Northwestern University

MEDIA CONTACT:
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).