An international team of scientists has found more evidence that massive
black holes are surrounded by a doughnut-shaped gas cloud which, depending
on our line of sight, blocks the view of the black hole in the center.
Using two European Space Agency orbiting observatories, INTEGRAL and
XMM-Newton, scientists looked “edge on” into this doughnut, called a torus,
to see features never before revealed in such clarity. They could infer
the doughnut structure and distance from the black hole by virtue of light
that was either reflected or completely absorbed. How the doughnut forms,
however, remains a mystery.
“By peering right into the torus, we see the black hole phenomenon in a
whole new light, or lack of light, as the case may be here,” said Dr.
Volker Beckmann of NASA Goddard Space Flight Center in Greenbelt, Md., the
lead author on an upcoming article in The Astrophysical Journal. “This
torus is not as dense as a Krispy Kreme doughnut, but it is far hotter (up
to a thousand degrees) and loaded with many more calories.”
Black holes are objects so dense and with gravity so strong that not even
light can escape from them. Scientists say that “supermassive” black holes
are located in the cores of most galaxies, including our Milky Way galaxy,
and contain the mass of millions to billions of suns confined within a
region no larger than our Solar System.
Supermassive black holes appear to be surrounded by a hot, thin disk of
accreting gas and, farther out, the thick doughnut-shaped
torus. Astronomers often view black holes that are aligned face-on or at a
slight angle in relation to Earth, thus avoiding the dark, enshrouding
torus to study the hot accretion disk.
Beckmann’s group took the path less trodden and observed a black hole with
a theorized torus directly in the line of sight. X-ray and gamma-ray
light, as detected by XMM and INTEGRAL, respectively, partially penetrates
the torus. The new view through the haze provides valuable insight into
the relationship among the black hole, its accretion disk and the doughnut.
The scientists observed a black hole in the spiral galaxy NGC 4388, which
is 65 million light years from Earth in the constellation Virgo. This
galaxy is called a Seyfert 2, referring to the type of black hole in the
core — that is, one that is enshrouded from our vantage point.
Seyfert 2 galaxies are usually faint to optical telescopes. The torus
model is one explanation. Another theory is that the central black hole,
for reasons unclear, is not actively accreting gas and is therefore
faint. (Accretion produces energy, or light.) NGC 4388 is relatively
close and therefore an unusually bright Seyfert 2, easy to study.
The new observation supports the torus model in several ways. Gas in the
accretion disk close to the black hole reaches high speeds and temperatures
(over 100 million degrees, hotter than the Sun) as it races toward the
void. The gas radiates predominantly at high energies, in the X-ray
wavelengths. This light, which is able to escape the black hole because it
is still outside of its border, ultimately collides with matter in the
torus. Some of it is absorbed; some of it is reflected at different
wavelengths, like sunlight penetrating a cloud; and the very energetic
gamma rays pierce through.
Beckmann’s group saw how different processes around a black hole produce
light at different wavelengths. For example, some of the gamma rays
produced close to the black hole get absorbed by iron atoms in the torus
and are reemitted at a lower energy. This in fact is how the scientists
knew they were seeing “reprocessed” light farther out. Also, because of
the line of sight towards NGC 4388, they knew this iron was from a torus on
the same plane as the accretion disk, and not from gas clouds “above” or
“below” the accretion disk.
Lower-energy X rays (below 2.5 kilo-electron volts) appear to be from a
diffuse emission far away from the black hole. Higher-energy X rays (above
2.5 keV) are directly related to black hole activity. The torus itself
appears to be several hundred light years from the black hole.
Dr. Beckmann said the observation could not gauge the diameter of the
torus, from inside to outside. Other scientists say that the doughnut
shape is more intact closer to the accretion disk, but that it cannot
maintain structural integrity farther away, perhaps resembling a doughnut
with part of its edges eaten away.
The result marks the clearest observation of an obscured black hole in
X-ray and gamma-ray “colors,” a swatch of energy nearly a million times
wider than the window of visible light, from red to
violet. Multiwavelength studies are increasingly important to
understanding black holes. XMM-Newton was launched in December 1999, and
INTEGRAL was launched in October 2002.
Dr. Beckmann is a visiting scientist at NASA Goddard through the University
of Maryland, Baltimore County. His coauthors on the Astrophysical Journal
article are: Dr. Neil Gehrels of NASA Goddard; Pascal Favre, Dr. Roland
Walter and Prof. Thierry Courvoisier of the INTEGRAL Science Data Centre in
Switzerland; Dr. Pierre-Olivier Petrucci of the Laboratoire d’Astrophysique
de Grenoble in France; and Dr. Julien Malzac of the Centre d’Etude Spatiale
des Rayonnements in France and the Institute of Astronomy, University of
Cambridge, U.K.
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