Trent J. Perrotto
Headquarters, Washington
202-358-0321
trent.j.perrotto@nasa.gov

RELEASE: 11-401

NASA'S SWIFT FINDS A GAMMA-RAY BURST WITH A DUAL PERSONALITY

WASHINGTON -- A peculiar cosmic explosion first detected by NASA's
Swift observatory on Christmas Day 2010 was caused either by a novel
type of supernova located billions of light-years away or an unusual
collision much closer to home, within our own galaxy. Papers
describing both interpretations appear in the Dec. 1 issue of the
journal Nature.

Gamma-ray bursts (GRBs) are the universe's most luminous explosions,
emitting more energy in a few seconds than our sun will during its
entire energy-producing lifetime. What astronomers are calling the
"Christmas burst" is so unusual that it can be modeled in such
radically different ways.

"What the Christmas burst seems to be telling us is that the family of
gamma-ray bursts is more diverse than we fully appreciate," said
Christina Thoene, the supernova study's lead author, at the Institute
of Astrophysics of Andalusia in Granada, Spain. It's only by rapidly
detecting hundreds of them, as Swift is doing, that we can catch some
of the more eccentric siblings."

Common to both scenarios is the presence of a neutron star, the
crushed core that forms when a star many times the sun's mass
explodes. When the star's fuel is exhausted, it collapses under its
own weight, compressing its core so much that about a half-million
times Earth's mass is squeezed into a sphere no larger than a city.

The Christmas burst, also known as GRB 101225A, was discovered in the
constellation Andromeda by Swift's Burst Alert Telescope at 1:38 p.m.
EST on Dec. 25, 2010. The gamma-ray emission lasted at least 28
minutes, which is unusually long. Follow-up observations of the
burst's afterglow by the Hubble Space Telescope and ground-based
observatories were unable to determine the object's distance.

Thoene's team proposes that the burst occurred in an exotic binary
system where a neutron star orbited a normal star that had just
entered its red giant phase, enormously expanding its outer
atmosphere. This expansion engulfed the neutron star, resulting in
both the ejection of the giant's atmosphere and rapid tightening of
the neutron star's orbit.

Once the two stars became wrapped in a common envelope of gas, the
neutron star may have merged with the giant's core after just five
orbits, or about 18 months. The end result of the merger was the
birth of a black hole and the production of oppositely directed jets
of particles moving at nearly the speed of light, followed by a weak
supernova.

The particle jets produced gamma rays. Jet interactions with gas
ejected before the merger explain many of the burst's signature
oddities. Based on this interpretation, the event took place about
5.5 billion light-years away, and the team has detected what may be a
faint galaxy at the right location.

"Deep exposures using Hubble may settle the nature of this object,"
said Sergio Campana, who led the collision study at Brera Observatory
in Merate, Italy.

If it is indeed a galaxy, that would be evidence for the binary model.
On the other hand, if NASA's Chandra X-ray Observatory finds an X-ray
point source or if radio telescopes detect a pulsar, that goes
against it.

Campana's team supports an alternative model that involves the tidal
disruption of a large comet-like object and the ensuing crash of
debris onto a neutron star located only about 10,000 light-years
away. The scenario requires the break-up of an object with about half
the mass of the dwarf planet Ceres. While rare in the asteroid belt,
such objects are thought to be common in the icy Kuiper belt beyond
Neptune. Similar objects located far away from the neutron star may
have survived the supernova that formed it.

Gamma-ray emission occurred when debris fell onto the neutron star.
Clumps of cometary material likely made a few orbits, with different
clumps following different paths before settling into a disk around
the neutron star. X-ray variations detected by Swift's X-Ray
Telescope that lasted several hours may have resulted from
late-arriving clumps that struck the neutron star as the disk formed.


In the early years of studying GRBs, astronomers had very few events
to study in detail and dozens of theories to explain them. In the
Swift era, astronomers have settled into two basic scenarios, either
the collapse of a massive star or the merger of a compact binary
system.

"The beauty of the Christmas burst is that we must invoke two exotic
scenarios to explain it, but such rare oddballs will help us advance
the field," said Chryssa Kouveliotou, a co-author of the supernova
study at NASA's Marshall Space Flight Center in Huntsville, Ala.

NASA's Swift was launched in November 2004 and is managed by Goddard.
It is operated in collaboration with several U.S. institutions and
partners in the United Kingdom, Italy, Germany and Japan.

For more information and video associated with this release, visit:

http://www.nasa.gov/swift