Astronomers link old stars and mysterious cosmic explosions (Forwarded)

Carnegie Institution of Washington
Washington, D.C.

Contact:
Edo Berger, Carnegie Observatories
626-304-0251

December 15, 2005

Astronomers link old stars and mysterious cosmic explosions

Pasadena, CA -- Cosmic gamma-ray bursts, the most powerful explosions in
the universe, have the extreme brilliance of a billion billion Suns and
occur several times a day. But they are not all created equal. For several
decades astronomers have known that two types exist -- long ones that last
for tens or hundreds of seconds, and short bursts, which last a few
milliseconds to a second. Intense research over the last decade has shown
that long bursts are the death throes of massive stars in distant, young,
and vigorously star forming galaxies. The origin of the short gamma-ray
bursts, however, has been shrouded in mystery until now. Edo Berger, a
Hubble post-doctoral fellow at the Carnegie Observatories, is lead author
of the first study that accurately pinpoints a short gamma-ray burst to an
old dead galaxy, implicating a population of old neutron stars as the
sources of these explosions. The study appears in the December 15, 2005,
issue of Nature.

"We had no idea if they explode in nearby galaxies, or the farthest
reaches of the universe, or even what kind of object was producing them,"
stated Berger. "Now, after eluding us for years, we have finally found out
what objects are giving rise to these explosions." he added.

Because short gamma-ray bursts are fainter than the long bursts, they have
been very difficult to localize until recently, with the advent of NASA's
Swift satellite and rapid follow-up by telescopes on the ground. Swift
detects and observes gamma-ray bursts in multiple wavelengths and alerts
astronomers who quickly point ground-based telescopes to catch the fading
afterglow -- the dying ember that glows for hours or days after the burst
of gamma-rays.

In this study, Berger led a team that discovered and monitored the
afterglow in radio, optical, and infrared wavelengths using telescopes at
Carnegie's Las Campanas Observatory in Chile and the Very Large Array in
New Mexico.They homed in on a gamma-ray burst (GRB 050724) localized by
Swift on July 24 and found that the emission came from a fairly close (3.5
billion light-years) old elliptical galaxy.

"When the images came in from the telescope in the middle of the night I
was shocked," said Berger." We finally determined a precise position for a
short burst, and it happened in the most unlikely of places, an old dead
galaxy." By the second night, the afterglow had completely faded away.

While long gamma-ray bursts result from the death of massive stars only a
few million years old, the fact that the short burst occurred in an old
elliptical galaxy, which has not formed new stars in several billion
years, suggests that a completely different population of sources was at
play. The objects giving rise to short bursts must be very old.

Theorists have long suspected that short gamma-ray bursts are the end
product of a class of binary stars composed of two old neutron stars, or a
neutron star and a black hole, which slowly approach each other as their
orbit shrinks over hundreds of millions of years. This process results
from the emission of gravitational waves, one of the major predictions of
Einstein's theory of general relativity. "Eventually, the two objects get
so close together that they just shred each other apart in a cataclysmic
explosion," said Tsvi Piran, a theoretical astrophysicist from Hebrew
University in Israel and a co-author on the paper.

Because GRB 050724 is the first short burst with detections across the
electromagnetic spectrum, the study reveals that the energy produced in
the burst was channeled out in narrow jets, similar to those observers
have seen in many long GRBs. But the energy was one thousand times lower.
"We have been simulating the explosions from neutron star collisions on
computers for years" stated Stephan Rosswog, a professor of astronomy at
the International University in Bremen, Germany. "It is exhilarating to
finally compare our results to actual data, and I am happy to see that
some of our predictions were validated," he added.

The next stage in the study of short bursts is to locate and study a large
number of these objects. "We have been studying the long GRBs for many
years, and each one produces a new surprise," said Berger. "I can't wait
to see what surprises the universe has in store for us with the short
GRBs."

In addition to Berger and Piran the international team that identified and
researched the afterglow and host galaxy of the short GRB 050724 includes
David Murphy and Eric Persson from Carnegie Observatories, Paul Price and
Len Cowie from the University of Hawaii, Brad Cenko, Avishay Gal-Yam,
Alicia Soderberg, Mansi Kasliwal, Doug Leonard, Brian Cameron, Shri
Kulkarni, Dae-Sik Moon, Derek Fox, and Fiona Harrison from Caltech, Dale
Frail from the National Radio Astronomy Observatory, Wojtek Krzeminski
from Las Campanas Observatory, Brian Lee from the University of Toronto,
Kathy Roth from the Gemini Observatory, Brian Schmidt, Joshua Rich, and
Bruce Peterson from Mount Stromlo Observatory, and Bryan Penprase from
Pomona College.

More information on Swift can be found at:
http://swift.gsfc.nasa.gov/

Information about the Carnegie Observatories is at:
http://www.ociw.edu/

Gamma-ray burst research at Carnegie and Caltech is supported in part by
funds from NASA. E.B. and A.G.Y. are supported by NASA through Hubble
Fellowship grants awarded by the Space Telescope Science Institute, which
is operated by AURA, Inc., for NASA. The VLA is operated by the National
Radio Astronomy Observatory, a facility of the National Science Foundation
operated under cooperative agreement by Associated Universities, Inc.

Image Caption:
[http://www.carnegieinstitution.org/I...ow_0512_06.jpg
(34KB)]
These infrared images of GRB 050724 and its host galaxy were taken with
the Carnegie Observatories' Magellan telescopes. The red color of the
elliptical host galaxy shows that no new stars have been forming for
several billion years, implicating old neutron stars, or a neutron star
and a black hole, as the source of short gamma-ray bursts (Image courtesy
Edo Berger / Carnegie Observatories.)