Astronomers Shed Light on Black Holes (Forwarded)

Northwestern University

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Megan Fellman, 847-491-3115

January 6, 2006

Astronomers Shed Light on Black Holes

EVANSTON, Ill. -- In the most comprehensive study of Sagittarius A* (Sgr
A*), the enigmatic supermassive black hole in the center of the Milky
Way Galaxy, astronomers -- using nine ground and space-based telescopes
including the Hubble Space Telescope and the XMM-Newton X-ray
Observatory -- have discovered that Sgr A* produces rapid flares close
to the innermost region of the black hole in many different wavelengths
and that these emissions go up and down together.

This insight into the frequent bursts of radiation observed shooting off
the black hole like firecrackers -- similar to solar flares -- will help
scientists better understand the dynamics of Sgr A* and the source of
its flares.

Farhad Yusef-Zadeh, professor of physics and astronomy at Northwestern
University, who led a team of 11 astronomers from around the world in
the study of Sgr A*, presented the team's results at a press conference
Jan. 10 at the American Astronomical Society meeting in Washington, D.C.

"We observed that the less energetic infrared flares occur
simultaneously with the more energetic X-ray flares as well the
submillimeter flares," said Yusef-Zadeh. "From this, we infer that the
particles that are accelerated near the black hole give rise to X-ray,
infrared and submillimeter emission. In addition, not all of the
material that approaches the black hole gets sucked in. Some of the
material may be ejected from the vicinity of the central black hole or
event horizon. Our observations hint that these flares have enough
energy to escape from the closest confines of the supermassive black
hole's sphere of influence."

Yusef-Zadeh and his team observed Sgr A* during two four-day periods in
2004, one in March and one in September. (2004 marked the 30th
anniversary of the discovery of Sgr A*, which has a mass equivalent to
3.6 million Suns and is located in the Sagittarius constellation.) The
campaign captured data across a wide spectrum, including radio,
millimeter, submillimeter, infrared, X-ray and soft gamma ray wavelengths.

The astronomers also determined that the real engine of the flare
activity is in the infrared wavelength. Using observations from Hubble's
Near-Infrared Camera and Multi-Object Spectrometer, they found infrared
activity 40 percent of the time, more than was observed at any other
wavelength.

"This is not something we expected," said Yusef-Zadeh. "Other black
holes in other galaxies don't show this flare activity. We believe it is
the dynamics of the captured material -- very close to the event horizon
of the black hole -- that produces the flares. And the flares are
fluctuating at low levels, like flickers. The flare radiation results
from fast-moving materials in the innermost region of the black hole.
It's a way of life for Sgr A*, this frequent low level of activity."

Because flares are variable and not constant, the study required a large
number of telescopes devoted to studying flare activity simultaneously.
The space-based telescopes used in this observation campaign were the
Hubble Space Telescope, the XMM-Newton X-ray Observatory and the
International Gamma-Ray Astrophysics Laboratory (INTEGRAL). The ground
telescopes used were Very Large Array (VLA) of the National Radio
Astronomy Observatory; Caltech Submillimeter Observatory (CSO);
Submillimeter Telescope (SMT); Nobeyama Array (NMA); Berkeley Illinois
Maryland Array (BIMA); and Australian Telescope Compact Array (ATCA).

In addition to Yusef-Zadeh, the Sgr A* study was conducted by Howard A.
Bushouse, Space Telescope Science Institute; Mark Wardle, Macquarie
University, Australia; Douglas Roberts, Northwestern University and
Adler Planetarium and Astronomy Museum; Craig Heinke, Northwestern
University; C. Darren Dowell, Jet Propulsion Laboratory, California
Institute of Technology; Geoffrey Bower, University of California,
Berkeley; Stuart Shapiro, University of Illinois at Urbana-Champaign;
Andrea Goldwurm and Guillaume Belanger, Service d'Astrophysique, France;
and Baltasar Vila-Vilaro, National Astronomical Observatory of Japan.

The research was supported by the National Aeronautics and Space
Administration and the National Science Foundation.

Images associated with this release,
http://www.northwestern.edu/newscent...1/flares0.html

IMAGE CAPTION:
[http://www.northwestern.edu/shared/c..._blackhole.jpg
(9KB)]
This image shows a simulation of what the environment of Sagittarius A*
(Sgr A*) looks like. The hole at the center is the event horizon, and
the green and white lines illustrate the expanding blobs of gas caused
by flares as they cool. (CREDIT: Goldston, Quataert and Igumenshchev, 2005).