Scientists
Piece Together The Most Distant Cosmic Explosion
It came from the edge of the visible universe, the most distant
explosion ever detected.
In this week's issue of Nature, scientists at Penn State University
and their U.S. and European colleagues discuss how this explosion,
detected on 4 September 2005, was the result of a massive star
collapsing into a black hole.
The explosion, called a gamma-ray burst, comes from an era soon
after stars and galaxies first formed, about 500 million to 1
billion years after the Big Bang. The universe is now 13.7 billion
years old, so the September burst serves as a probe to study the
conditions of the early universe.
"This was a massive star that lived fast and died young," said
David Burrows, senior scientist and professor of astronomy and
astrophysics at Penn State, a co-author on one of the three reports
about this explosion published this week in Nature. "This star was
probably quite different from the kind we see today, the type that
only could have existed in the early universe."
The burst, named GRB 050904 after the date it was spotted, was
detected by NASA's Swift satellite, which is operated by Penn
State. Swift provided the burst coordinates so that other
satellites and ground-based telescopes could observe the burst.
Bursts typically last only 10 seconds, but the afterglow will
linger for a few days.
GRB 050904 originated 13 billion light years from Earth, which
means it occurred 13 billion years ago, for it took that long for
the light to reach us. Scientists have detected only a few objects
more than 12 billion light years away, so the burst is extremely
important in understanding the universe beyond the reach of the
largest telescopes.
"Because the burst was brighter than a billion suns, many
telescopes could study it even from such a huge distance," said
Burrows, whose analysis focuses mainly on Swift data from its three
telescopes, covering a range of gamma-rays, X-rays, and
ultraviolet/optical wavelengths, respectively. Burrows is the lead
scientist for Swift's X-ray telescope.
The Swift team found several unique features in GRB 050904. The
burst was long, lasting about 500 seconds; and the tail end of the
burst exhibited multiple flares. These characteristics imply that
the newly created black hole didn't form instantly, as some
scientists have thought, but rather it was a longer, chaotic
event.
Closer gamma-ray bursts do not have as much flaring, implying that
the earliest black holes may have formed differently from ones in
the modern era, Burrows said. The difference could be because the
first stars were more massive than modern stars. Or, it could be
the result of the environment of the early universe when the first
stars began to convert hydrogen and helium (created in the Big
Bang) into heavier elements.
GRB 050904, in fact, shows hints of newly minted heavier elements,
according to data from ground-based telescopes. This discovery is
the subject of a second Nature article by a Japanese group led by
Nobuyuki Kawai at the Tokyo Institute of Technology.
GRB 050904 also exhibited time dilation, a result of the vast
expansion of the universe during the 13 billion years that it took
the light to reach us on Earth. This dilation results in the light
appearing much redder than when it was emitted in the burst, and it
also alters our perception of time as compared to the burst's
internal clock.
These factors worked in the scientists' favor. The Penn State team
turned Swift's instruments onto the burst about 2 minutes after the
event began. The burst, however, was evolving as if it were in
slow-motion and was only about 23 seconds into the bursting. So
scientists could see the burst at a very early stage.
Only one quasar has been discovered at a greater distance. Yet,
whereas quasars are supermassive black holes containing the mass of
billions of stars, this burst comes from a single star. The
detection of GRB 050904 confirms that massive stars mingled with
the oldest quasars. It also confirms that even more distant star
explosions---perhaps from the first stars, theorists say--can be
studied through a combination of observations with Swift and other
world-class telescopes.
"We designed Swift to look for faint bursts coming from the edge of
the universe," said Neil Gehrels of NASA Goddard Space Flight
Center in Greenbelt, Md., Swift's principal investigator. "Now
we've got one and it's fascinating. For the first time we can learn
about individual stars from near the beginning of time. There are
surely many more out there."
Swift was launched in November 2004 and was fully operational by
January 2005. Swift carries three main instruments: the Burst Alert
Telescope, the X-ray Telescope, and the Ultraviolet/Optical
Telescope. Swift's gamma-ray detector, the Burst Alert Telescope,
provides the rapid initial location and was built primarily by the
NASA Goddard Space Flight Center in Greenbelt and Los Alamos
National Laboratory and constructed at GSFC. Swift's X-Ray
Telescope and UV/Optical Telescope were developed and built by
international teams led by Penn State and drew heavily on each
institution's experience with previous space missions. The X-ray
Telescope resulted from Penn State's collaboration with the
University of Leicester in England and the Brera Astronomical
Observatory in Italy. The Ultraviolet/Optical Telescope resulted
from Penn State's collaboration with the Mullard Space Science
Laboratory of the University College-London. These three telescopes
give Swift the ability to do almost immediate follow-up
observations of most gamma-ray bursts because Swift can rotate so
quickly to point toward the source of the gamma-ray signal.
