 February 9, 2007
Science Daily —
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.”
Note: This story has been adapted from a news release issued by Royal Astronomical Society.
http://www.sciencedaily.com/releases/2007/02/070208080143.htm
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