Campus News
Gamma Ray Bursts May Come From “Failed” Supernova Explosions
SAN DIEGO, CA–Astronomers at the University of California, Santa Cruz, have developed a theoretical model that may explain recent observations of tremendously powerful gamma ray bursts, the most luminous phenomena in the universe. Andrew MacFadyen, a graduate student researcher, and Stan Woosley, a professor of astronomy and astrophysics, based their model on a scenario in […]
SAN DIEGO, CA–Astronomers at the University of California, Santa Cruz, have developed a theoretical model that may explain recent observations of tremendously powerful gamma ray bursts, the most luminous phenomena in the universe. Andrew MacFadyen, a graduate student researcher, and Stan Woosley, a professor of astronomy and astrophysics, based their model on a scenario in which the processes leading to a supernova explosion fail, resulting instead in the formation of a black hole. MacFadyen is presenting their findings today to the American Astronomical Society meeting in San Diego.
Gamma ray bursts-short blasts of very intense radiation-have puzzled astronomers for more than 25 years. Over 2,000 have been observed and their random distribution in the sky indicates they originate outside our galaxy. But they last only seconds, making it hard to pinpoint their origin and determine how far away they are.
"They happen every day and are very bright, but until recently no one had a clue about what they were," says Woosley.
Because gamma rays do not penetrate the Earth’s atmosphere, they can only be detected by satellites. Last year astronomers found that gamma ray bursts are accompanied by a faint afterglow of visible light and radio emissions detectable by ground-based telescopes. Since then, researchers have succeeded in pinpointing the origins of three bursts.
Many of the competing theories developed to explain gamma ray bursts crumbled in the aftermath of an extremely distant and enormously powerful burst recorded in December 1997. After determining that it originated in a galaxy 12 billion light-years from Earth, a team of researchers from several institutions calculated that this gamma ray burst released a phenomenal amount of energy. For six seconds it appeared as luminous as 10 billion billion suns, Woosley says.
"As theoretical astrophysicists, our job is to explain what powers these enormous explosions, so we build mathematical models of situations that could result in such observations," says Woosley.
MacFadyen began working two years ago to refine a model Woosley had first proposed in 1993. The model starts with an aging massive star, at least 25 times the mass of the Sun, in which the forces that ordinarily would lead to a supernova explosion somehow go awry. The star is also presumed to be a member of a binary system (two companion stars) and to have lost its hydrogen envelope to its companion. As a result, when the star dies it is a bare core of helium and heavy elements (known as a Wolf-Rayet star) about 10 times the mass of the sun.
A typical supernova is thought to occur in a star initially 8 to 25 times the mass of the Sun as it exhausts the fuel in its core, says MacFadyen. As the star runs out of energy to counteract the force of gravity, the core of the star starts to implode, compressing its mass into a dense neutron star and releasing an enormous amount of energy in a supernova explosion. Driving the explosion, according to the leading theory, are neutrinos emitted by the forming neutron star and captured by the collapsing stellar mantle.
"But when our colleagues run the computer models based on this theory, the very massive stars don’t always explode," MacFadyen says.
If it doesn’t explode in a supernova, the star collapses into something much denser than a neutron star, namely a black hole. Because the star is spinning, it forms a disk of rotating material being pulled into the black hole. Counteracting the gravitational pull of the black hole is the angular momentum of the spinning matter. Matter along the axis of rotation, however, with little or no angular momentum, disappears into the black hole leaving a central area of low density.
"The result is like a huge spinning doughnut with the black hole in the middle," MacFadyen says.
As matter from the disk collapses into the black hole it releases energy, which escapes in a jet through the area of low density along the axis of rotation. This high-velocity jet, as it collides with gas around the star, would produce the gamma ray burst, MacFadyen says.
Failure of the engine that normally powers a supernova is probably a rare event and may depend on the mass of the star or other variables in the initial conditions, MacFadyen says. Other theorists have proposed similar models for gamma ray bursts involving black holes, and the explosion of energy they predict has been dubbed a hypernova. In some respects, these models are similar to current models for quasars, which are powered by much larger black holes, says Woosley.
"According to these models, a gamma ray burst is like a miniature-sized quasar," he says.
MacFadyen and Woosley’s model does not address the exact mechanisms by which the energy released is converted into gamma rays, Woosley notes. Their computer calculations show that energy deposited in the vicinity of the black hole is channeled along the evacuated rotational axis into tightly focused relativistic jets that escape the star with ease. Slower-moving matter is also ejected from the star.
These results appear to correspond well with recent astronomical observations. On April 25, 1998, astronomers in the Southern Hemisphere observed a supernova (SN 1998bw) that appeared to coincide in timing and location with a gamma ray burst (GRB980425). If confirmed, these preliminary findings would show that some gamma ray bursts are accompanied by a supernova-like event.
"If I could have asked for an observation to be made, this would be it, because it is exactly what our model would predict-jet formation and a gamma-ray burst from a stellar explosion in a massive helium star," MacFadyen says.