Black Holes Aren't So Black (Forwarded)

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For Release: Monday, October 3, 2005

Release No.: 05-32

Black Holes Aren't So Black

Cambridge, MA -- Common wisdom holds that we can never see a black hole
because nothing can escape it -- not even light. Fortunately, black holes
aren't completely black. As gas is pulled into a black hole by its strong
gravitational force, the gas heats up and radiates. That radiation can be
used to illuminate the black hole and paint its profile.

Within a few years, astronomers believe they will be able to peer close to
the horizon of the black hole at the center of the Milky Way. Already,
they have spotted light from "hot spots" just outside the black hole.
While current technology is not quite ready for the final plunge, Harvard
theorists Avery Broderick and Avi Loeb (Harvard-Smithsonian Center for
Astrophysics) already have modeled what observers will see when they look
into the maw of this monster.

"It will be really remarkable when observers can see all the way to the
edge of the Milky Way's central black hole -- a hole 10 million miles in
diameter that's more than 25,000 light-years away," said Broderick.

All it will take is a cross-continental array of submillimeter telescopes
to effectively create a single telescope as large as the Earth. This
process, known as interferometry, has already been used to study longer
wavelength radio emissions from outer space. By studying shorter
wavelength submillimeter emissions, astronomers could get a
high-resolution view of the region just outside the black hole.

"The Holy Grail of black hole astronomy is within our grasp," said
Broderick. "We could see the shadow that the black hole casts on
surrounding material, and determine the size and spin of the black hole
itself."

Infrared observations using existing and near-future interferometric
instruments also offer the possibility of imaging the core of our Galaxy
in incredible detail, with resolutions better than one milli-arcsecond.

"Submillimeter and infrared observations are complementary," said
Smithsonian astronomer Lincoln Greenhill of the Center. "We need to use
both to tackle the problem of getting high-resolution observations. It's
the only way to get a complete picture of the Galactic center."

The black hole at the center of the Milky Way is the best target for
interferometric observations because it spans the largest area in the sky
of any known black hole. Nevertheless, its angular size of tens of
micro-arcseconds poses a major challenge to observers, requiring
resolution 10,000 times better than the Hubble Space Telescope provides in
visible light.

"When astronomers achieve it, that first image of the black hole's shadow
and inner accretion disk will enter textbooks, and will test our current
notions on gravity in the regime where spacetime is strongly curved," said
Loeb.

"Ultimately, we want to test Einstein's general theory of relativity in
the strong field limit -- within a strong gravitational field like that of
a black hole," said Broderick.

In preparation for that observational leap, Broderick and Loeb created a
computer program to simulate the view. Emissions from the Galaxy's central
black hole are known to fluctuate, probably due to clumps of material
being swallowed. The researchers modeled those clumps of hot gas and
predicted the up-close appearance. They also summed the total light from
the "hot spots" to simulate low-resolution observations possible with
current technology.

New observational results are starting to come out and already are proving
consistent with Broderick and Loeb's prediction.

"Observations to date only span a limited time interval," said Loeb. "With
routine monitoring, astronomers will be able to collect many examples of
flares and start deriving the characteristics of the black hole itself."

A paper on the hot-spot modeling has been accepted for publication by the
Monthly Notices of the Royal Astronomical Society and is available online
at
http://arxiv.org/abs/astro-ph/0506433

A second paper modeling the accretion disk has been submitted to The
Astrophysical Journal Letters and is available online at
http://arxiv.org/abs/astro-ph/0508386

A third paper combining the accretion disk with hot spots has been
submitted to Monthly Notices of the Royal Astronomical Society and is
available online at
http://arxiv.org/abs/astro-ph/0509237

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.

Note to editors: Theoretical animations to accompany this release are
online at
http://www.cfa.harvard.edu/press/pr0532image.html