Cosmologists solving mysteries of gamma-ray burstsBy Steve Koppes
Astrophysicist Don Lamb has just completed one of the most scientifically explosive months of his career.
On Sept. 12, scientists announced that NASA’s Swift satellite and ground-based telescopes had discovered the most distant exploding star on record. The exploding star, called a gamma-ray burst, or GRB, confirmed a 1999 prediction made by Lamb, the Louis Block Professor in Astronomy & Astrophysics, and Daniel Reichart (M.S. ’98, Ph.D. ’00), now a professor at the University of North Carolina, Chapel Hill.
And on Oct. 6, the journal Nature published two papers, including one co-authored by Lamb, which solve the last remaining mystery surrounding the causes of gamma-ray bursts, the most powerful explosions in the universe.
Both developments herald even more dramatic scientific discoveries to come. “Now the fun begins,” Lamb said.
What produces GRBs baffled scientists for more than three decades after their discovery in 1969. GRBs last anywhere from fractions of a second to many minutes. They occur almost daily, come from any direction in the sky and shine billions of times brighter than their host galaxies.
In 2003, solid evidence began to emerge linking long GRBs to the collapse of massive stars. Now Lamb and his colleagues have shown that the short versions of these bursts result from the collision of closely orbiting compact stars, or one of those stars and a black hole.
“The mystery of short gamma-ray bursts is solved,” Lamb said. “Even the most conservative person would say it’s largely solved.”
The Oct. 6 Nature paper details the discovery of a GRB that was found by NASA’s High-Energy Transient Explorer 2 satellite on Saturday, July 9. The paper’s 33 co-authors include Lamb, who is HETE-2’s mission scientist; Carlo Graziani, Senior Research Associate in Astronomy & Astrophysics; and Timothy Donaghy, a Chicago graduate student in physics.
A second paper in the same issue of Nature, written by a separate team, will present key follow-up observations of the July 9 burst recorded by NASA’s Chandra X-ray Observatory and Hubble Space Telescope and a variety of ground-based telescopes.
The papers show that short GRBs result from colliding neutron stars. The burned-out cores of dead stars, neutron stars are extremely compact, measuring less than 10 miles in diameter. As for the future, Lamb noted that merging neutron stars are powerful sources of gravitational waves. Albert Einstein incorporated gravitational waves into his 1916 general theory of relativity.
No direct measurement of gravitational waves has ever been made. The relative proximity of short GRBs to Earth means future experiments could detect the gravitational waves they emit. The search for gravitational waves is in progress at the Laser Interferometer Gravitational-Wave Observatory, known as LIGO, in Washington and Louisiana. “The first gravitational wave source that LIGO sees may well be a short gamma-ray burst,” Lamb said.
Lamb also anticipates that Swift will detect GRBs at even greater distances than the Sunday, Sept. 4 burst. These bursts would allow scientists to probe more deeply into the past of the universe, which is an estimated 13.7 billion years old.
In astronomical terms, the Sunday, Sept. 4 GRB had a redshift of 6.29. Redshift is a measure of the amount that light from a distant object has shifted toward the red end of the light spectrum by the expansion of the universe. The higher the redshift, the greater the distance and the younger the universe was when the light was emitted.
A redshift of 6.29 translates to a time when the universe was approximately a billion years old. At a 1999 scientific meeting and in a 2000 paper published in the Astrophysical Journal, Lamb and Reichart predicted that Swift would be able to detect GRBs at such ancient times. Back then, astrophysicists generally thought that the most distant GRBs would be found at a redshift of 2 or 3. They assumed that beyond that distance there would be no GRBs because few massive stars would have formed by then, when the universe was only two or three billion years old.
But Lamb and Reichart’s calculations, based on emerging cosmological evidence, showed that the bursts should occur as far away as a redshift of 20, when the universe was only a few hundred thousand years old. The most distant known object today is a galaxy at a redshift of 6.5.
“This is uncharted territory,” Reichart said. “We are finally starting to see the remnants of some of the oldest objects in the universe.”
Reichart led the team that discovered the afterglow of the Sunday, Sept. 4 GRB, which culminated in the confirmation of his and Lamb’s earlier prediction. “Dan and I have been patiently waiting six years for this discovery,” Lamb said.