A neutron star halfway across the Milky Way galaxy is
ready for its close-up. A rare and massive explosion on this
star illuminated the region and allowed scientists to view
details never seen before, virtually bringing the scientists to
the action occurring just a few miles above the star’s surface.
Scientists at NASA and the Canadian Institute for Theoretical
Astrophysics (CITA) report their findings in the current issue
of Astrophysical Journal Letters. The action was captured
second-by-second in movie-like fashion through a process called
spectroscopy with NASA’s Rossi X-ray Timing Explorer.
A neutron star is the dense, core remains of an exploded star
at least eight times more massive than the sun. The neutron
star contains about a sun’s worth of mass packed in a sphere
only about 10 miles (16 kilometers) in diameter.
Often neutron stars are in binary (two-star) systems. Gas from
the nearby companion star can funnel towards the neutron star,
attracted by the star’s strong gravity. The gas spirals toward
the neutron star like water going down a drain, forming what
scientists call an accretion disk.
This is the first time we have been able to watch the inner
regions of an accretion disk, in this case literally a few
miles from the neutron star’s surface, change its structure in
real time,” said Dr. David Ballantyne of CITA at the University
of Toronto. “Accretion disks are known to flow around many
objects in the universe, from newly forming stars to the giant
black holes in distant quasars. Details of how such a disk
flows could only be inferred up to now.”
Under normal conditions, accretion disks appear far too minute
to resolve with even the most powerful telescopes. The
explosion occurred on a neutron star named 4U 1820-30, 25,000
light years from Earth. It poured out more energy in three
hours than the sun does in 100 years. The region was
illuminated in such a way, the scientists could see details as
fine as the accretion disk buckling from the explosion and then
slowly recovering its original form after approximately 1,000
seconds.
Such explosions are the result of accretion. As matter from the
companion star crashes down on the neutron star, it builds up a
10 to 100 yard layer of material comprised mostly of helium.
The fusion of the helium into carbon and other heavier elements
releases enormous energy and powers a strong burst of X-ray
light, far more energetic than visible light. Such bursts can
occur several times a day on a neutron star and last for about
10 seconds.
Ballantyne and his colleague, Dr. Tod Strohmayer of NASA’s
Goddard Space Flight Center in Greenbelt, Md., observed a
“superburst.” These are much more rare than ordinary, helium-
powered bursts and release 1000 times more energy. Scientists
say superbursts are caused by a buildup of nuclear ash in the
form of carbon from the helium fusion. Current thinking
suggests it takes several years for the carbon ash to build up
to such an extent that it begins to fuse.
The superburst was so bright and long, it acted like a
spotlight beamed from the neutron star surface onto the
innermost region of the accretion disk. The X-ray light from
the burst illuminated iron atoms in the accretion disk, a
process called fluorescence. The Rossi Explorer captured the
characteristic signature of the iron fluorescence, its
spectrum. This, in turn, provided information about the iron’s
temperature, velocity and location around the neutron star.
“The Rossi Explorer can get a good measurement of the
fluorescence spectrum of the iron atoms every few seconds,”
Strohmayer said. “Adding up all this information, we get a
picture of how this accretion disk is being deformed by the
thermonuclear blast. This is the best look we can hope to get,
because the resolution needed to actually see this action as an
image, instead of spectra, would be a billion times greater
than what the Hubble Space Telescope offers.”
The scientists said the bursting neutron stars serve as a
laboratory to study accretion disks, which are seen (but in
less detail) throughout the universe around nearby stellar
black holes and exceedingly distant quasar galaxies. Stellar
black holes with accretion disks do not produce X-ray bursts.
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