Supercomputers simulate inner workings of one theory for the cause of bursts
Nov. 3, 1999: Supercomputers are being used to take the human mind on a voyage that no space probe can ever take, to a ringside seat near the center of a dying star in the minutes before it becomes a gamma-ray burst that is seen across the universe.
This virtual seat really is ringside because it places the astrophysicist just outside a swirling accretion disk of matter surrounding the birth of a black hole. And all around, the star is preparing itself for a most spectacular light show.
Welcome to collapsar, the collapsing star scenario that is one of the leading contenders as the cause of gamma-ray bursts.
The collapsar theory was proposed in 1993 by Dr. Stan Woosley of the University of California at Santa Cruz. Since 1996 his work on the collapsar has been done jointly with a UC Santa Cruz graduate student, Andrew MacFadyen. In particular, the multi-dimensional graphics shown here are from MacFadyen's thesis.
Woosley has been well established in astrophysics owing to his work for the last 20 years on the evolution and explosion of massive stars. He reviewed and discussed his concept Oct. 21 during the theory session of the Fifth Biennial Huntsville Gamma Ray Burst Symposium.
By the time of the second symposium, in 1993, more than 150 different models had appeared in scientific literature as possible causes of bursts. Since then scientists have winnowed and refined the possibilities. Woosley cautioned against the notion of arriving at a single solution.
"We shouldn't try to explain everything with one model," he said. "We should push the model as far as possible, but not be surprised that the universe has more going on than just one answer."
Still, the collapsar is an extremely attractive model that fits a wide range of observed gamma-ray bursts. A key requirement for any burst model is how to produce an immense amount of energy in a few seconds.
Before the Burst and Transient Source Experiment aboard the Compton Gamma Ray Observatory piled up evidence that bursts are quite distant, it was possible to build a burst model that only shed modest amounts of energy. But observations of optical and other counterparts since the famous Feb. 28, 1997, burst (GRB 970228) have established that bursts are near the edge of the observable universe.
That means that that their brightness here, in gamma rays, translates into frightening energies at the source. The total is about 1054 ergs. In ordinary terms, it's 1,000 times as much energy in a few seconds as the sun will produce in its estimated 10-billion-year life. The number is also equal to the energy yield if all the sun's mass was converted to energy.
The total energy of 1054 ergs assumes the burst is not beamed but is isotropic or evenly distributed, MacFayden explained. But if the energy is beamed by the jets erupting from the poles, then the burst requires "only" 1 percent as much power, 1052 ergs (or, a mere 10 times as much as our sun will generate in its entire life), and only 1 percent of the sky will be illuminated by the blast. This also implies that bursts are 100 times more common than we detect because only 1 burst in 100 will be pointed at Earth.
Either way, the energy requirerments help select which models survive.
"The models these days, because of the energy numbers, involve gravitational collapse," Woosley said. Another leading theory is the merger of two neutron stars (sometimes written n*+n*), or a neutron star and a black hole (n*+BH), as they revolve around each other. But it has some shortfalls that the collapsar easily satisfies, Woosley said. The chief argument against merging n*+n* and n*+BH right now is that the event should take place far outside galaxies because they are born with high velocities that would take them outside the galaxy before their orbits decayed to allow merger. So far, most optical counterparts for bursts have been associated with faint galaxies.
But he also cautioned that "even this simple model can get you into a lot of complexity."
Stars can take several different routes to oblivion as they fuse the last of the hydrogen in their cores. Our sun will expand to a red giant then shrink to become a white dwarf that slowly fades away. Stars having 10 to 25 solar masses will go into a series of hotter, faster burn cycles that use the ash of one cycle as fuel for the next. The end products are silicon and, a few hours later, iron and nickel.
At this point fusion to heavier elements absorbs more energy than it yields. The furnace is switched off and the star collapses.
The shock that is initially produced following the collapse of the iron core gets its energy from neutrinos (fundamental particles moving at or near the seed of light) emitted by the contracting proto-neutron star which might have a radius of 50 km.
But what happens if the star is a real heavyweight? It fails as a supernova, Woosley contends, and follows either of two paths, prompt or delayed black hole formation.
In the prompt formation case, the shock that usually blows up a supernova fails to be launched. Within a second the neutronized core is converted into a black hole that accretes infalling material thru a disk at about 1/10th of a solar mass per second.
In the delayed formation case, a shock is launched, and is successful in the sense that it never stops moving out and would have made it to the surface of the star, given time. However the shock fails to eject all the star. Enough falls back over about 100 seconds to make a black hole that accretes material through a disk but at about 1/10th of a solar mass per second.
Wrapped around the black hole is an accretion disk of nuclei whose trip inward is delayed by their own angular momentum, like water circling the drain before disappearing.
About 100 seconds after the supernova blast wave departed, it returns with a vengeance. That energy has to go somewhere, so the star blows its top - and bottom.
As the avalanche of stellar material forms the black hole, it also tunnels outward along the star's rotational poles at about 1/3rd the speed of light. The star's mass presses back, so the jet forms a sharp cone with an angle of about 20 degrees.
"That gives a very powerful jet," Woosley said. When the jet erupts like a hellish geyser from the star's poles, the geyser quickly envelopes the whole star with a shock wave that rips the star apart.
"You have no idea anything is happening until the flare erupts from the surface of the star," he said, "because the jet has been capped while it's drilling through the star."
From that point the collapsar becomes a "relativistic fireball" with material hurled nearly at the speed of light into the solar wind the star had been blowing off until a few minutes ago and then into the interstellar medium.
Woosley admits that the collapsar theory is still a work in progress and depends on conjectures "both reasonable and unproven." Nor does the model yet predict all types of bursts, such as short, fast bursts lasting a few seconds.
Meanwhile, Woosley and his colleagues continue to refine their models in supercomputers with advanced programs that describe the behavior of fluids under extreme conditions. No spacecraft will ever have so good a ringside seat. Never mind getting inside a star. Gamma-ray bursts appear to be a creature of the past, occurring early in the universe when it was easier to make supermassive stars. Bursts, as we see them today, are from the cosmic fossil record.
So, like the creatures of Jurrasic Park,the inner workings of gamma-ray bursts - be it collapsars or some other model - will be something we can view only through the eyes of a computer.