Matter Found Falling in Black Hole
02:43 PM ET 08/17/99
WASHINGTON (AP) - NASA scientists have detected what they say may be the first
actual evidence of matter falling into a black hole in space. Using the Advanced
Satellite for Cosmology and Astrophysics, Paul Nandra and colleagues at Goddard Space
Flight Center in Greenbelt, Md., observed the emission of X-rays from iron atoms in the
gas swirling around a central, dense object.
Along with the X-rays was a rare light emission from iron atoms traveling away
from the viewer at 6.5 million miles per hour - possibly a mark of material being drawn
into a black hole, Nandra reported in an upcoming edition of Astrophysical Journal
Letters. This differs from the previously observed phenomena regarding black holes.
``You often see evidence of matter flying out of a black hole, but never falling
in,'' Nandra, an astrophysicist, said in a statement. ``The evidence is pretty good.''
The data came from five days of observation of galaxy NGC 3516 about 100 million
light years away. It is thought to harbor a massive black hole.
In a black hole, the force of gravity is so powerful that nothing, not even
light, can escape its pull.
Buried in the typical X-ray emissions from matter around black holes, however,
was a rare glimpse of a red-shifted light from iron atoms. That suggests matter is moving
away from the viewer, in this case at about 6.5 million miles per hour, towards the black
hole, Nandra said.
The emission spectrum is a breakdown of light into its component colors, much
like a prism separates white light into a rainbow. This provides information about
the type of gas emitting the light, as well as its velocity, temperature and pressure. The
emission from NGC 3516 comes from a blur of gas molecules or atoms moving in many
directions, he said.
Because Nandra detected a red-shift, he knew the matter was moving away from
Earth because of the Doppler effect. It is the same phenomenon that distorts the sound of
the horn on a speeding car as it approaches and passes. The Doppler effect distorts light
waves as well as sound waves. Objects moving away emit light with wavelengths shifted
towards red light, those approaching appear more blue.
_______________________
The existence of black holes is one of the most robust predictions in
Einstein's theory of General Relativity. Yet simply finding an isolated black-hole, much
less measuring its properties of mass, charge, or spin can be difficult, since the
gravitational pull of a black hole is so large that light itself cannot escape. Therefore,
the best hope of observing black holes and learning about their properties are by studying
the interactions with its environment.
Using data from several NASA satellites, Dr. Shuang Nan Zhang of the Universities
Space Research Association at NASA/Marshall Space
Flight Center and collaborators Dr. Wei Cui of the Massachusetts Institue of
Technology and Dr. Wan Chen from the University of Maryland at College Park and
NASA/Goddard Space Flight Center have measured the spins of several black holes, by
accurately measuring the size of the last stable orbit of material around the black hole.
In the past several
years, evidence has been accumulated that there are indeed such things as black holes in
the Universe. Not only do they exist in the center of many galaxies with masses between
millions and billions times the Sun, such as in this Hubble Space Telescope photo of the
center of the galaxy M87, about a dozen of them have been found fairly nearby in our own
Galaxy with masses between 3 and 30 times that of the Sun.
These lighter ones, usually called stellar mass black holes, all reside in
so-called "binary systems," with the black hole's companion star being rather
ordinary. By examining the relative movements of both objects in a binary system, one can
infer the mass of the invisible object - the black hole candidate. The black hole
candidate is believed to be a real black hole, when the amount of mass more than three
times that of the Sun is found to be concentrated within a radius of about 10 kilometers,
since theoretically no other objects may have such a high degree of mass concentration.
With a fairly robust way to measure the mass of a black hole, the next problem
is to study the other properties such as the charge and the spin. Currently astronomers
have not figured out on how to observe the black hole charge. "Tremendous progress,
however, has been made in the study of the black hole spin," said Dr. Zhang.
"Relativity theory tells us that there should be a last-stable-orbit around the black
hole. Material inside this orbit cannot survive and is consumed by the black hole. The
size of this orbit is related to the spin of the black hole, so by looking at the material
that occupies this orbit, and measuring its extent, we can learn about how fast the black
hole is spinning."
Using this technique, Dr. Zhang and his colleagues have measured the spins of
several black holes. In their recently submitted publication, Zhang and colleagues
describe that two of the black holes they studied are rapidly spinning, while others are
not. "Coincidentally these two rapidly spinning black holes also occasionally eject
relativistic jets," said Zhang. A relativistic jet is a stream of high-speed material
ejected from the black hole region at nearly the speed of light. Their continued
observations of black holes promise to provide more insight into these fascinating
objects.
For more information on spinning black holes, please contact:
Dr. Shuang Nan Zhang
Space Sciences Laboratory
NASA/Marshall Space Flight Center
USRA Mail Code ES-81
zhang@sslmor.msfc.nasa.gov
Related Links:
Strongest
Explosion Ever Observed
Powerful
Telescope Set for Launch
Astronomers
Baffled by Space Light
