Press Release: GRBs and Massive Stars
[Editors:  Original illustration and Keck Telescope images available at
http://astro.caltech.edu/~jsb]

Embargoed for release at 2 p.m. EDT, Wednesday, September 29, 1999

Largest explosions in the universe may come from the death of massive stars


PASADENA-Cosmic gamma-ray bursts, the brightest known explosions in
the universe, may come from the fiery deaths of very massive stars in
supernova explosions, a team of astronomers said today.

In a paper to appear today in the international journal Nature, the
international team led by the California Institute of Technology
presents evidence that the gamma-ray burst of March 26, 1998 (GRB
980326) is apparently associated with a supernova explosion.

This would then indicate that some gamma-ray bursts are associated
with the formation of black holes during the fiery deaths of very
massive stars.  If true, this would be some of the first direct
evidence for what produces gamma-ray bursts.

As a consequence, the team suggests that a burst of gamma rays are
seen when one of the jets from the supernova's central black hole is
pointed directly toward Earth.  Gamma-ray bursts are brilliant flashes
of high-energy radiation that occur at seemingly random times and from
random places in the sky.

While these objects have been known since 1967, it was only recently
demonstrated that these bursts originate from galaxies in the very
distant universe and are by far the most brilliant bursts in the
universe.  This breakthrough was made possible due to the launch of
the Italian-Dutch satellite BeppoSAX in 1996, which for the first time
pinpointed the location of the bursts with a sufficient accuracy to
enable their detailed studies with ground-based telescopes such as the
W. M. Keck Telescope.

Despite the strides, scientists were still left wondering what
produces these spectacular explosions.  Various theories of their
possible origins are still vigorously debated.  There are currently
two popular models, both suggesting that the bursts originate in a
formation of a black hole.  In one model, two massive objects such as
neutron stars or black holes (both of which may be end-products of
previous supernova explosions) coalesce, forming a single massive
black hole.

In the second model, such a black hole is produced in a catastrophic
collapse of the core of a massive star.  In this model, one then
expects two sources of light: the "afterglow'' emission from the
gamma-ray burst itself and light from the exploding star, a supernova.
The afterglow rapidly declines whereas the supernova explosion gains
in brightness over a period of a few weeks, and then gradually fades
away.

The new study reports on the observations of GRB 980326 carried out at
the W. M. Keck Observatory's 10-m telescope located atop Mauna Kea,
Hawaii.  As in many other cases, a visible light afterglow was found
following the burst, which then rapidly faded away.  However, the
Caltech-led team discovered something never previously observed-a
dramatic rebrightening of optical emission at the position of the
gamma-ray burst.

Normally, the optical light of a gamma-ray burst vastly outshines its
host galaxy for weeks.  When the light from the gamma-ray burst fades,
the apparent total brightness remains constant: all that remains is
the light from the host galaxy.

Shrinivas R. Kulkarni of the Caltech team explains, "A month after GRB
980326, it looked as though the host galaxy was dominating the light."
However, the next time the team observed, some eight months after the
burst, the "galaxy" was gone.

"Galaxies do not just disappear, so we were astonished," Kulkarni
said.  "Clearly, what we were seeing is a new source of light
brightening one month and then fading away. This is something quite
new."

This unexpected rebrightening is now believed to be due to the
underlying supernova created in the explosion of the massive star.
The team had also obtained spectra of the object at different times,
and that provided additional clues.

"The spectrum of the source right after the burst was blue, which is
common," said S. George Djorgovski of Caltech.  "But after a month it
was very red, which was unexpected.

"That alone suggested that we were looking at some different
phenomenon happening at the same location, but with a time delay of a
few weeks."

Both the rebrightening and the spectrum changes are naturally
explained by the presence of a supernova.  The intensity of the
apparent re-burst matches the peak brightness of a supernova seen in a
distant galaxy, and its red spectrum also has the right color.

This represents the most direct evidence to date in favor of the
massive supernova model.  In this scenario, a black hole is quickly
formed in the center of a massive star whose core is unable to support
itself against gravity.

When the star explodes, powerful jets from the central black hole
emerge along the original axis of rotation, and gamma rays are created
by the jets.  If the jets are not pointed toward Earth, then we see
only a supernova and the effects of the exploding star.  But gamma
rays as well as the light from the supernova arrive at Earth if the
jets are pointing in our direction.

Joshua S. Bloom, a graduate student at Caltech and lead author of the
paper said, "This appears to be the smoking gun for the origin of some
gamma-ray bursts, a perfect marriage of the two brightest events in
the universe.  It is wonderful to be a part of such a discovery."

Gamma-ray bursts, since their discovery some 30 years ago, have over
150 theoretical models about their possible origins, but only a
handful can come close to describing the true trigger of the bursts.

"It is possible that there are other causes for gamma-ray bursts such
as the coalescence of neutron stars," Bloom said.  "Undoubtedly,
astronomers will focus on unearthing new classes in the years to
come."

Early reports of the results created some excitement in the
astronomical community.  Two other groups, from universities of
Amsterdam and Chicago, in view of the work presented by the Caltech
team, have reanalyzed the data on some other gamma-ray bursts.  They
appear to find good evidence for an underlying supernova in another
well-studied gamma-ray burst.

"It is encouraging to have had such a resounding reception to an
unexpected result," said Kulkarni.  "Even some of the initial skeptics
seem to be converted by these results."

Other members of the Caltech team are graduate student
A. C. Eichelberger; postdoctoral scholars P. Côté,
J. P. Blakeslee, and S. C. Odewahn; and Assistant Professor
F. A. Harrison.

In addition to the members of the Caltech team, the other coauthors
include M. Feroci of the BeppoSAX team; D. A. Frail of the National
Radio Observatory; A. V. Filippenko, D. C. Leonard, A. G.  Reiss,
H. Spinrad, D.  Stern, A. Bunker, B. Grossan, S. Perlmutter, and R. A.
Knop of the University of California at Berkeley; A. Dey of the
National Optical Astronomy Observatory; and I. M. Hook of the European
Southern Observatory.

For more information please contact:

Robert Tindol
Caltech Media Relations
626-395-3631
tindol@caltech.edu

Joshua S. Bloom
Caltech Astronomy Department
626-395-4987 or 626-695-1243
jsb@astro.caltech.edu

Prof. Shrinivas Kulkarni
Caltech Astronomy Department
626-395-4010
srk@astro.caltech.edu

Prof. S. George Djorgovski
Caltech Astronomy Department
626-395-4415
george@astro.caltech.edu

Visit the Caltech Media Relations Web site at:
http://www.caltech.edu/~media/home.html