Magnetic Explosions in the Distant Universe (Forwarded)

Royal Astronomical Society Press Notice
London, U.K.

CONTACTS:

Professor Pawan Kumar
Department of Astronomy
University of Texas at Austin, USA
Tel: (+1) 512-471-3412

Dr. Paul O'Brien
Department of Physics & Astronomy
University of Leicester, UK
Tel: (+44) (0)116 252 5203

Dr. Silvia Zane
Mullard Space Science Laboratory
University College London, UK
Tel: (+44) (0)1483 204282

Inquiries regarding the Swift satellite should be forwarded to:

Professor Neil Gehrels
Goddard Space Flight Center, USA
E-mail: gehrels @ milkyway.gsfc.nasa.gov

Professor David Burrows
Pennsylvania State University, USA
E-mail: dxb15 @ psu.edu

Wednesday, 07 February 2007

RAS PN07/02

Magnetic Explosions in the Distant Universe
Contributed by Peter Bond

A new theory to explain the high-energy gamma-ray emissions from
collapsing stars has been put forward by an international team of
researchers. Their results will be published shortly in the Monthly
Notices of the RAS.

Long duration gamma-ray bursts (GRBs), first discovered in the 1970s, are
the most explosive events in the Universe. Finding out what happens during
these cataclysmic events is a major challenge, partly because they usually
occur at the edge of the visible Universe and partly because the bursts
last only a matter of seconds.

Observations accumulated over the last decade have led to a consensus that
at least some GRBs mark the death throes of a giant star as its core
collapses to form a black hole. Until now, it has generally been thought
that the black hole ejects a jet of plasma (extremely hot gas) which is
blasted outwards at close to the speed of light.

This theory is called into question by a new study led by Pawan Kumar from
the University of Texas. The work has been accepted for publication in the
journal, 'Monthly Notices of the Royal Astronomical Society'.

MAGNETIC OUTFLOW

Scientists have long speculated that the gamma-ray emission we see comes
from fluctuations in the speed of the ejected material. The faster and
slower ejecta collide, producing shocks in the jet which result in the
emission of gamma-rays. Although this internal shock model is the standard
explanation, it relies on the jet consisting of ordinary matter -- the
same sort of material that we are made from -- or what scientists call
baryons.

Now, however, Pawan Kumar and colleagues have cast doubt on this model.
Instead of the GRBs being generated by internal shocks, Kumar's team finds
that the jet is actually a powerful magnetic outflow which transports huge
amounts of energy away from the collapsed star.

Using data from the Swift satellite, Professor Kumar's team has analysed a
sample of 10 gamma-ray bursts that were recorded between January 2005 and
May 2006. In each case, Swift collected gamma-ray, X-ray and optical light
immediately after the explosions were detected. Such multi-wavelength
observations are essential if the researchers are to understand what
happens after the brief burst fades and the source object is only visible
in X-rays or visible light.

"Swift is uniquely capable of such simultaneous multi-wavelength
observations," said Neil Gehrels of NASA's Goddard Space Flight Center,
Principal Investigator for the Swift satellite.

The new study reveals the physical process responsible for the generation
of gamma-ray radiation and the distance from the black hole where this
radiation is produced.

"The gamma-ray source is located about 10 billion km from the black hole,
or 100 times further than previously thought," said Professor Kumar. "This
and several other lines of evidence put forward in our work suggest that
the outflow is dominated by the magnetic field."

The data indicate that a magnetic jet decays into gamma-rays. The
subsequent interaction (of the jet) with the surrounding gas causes
intense heating and this produces an afterglow that is seen at X-ray and
visible light wavelengths.

Dr. Paul O'Brien from the University of Leicester, a co-investigator on
the project, said, "In just a few seconds gamma-ray bursts emit as much
energy as the Sun does in 10 billion years. The Swift observations are
telling us that this emission is due to an outflow in which magnetic
fields transport the energy. If confirmed, this will alter our view of how
these objects work."

"Using the Swift data we can accurately measure the times when the prompt
emission stops and the afterglow becomes visible," said Richard
Willingale, also from the University of Leicester. "These times constrain
the distance of the emitting region from the black hole and hence the
physical processes involved."

Since its launch on 20 November 2004, Swift has observed over 200
gamma-ray bursts and provides prompt data on almost all of them.

"Swift can turn and observe a gamma-ray burst with its X-ray and optical
telescopes in just a few tens of seconds," said Professor David Burrows
from Pennsylvania State University, lead investigator for the X-ray
telescope on Swift. "This capability allows us to capture a snapshot of
the early emission which carries information on the physical processes
involved."

Dr Silvia Zane, from the Mullard Space Science Laboratory said, "This is
going to revolutionise our understanding of the cause of such explosions."

NOTES FOR EDITORS

Swift, a medium-class explorer mission, is managed by NASA Goddard. Swift
is a NASA mission with participation of the Italian Space Agency and the
Particle Physics and Astronomy Research Council in the United Kingdom. It
was built in collaboration with national laboratories, universities and
international partners, including Penn State University; Los Alamos
National Laboratory in New Mexico; Sonoma State University, Rohnert Park,
Calif.; Mullard Space Science Laboratory in Dorking, Surrey, UK; the
University of Leicester, UK; the Brera Observatory in Milan; and the ASI
Science Data Centre, Italy.

In the UK, Swift is funded by The Particle Physics and Astronomy Research
Council (PPARC), which is the UK's strategic science investment agency. It
funds research, education and public understanding in four broad areas of
science: particle physics, astronomy, cosmology and space science.

The UK role in Swift has been to provide core elements of the narrow field
instruments (the X-ray telescope and the UV/Optical telescope), utilising
mature technology already developed for the ESA XMM-Newton mission, and
the JeT-X instrument. The University of Leicester provided the lead role
in the X-ray telescope design, focal plane camera assembly and runs the UK
SWIFT Science Data Centre. Mullard Space Science Laboratory-UCL provided
the major part of the UV/Optical telescope.

FURTHER INFORMATION

For more information on Swift:
http://www.swift.ac.uk/
and
http://swift.gsfc.nasa.gov

For a list of Swift's significant observations see:
http://swift.gsfc.nasa.gov/docs/swift/results/releases/

Swift Observes an Unusual Bang in the Far Universe (RAS press release):

http://www.ras.org.uk/index.php?opti...985&Item id=2

Further information and animations of gamma ray bursts:
http://imagine.gsfc.nasa.gov/docs/sc...l1/bursts.html
and

http://www.nasa.gov/vision/universe/...ultimedia.html

Animation of exploding stars:

http://www.nasa.gov/vision/universe/...ia.html#beauty