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Quasar light variability linked to black hole mass (Forwarded)



 
 
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Old January 8th 07, 11:42 PM posted to sci.space.news
Andrew Yee[_1_]
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Default Quasar light variability linked to black hole mass (Forwarded)

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University of Illinois at Urbana-Champaign
Champaign, Illinois

Contact:
James E. Kloeppel, Physical Sciences Editor
217-244-1073

Released: 1/8/07

Quasar light variability linked to black hole mass

CHAMPAIGN, Ill. -- Quasars are some of the most luminous and distant objects
in the universe -- and appear to have something in common with ordinary
light bulbs, say researchers at the University of Illinois at
Urbana-Champaign and at the National Center for Supercomputing Applications.

Quasars were discovered 40 years ago, when astronomers noticed that these
objects -- thought at the time to be stars -- were emitting far more radio
waves than expected. Researchers found, upon further inspection, that these
objects represented a new class of extremely energetic astronomical object.

Astronomers now believe quasars are young galaxies powered by supermassive
black holes at their centers. These black holes can be millions or billions
of times more massive than our sun.

"With such enormous sources of energy, quasars are among the brightest
objects in the universe, some giving off thousands of times more light than
our own Milky Way galaxy from a region slightly larger than our solar
system," said Brian Wilhite, an astronomer at Illinois and a researcher at
NCSA. "Astronomers have also determined that quasars are incredibly
variable, with some quasars quadrupling in brightness in the span of just a
few hours."

Although rarely that dramatic, variability in light output is seen in nearly
all quasars, with average quasars changing in brightness by 10 to 15 percent
over the course of one year, Wilhite said. Astronomers have yet to pin down
the exact mechanism that drives these changes.

Recently, Wilhite and other researchers at Illinois and NCSA found that this
variability is related to both the mass of the black hole at the center of
the quasar, and to the efficiency of the quasar at converting gravitational
potential energy into light energy.

Using data obtained by the Sloan Digital Sky Survey, the researchers
monitored the brightness and estimated the central black hole mass of more
than 2,500 quasars, observed over a period of four years. They found that,
for a given brightness, quasars with large black hole masses are more
variable than those with low black hole masses.

"Quasars with more massive black holes have more gravitational energy that
can potentially be extracted, which we would see in the optical as light,"
said Wilhite, who will present the team's findings at the American
Astronomical Society meeting in Seattle on Monday (Jan. 8).

"If two quasars have the same brightness, the one with the larger black hole
mass is actually less efficient at converting this gravitational energy into
light," Wilhite said. "We have found that these less-efficient quasars have
more variable light output. It could be a little like flickering light bulbs
-- the bulbs that are the most variable are those that are currently the
least efficient."

The researchers' findings mark the first time that changes in light output
of quasars have been related to their efficiency, and could prove useful in
helping astronomers decipher the underlying physics that causes quasars to
vary so wildly.

Collaborators on the project include astronomy professor Robert Brunner at
Illinois and NCSA, Illinois undergraduate student Catherine Grier, and other
members of the Sloan Digital Sky Survey collaboration. The study uses public
data from the Sloan Digital Sky Survey, which is managed by the
Astrophysical Research Consortium for the participating institutions, which
are listed on the SDSS Web site.

This work was funded by NASA and the U. of I.

Editor's note: To reach Brian Wilhite, call 217-244-1325.

Graphics are available at
http://hunter.astro.uiuc.edu/PR
 




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