Scientist Watch "Movie" of Neutron Star Explosion in Real-Time
David Ballantyne, CITA
issued by the University of Toronto
February 23, 2004, Toronto, Ontario -- Scientists at the Canadian Institute for Theoretical Astrophysics (CITA)
and NASA have captured unprecedented details of the swirling flow of gas
hovering just a few miles from the surface of a neutron star, itself a
sphere only about ten miles across.
A massive and rare explosion on the surface of this neutron star -
pouring out more energy in three hours than the Sun does in 100 years -
illuminated the area and allowed the scientists to spy on details of the
region never before revealed. They could see details as fine as the ring
of gas swirling around and flowing onto the neutron star as this ring
buckled from the explosion and then slowly recovered its original form
after approximately 1,000 seconds.
All of this was occurring 25,000 light years from Earth, captured
second-by-second in movie-like fashion through a process called
spectroscopy with NASA's Rossi X-ray Timing Explorer.
Dr. David Ballantyne of CITA at the University of Toronto and Dr. Tod
Strohmayer of NASA's Goddard Space Flight Center in Greenbelt, Md.,
present this result in an upcoming issue of Astrophysical Journal
Letters. The observation provides new insight into the flow of a neutron
star's (and perhaps a black hole's) "accretion disk," usually far too
minute to resolve with even the most powerful telescopes.
"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 Ballantyne.
"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."
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 no larger than Toronto. An
accretion disk refers to the flow of hot gas (plasma) swirling around
neutron stars and black holes, attracted by the strong gravity of the
region. This gas is often supplied by a neighboring star.
As matter crashes down on the neutron star it builds up a 10- to
100-meter 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. (Nuclear fusion is the same process that powers the Sun.)
Such bursts can occur several times a day on a neutron star and last for
about 10 seconds.
What Ballantyne and Strohmayer observed on this neutron star, named 4U
1820-30, was a "superburst". These are much more rare than ordinary,
helium-powered bursts and release a thousand times more energy.
Scientists say these superbursts are caused by a buildup of nuclear ash
in the form of carbon from the helium fusion. Current thinking suggests
that is takes several years for the carbon ash to buildup to such an
extent that it begins to fuse.
The superburst was so bright and long that it acted like a spotlight
beamed from the neutron star surface and 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
-- that is, 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) through the
Universe around nearby stellar black holes and exceedingly distant
quasar galaxies. Stellar black holes with accretion disks do not produce
The Rossi Explorer was launched in December 1995 to observe
fast-changing, energetic and rapidly spinning objects, such as
supermassive black holes, active galactic nuclei, neutron stars and
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