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BLACK
HOLES MAY TAKE SPACE FOR A SPIN
As
if black holes couldn't be more menacing, astronomers now
have observational evidence that at least some of them spin
about like whirlpools, wrapping up the fabric of spacetime
with them.
Dr.
Tod Strohmayer of NASA's Goddard Space Flight Center, Greenbelt,
MD, has studied one such black hole system with NASA's Rossi
X-ray Timing Explorer and found unique patterns in the X-ray
radiation that have previously only been seen in spinning
neutron stars. With these new parameters, he could verify
that a black hole, like a neutron star, can spin.
The
observation also challenges theories about neutron star radiation.
Strohmayer presents his findings today at the April 2001 Meeting
of the American Physical Society in Washington, D.C. Computer
animation illustrating the discovery will be broadcast at
noon today on NASA TV.
"Almost
every kind of object in space spins, such as planets, stars,
and galaxies," said Strohmayer. "With black holes, it's much
harder to directly see that they are spinning, because they
don't have a solid surface that you can watch spin around.
We can, however, see the light emitted from matter plunging
into the black hole. The matter, cannibalized from the black
hole's nearby binary companion star, whips frantically around
the black hole before it is lost forever."
The
black hole that Strohmayer observed is the stellar variety,
which is formed from a collapsed star. When stars at least
10 times more massive than our Sun exhaust their fuel supply,
they no longer have the energy to support their tremendous
bulk. These stars explode their outer shell of gas in an event
called a supernova. The remaining bulk, still several times
more massive than the Sun, collapses into a single point of
infinite density, called a singularity.
Neutron
stars form through a similar process, only from a slightly
less massive star that collapses into a dense chunk as heavy
as the Sun yet only ten miles across. Astronomers can "see"
neutron stars and black holes by virtue of the hot, glowing
gas -- often pulled from a neighboring star -- that orbits
these objects at near light speed.
The
Rossi Explorer has long recorded a certain type of X-ray flickering
from neutron stars called quasiperiodic oscillations, or QPOs,
caused by hot gas dancing around the neutron star in a lively
orbit. Astronomers think that these oscillations are produced
by motions of matter very near the innermost stable orbit
-- the closest orbit a blob of gas can maintain before falling
pell-mell into the central object.
Many
astronomers believe the QPO frequency is related to the mass
of the central object and that one can estimate the location
of the innermost stable orbit from the QPO frequency.
Strohmayer's
target was GRO J1655-40, a microquasar 10,000 light years
from earth. (The black hole menace continues: A microquasar
is a specific type of black hole with jets of high-speed particles
shooting perpendicularly from the plane of matter that orbits
it.) Strohmayer observed two QPOs, a previously detected one
at about 300 Hz and a newly detected one at 450 Hz. (A hertz,
Hz, is a unit of frequency equal to one cycle per second.)
GRO
J1655-40 is a well-studied system, and the black hole mass
has been established at 7 solar masses from earlier optical
observations. Applying Einstein's theory of general relativity,
this mass implies an innermost stable orbit of 64 kilometers
(about 40 miles) if the black hole were not spinning.
A
QPO at 450 Hz, however, implies an innermost stable orbit
with a radius less than or equal to 49 km (about 30 miles).
The only way that gas can maintain a stable orbit that close
to the black hole, Strohmayer said, is if the black hole is
spinning.
"A
spinning black hole modifies the fabric of spacetime near
it," said Strohmayer. "The spinning allows matter to orbit
at a closer distance than if it were not spinning, and the
closer matter can get the faster it can orbit. For GRO J1655-40
we can now say that the only way for it to produce the 450
Hz QPO is if it is spinning."
The
first detected QPO, at 300 Hz, was coincidentally in tune
with a 64-km innermost stable orbit. It was the detection
of the higher frequency QPO at 450 Hz that implied spin.
Strohmayer's
finding also marks the first detection of paired QPOs from
a black hole. Neutron stars often exhibit paired QPOs, and
this is thought to be a result of radiation coming from the
solid neutron star surface. Strohmayer's detection of paired
QPOs from an object with no solid surface, therefore, challenges
these important theories of how neutron stars produce these
QPOs.
The
spin of a black hole would be caused by the angular momentum
of the star that formed it, Strohmayer said, particularly
if that progenitor is a spinning neutron star. Neutron stars
can theoretically collapse into black holes if they accrete
enough matter. Alternatively, a black hole could form directly
during a supernova.
Compared
to a non-spinning black hole, a spinning black hole also has
a smaller event horizon, the theoretical boundary around a
black hole beyond which nothing can escape. Proposed X-ray
space-science missions such as Constellation-X would be able
to study black hole spin in greater detail, observing how
a spinning black hole more strongly warps spacetime and stretches
light emitted by hot gas, a test of general relativity.
The
Rossi Explorer, launched in December 1995, is a unique X-ray
satellite designed to record rapidly fluctuating objects,
such as pulsars and neutron stars, which can spin over a thousand
times per second.
MEDIA
CONTACT LIST
Dolores
Beasley/Space Science PIO/NASA Headquarters
(202)
358 1753
Nancy Neal/SEU PIO/Goddard Space Flight Center
(301) 286 0039
Bill
Steigerwald/Technical Writer/Goddard Space Flight Center
(301) 286 5017
Wade Sisler/Executive Video Producer/Goddard Space Flight
Center
(301) 286 6256
Rachel Weintraub/Associate Video Producer/Goddard Space
Flight Center
(301) 286 0918
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