University of Chicago scientists this week will challenge the
reigning theory about the puzzling nature of gamma-ray bursts, which
signal the birth of black holes.

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

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
will present the challenge in a series of papers this week (Sept. 9,
11 and 12) at the 2003 GRB Conference in Sante Fe, N.M.

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

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 supernovas,
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 supernovas, 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

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

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

“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 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 mission, have proposed
that NASA extend the mission, which is scheduled to end Jan. 31,
2004. Lamb noted that the $25 million HETE 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

The 2003 Gamma-Ray Burst conference Web site is available at