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

On Mon, 03 Oct 2005 11:16:30 -0400, Andrew Yee
quoted, in part:

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.

I have thought that we ought to design space probes similar to the
Hubble Space Telescope, but designed so that they can operate
continuously for a hundred years without any maintenance whatever... and
then put one in orbit around Uranus and another around Neptune.

This would allow obtaining stellar parallaxes for more distant objects.

While optical interferometry at such a scale would be extremely
difficult (the coherence length of the light from stars, for one thing,
would restrict the possible directions of view) this suggests putting
radio telescopes in such distant locations would yield important
scientific knowledge.

John Savard
http://home.ecn.ab.ca/~jsavard/index.html
http://www.quadibloc.com/index.html
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"John Savard" wrote in message
...
On Mon, 03 Oct 2005 11:16:30 -0400, Andrew Yee
quoted, in part:

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.

I have thought that we ought to design space probes similar to the
Hubble Space Telescope, but designed so that they can operate
continuously for a hundred years without any maintenance whatever...

Aye, there's the rub....