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Title: Astronomy/Stars/Novae and Supernovae - When supernovae go hyper: NASA article Magazine-style article written by NASA on the explanation for gamma-ray bursts surrounding 'hypernova' type supernovae.
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When supernovae go hyper: A mystery surrounds an exploding star
Space
Science News home
When stars go hyper
Different kind of nova ends
not with a whimper, but with a bang
October 21, 1998: Revolutions often start with
a bang. Such a blast may have been delivered this spring by a
supernova that may force scientists to rethink how stars die.
For now, observers have a classic situation - evidence that points
in different directions and dares anyone to find a single answer.
"All of this may lead to a revolution in our thinking
about how core-collapse supernovae are produced," wrote
Dr. Eddie Baron of the University of Oklahoma in the Oct. 15
issue of the prestigious science journal, Nature. Questions
to be answered include what causes "ordinary" supernovae,
is there a limit in their energy release, and when does core
collapse cause a gamma-ray burst?
 Before and during: In a May 15, 1985 image of galaxy ESO 184-G82
shows nothing unusual in a 120-minute, red light exposure by
the United Kingdom Schmidt Telescope in Australia. The image
at right is a color composite of three short-exposure green,
red, and near-infrared images obtained with the multi-mode instrument
at the ESO's 3.58-m New Technology Telescope (NTT) at La Silla
on May 4, 1998. SN1998bw is the very bright, bluish star at the
center (arrow), located on an arm of spiral galaxy ESO 184-G82.
Several other galaxies are in the field. In both photos, the
field of view measures 3.6 x 3.6 arcmin; north is up and east
is left. Note that while the brighter objects are more prominent
on the older, long-exposure Schmidt photo (right), considerably
more details are in the NTT image (left). Images courtesy European
Southern Observatory. Links to 463x365-pixel,
44KB JPG.
Baron was commenting on three Nature papers describing,
as the first paper is titled, "An unusual supernova in the
error box of the gamma-ray burst of 25 April 1998." The
lead author is Titus J. Galama, a graduate student at the Astronomical
Institute at the University of Amsterdam, The Netherlands. Among
his many co-authors are Dr. Jan van Paradijs of the University
of Amsterdam and the University of Alabama in Huntsville (UAH),
Drs. Chryssa Kouveliotou, Craig Robinson and Thomas Koshut of
the Universities Space Research Association at Marshall Space
Flight Center, and Dr. Marc Kippen, also of UAH.
The excitement started about 140 million years ago, near the
end of the Jurassic period when dinosaurs were in their prime,
but the news just arrived on April 25, 1998, when instruments
aboard several spacecraft recorded a strong flash of gamma radiation.
Optical astronomers pointing powerful ground-based telescopes
to the direction of the X-rays from the gamma-ray burst discovered
a supernova in a distant galaxy. Scientists estimate that the
blast, perhaps 10 times greater than an ordinary supernova, collapsed
the core of the dying star into a black hole. At the same time,
it slammed enough matter - mostly radioactive nickel 56 weighing
70 percent as much as our sun - outward to create a shock wave
of gamma radiation.
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This raises the possibility that "hypernovae" might
be a significant cause of gamma-ray bursts. Scientists have puzzled
over gamma-ray bursts since the late 1960's when they were discovered.
Not until the last couple of years, though, were they able to
conclude that the bursts were from cosmological distances, that
is, from far outside our own galaxy.
Two cross-section images depict
the likely explanations for the April 25, 1998 supernova. It
could have blown up in all directions (left) - called isotropic
- in which case it's almost impossibly powerful in order to account
for the energy output. Or, it could have squirted most of the
matter in jets long the magnetic poles - in which case it's most
unlikely that we would be lined up to see it just right. In either
case, the blast produces a black hole surrounded by a swirling
disk of matter crowding its way into the hole and oblivion. Links
to 1000x658-pixel, 59KB
JPG. Also available: 2821x1857-pixel,
259KB JPG. Credit: NASA/Marshall Space Flight Center, after
Baron (Nature, 395: 635, 1998).
The Burst and Transient Source Experiment (BATSE) aboard the
Compton Gamma Ray Observatory has recorded more than 2,000 bursts
- about one a day - since its launch in 1991. A key finding is
that the bursts are randomly distributed across the sky. This
means they have to be peppered at random throughout the universe
since a link to our galaxy would show them clustered along the
galactic plane, where most of the stars of our Milky Way are
located.
The discovery of an optical companion to a burst recorded
on Feb. 28, 1997, linked bursts with distant galaxies. The distances
of the GRB host galaxies were subsequently measured in several
cases. Just what causes them, though, remains open to debate.
Supernovae had been ruled out as being not quite powerful enough,
especially given the distances involved.
Observations of an optical component to the April 25, 1998,
burst (GRB980425) indicate that a special set of supernovae -
the hypernova - might be a contributor.
GRB980425 was detected by BATSE, the Dutch-Italian BeppoSAX
(X-ray astronomy) satellite, and instruments on other spacecraft.
With BeppoSAX scientists were able to image the part of the sky
where the burst came from. Within this image, both the European
Southern Observatory (ESO) at La Silla (about 600 km north of
Santiago, Chile), the National Radio Astronomy Observatory in
Socorro, N.M., and others went hunting.
They soon found their quarry.
Circling the drain
Trying to figure out what happens inside a black hole is a
bit like Alice trying to make sense of what she found down the
rabbit's hole. Nothing works the way we think it should.
 When
a star is massive enough, at the end of its life it can generate
enough inward force to compress the core beyond the stage of
a super-dense neutron star and seemingly erase the laws of physics
that let subatomic particles maintain their identities.
Left: An artist's concept depicts a black hole at the
center of a warped disk of material that is swirling into the
disk. Whether SN1998b blew out in all directions or beamed its
energy like a lighthouse, the end result most likely is similar
to this. Illustration by James Gitlin, Space Telescope Science
Institute. Click here for details and a high-resolution
copy of the image.
All that's left is a deep gravity well strong enough to keep
light from escaping. What happens at the center is anyone's guess,
literally, because the laws of physics, as we understand them,
do not work under such extreme conditions.
What we can see and measure are the effects of the black hole
on space and matter around the hole. Materials swirl inward,
like water circling the drain. As they near the event horizon
- the point of no return - they run into each other, heating
the gas to glow in radio, then visible light, ultraviolet, and
finally, X-rays.
The UK Schmidt Telescope in Australia had surveyed the area
as one of its first tasks in the 1970s, so it had a good set
of "before" images in hand. In an arm of the spiral
galaxy tagged ESO 184-G82, scientists found a brilliant star
that was not in previous images. This was dubbed SN1998bw.
At such close range, GRB980425 could only have about 1 percent
of 1 percent (0.0001) of the raw power of more distant bursts.
But its light curve in radio waves was striking, indicating that
the shock wave from the blast was moving very close to the speed
of light. Indeed, it's the brightest supernova ever seen in radio
waves.
"There is a significant chance that the GRB and the supernova
explosion are associated", said Kouveliotou. "SN1998bw
is a rare type of supernova, both in its optical and in its radio
properties". However, "Nobody would have picked up
GRB980425 as somehow special on the basis of its gamma-ray properties
alone", said van Paradijs.
If GRB980425 and SN1998bw are associated, the scientists wrote,
then GRB980425 is a rare type of burster, and SN1998bw is a rare
type of supernova.
The rarity that might fit this pigeonhole is the hypernova,
an idea that has been around for a few years but not yet confirmed.
Web Links
Einstein
was right! A Nov. 6,
1997, story about how black holes dance to "The Time Warp."
External links (non-NASA)
European Southern Observatory announcement and details and more pictures
Press
releases
from the University of Amsterdam and the European Southern Observatory.
University of Amsterdam has additional
information.
Nature The full articles are available from
Nature magazine (Note: Nature's contents will change
on Oct. 22, thus requiring you to use their search function to
locate the stories):
How big do stellar explosions
get? Eddie Baron.
Radio emission from the unusual
supernova 1998bw and its association with the -ray burst of 25
April 1998. S.R. Kulkarni,
et al.
An unusual supernova in the
error box of the gamma-ray burst of 25 April 1998. T.J. Galama, et al. (including Jan
van Paradijs, UAH; Chryssa Kouveliotou, USRA/MSFC; Thomas Koshut,
USRA/MSFC; Marc Kippen UAH)
A hypernova model for the
supernova associated with the gamma-ray burst of 25 April 1998. K. Iwatomo, et al (including Jan van
Paradijs, UAH; Chryssa Kouveliotou, USRA/MSFC).
"The extremely large energy suggests the existence of
a new mechanism of massive star explosion that can also produce
the relativistic shocks necessary to generate the observed gamma
rays," wrote a team led by K. Iwatomo of the University
of Tokyo, and including Kouveliotou. (The hypernova bears no
relation to the magnetars which Kouveliotou discovered earlier
this year. Magnetars are highly magnetized neutron stars. A hypernova
would leave behind nothing to form a magnetar, just a black hole
in space.)
Models of the light curves indicate that it started with the
core collapse of a star about 40 times as massive as our sun.
It was spinning rapidly - possibly due to a binary companion
spiraling into the star - and had a strong magnetic field.
It had already burned through all its nuclear fuel, converting
hydrogen into helium "ash," and the ash into heavier
elements until all that was left was silicon ash. In a final
fury, it burned this into nickel 56 and collapsed on itself,
compacting the core into a black hole and blasting the outer
layers into space.
The authors outlined two possible paths at this point. First,
the star blasted outward in all directions, making for a phenomenally
powerful blast, about 30 times greater than any recorded supernova.
Or, it focused most of the energy into one direction, which happened
to be aimed at Earth - a most unlikely event.
Either way, astronomers now have a new mystery to study.
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Author: Dave
Dooling
Curator: Linda Porter
NASA Official: Gregory
S. Wilson
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Magazine-style | article | written | by | NASA | on | the | explanation | for | gamma-ray | bursts | surrounding | 'hypernova' | type | supernovae. |
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http://science.msfc.nasa.gov/newhome/headlines/ast21oct98_1.htm
When supernovae go hyper: NASA article 2009 January
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Magazine-style article written by NASA on the explanation for gamma-ray bursts surrounding 'hypernova' type supernovae.
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