Death of a Star: Scientists Watch Supernova in Real Time





Scientists
are studying a strange explosion that appeared on February 18, 2006,
about 440 million light years away in the constellation Aries. The
"before" image on the left is from the Sloan Digital Sky Survey. The
"after" image on the right is from NASA Swift's Ultraviolet/Optical
Telescope. The pinpoint of light from this star explosion outshines the
entire host galaxy. Most other sources are foreground stars. Each image
is 5 arcminutes by 5 arcminutes. Coordinates for this burst are as
follows: RA: 03:21:39.71
(Credit: SDSS (left), NASA/Swift/UVOT (right))



For the first time a star has been observed in real-time as it goes
supernova a mind bogglingly powerful explosion as the star ends its
life, the resulting cosmic eruption briefly outshining an entire
galaxy. UK scientists, in collaboration with international colleagues,
used NASA's Swift satellite and a combination of orbiting and
ground-based observatories to catch a supernova in the act of
exploding. The results, including an associated and intriguing Gamma
Ray Burst [GRB], appear in 31 August issue of Nature.

The event began on the 18th February, 2006, in a star forming galaxy
about 440 million light-years away toward the constellation Aries. At
that time it was immediately realised that this was an unusual
gamma-ray burst, about 25 times closer and 100 times longer than a
typical gamma-ray burst. The burst lasted for almost 40 minutes as
opposed to a typical GRB of a few milliseconds to tens of seconds.
Because the burst was so long Swift was able to observe the bulk of the
explosion with all three of its instruments: the Burst Alert Telescope,
which detected the burst and relayed the location to ground
observatories within 20 seconds; the X-ray telescope [XRT] and
Ultraviolet/Optical Telescope [UVOT], which provide high-resolution
imagery and spectra across a broad range of wavelengths.



"The fact that Swift can re-point very fast, slewing round to bring the
XRT and UVOT to bear on the burst allowed us to get onto it very
quickly indeed, enabling us to observe the critically important early
behaviour of the event" remarked Dr. Alex Blustin from University
College Londons Mullard Space Science Laboratory [UCL/MSSL],

Careful, multi-wavelength analysis of space and ground-based observations has now revealed exactly what took place.



The exceptionally long burst, in the form of a jet of high-energy
X-rays, pierced through the doomed star from its core and sent out a
warning within minutes that a supernova was imminent. As the GRB faded
away the massive star blew itself into smithereens.

"This GRB was the most extraordinary evolving object yet seen by
Swift," said team member Dr. Paul OBrien at the University of
Leicester. "The three on-board telescopes all detected a slowly
brightening then fading object. The results suggest a broad jet
expanded into the surroundings but it was accompanied by a
slower-moving and incredibly hot - two million degree - bubble of gas
produced from the shock-wave of the exploding star."



 


The
collapsing star scenario that is one of the leading contenders as the
cause of gamma-ray bursts. Dr. Stan Woosley of the University of
California at Santa Cruz proposed the collapsar theory in 1993. This
artist's concept of the collapsar model shows the center of a dying
star collapsing minutes before the star implodes and emits a gamma-ray
burst that is seen across the universe. Credit: NASA/Dana Berry



Swift's three telescopes - covering gamma ray, X-ray, ultraviolet and
optical wavelengths - captured X-rays fading to ultraviolet and then
optical light, evidence of the shock wave from the explosion pushing
exploded star material into the surrounding medium. Dr. Alex Blustin
and colleague Dr. Mat Page, also from UCL/MSSL, conducted the analysis
of Swifts ultra violet and optical data that tracked the expansion of
the shock wave from the explosion.



Paul OBrien added, "This is the first time such an extraordinary event
has been seen from a GRB. The thermal component of the supernova shock
wave was clearly seen in this case as the GRB itself was fairly modest,
some 100 times less than a typical GRB - a mere ten million billion
times the power of the Sun!"



UK astronomers from the Universities of Leicester and Hertfordshire
were part of a group led by Italy's National Institute for Astrophysics
that used the European Southern Observatory's 8.2-metre Very Large
Telescope [VLT] in Chile and the University of California's Lick
Observatory Shane 3-metre telescope to obtain regularly-sampled optical
spectroscopy of the shock wave. Two days later the classical supernova,
a glowing cloud of gas powered by the decay of radioactive debris from
the dead star, was beginning to outshine the fading shock wave.



Dr Andrew Levan, University of Hertfordshire said, "As well as studying
the early evolution of the supernova for the first time these
observations also show how the material ejected in the explosion evolve
in the following days and weeks, the timescales on which supernovae are
normally studied." Dr. Levan added, "This shows that the supernova
associated with this GRB is a transition object, brighter than most
supernovae in the universe, but fainter than those previously seen with
GRBs. Understanding the reasons for this is a crucial step in
understanding why only a small percentage of massive stars can create
GRBs."



"Usually these events are not detected until after the supernova has
brightened substantially in the optical wavelength, many days after the
initial explosion," commented Prof. Keith Mason, UK lead investigator
for the UVOT telescope on Swift and CEO of the Particle Physics and
Astronomy Research Council [PPARC], "but on this occasion we were able
to study the remarkable event in all its glory from the very beginning."



See:  http://www.pparc.ac.uk/Nw/Md/Artcl/GRB_supernova.asp