolution remains a matter of speculation.
“One promising explanation of why the jet opening angle varies is how much spin there is in the core of a star when it collapses to a black hole,” Lamb said. In this scenario, a rapidly spinning core would produce a gamma-ray burst with a narrow jet. A slowly spinning core would result in an X-ray flash with an extremely wide jet.
“Maybe the cores were rotating much more rapidly in stars formed earlier,” Lamb explained, while stronger magnetic fields have applied the brakes to the spinning cores of modern stars.
Once the evolutionary scenario was introduced into the team’s computer simulations, everything fell into place, Lamb said. The computer simulations, conducted by Donaghy, a graduate student in physics, closely matched the findings of HETE-2 and BeppoSAX, along with the data from a third source, the Burst and Transient Source Experiment aboard NASA’s now-defunct Compton Gamma-Ray Observatory.
As for the future, Lamb and George Ricker of the Massachusetts Institute of Technology, who heads the HETE-2 mission, have proposed that NASA extend the mission, which is scheduled to end Jan. 31, 2004. Lamb noted that the $25 million HETE-2 satellite has discovered the source of 48 gamma-ray bursts since it was launched in 2000. The Italian Space Agency’s $400 million BeppoSAX satellite, by comparison, has documented 52 since its launch in 1996.
“In bucks per burst, HETE is a bargain,” Lamb said.
Copies of the Chicago team’s papers are available at: http://astro.uchicago.edu/home/web/lamb/jet.dir/
The 2003 Gamma-Ray Burst conference web site is available at http://grb2003.lanl.gov/.
Adapted with permission from a press release issued September 11, 2003, by the University of Chicago News Office.