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Massive Black Hole Smashes Record
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10.30.07
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Using two NASA satellites,
astronomers have discovered the heftiest known black hole to
orbit a star. The new black hole, with a mass 24 to 33 times
that of our Sun, is more massive than scientists expected for
a black hole that formed from a dying star.
Image right: In this artist’s portrayal of the IC 10
X-1 system, the black hole lies at the upper left and its
companion star is on the right. The two objects orbit around a
center of gravity once every 34.4 hours. The stellar companion
is a type known as a Wolf-Rayet star. Such stars are highly
evolved and destined to explode as supernovae. The black hole
companion is shedding its outer envelope in a powerful wind,
and some of this gas is captured by the black hole’s powerful
gravity. +
Click for high resolution (1.2 Mb) JPG Credit: Aurore
Simonnet/Sonoma State University/NASA.
The newly
discovered object belongs to the category of "stellar-mass"
black holes. Formed in the death throes of massive stars, they
are smaller than the monster black holes found in galactic
cores. The previous record holder for largest stellar-mass
black hole is a 16-solar-mass black hole in the galaxy M33,
announced on October 17.
"We weren’t expecting to find
a stellar-mass black hole this massive," says Andrea Prestwich
of the Harvard-Smithsonian Center for Astrophysics in
Cambridge, Mass., lead author of the discovery paper in the
November 1 Astrophysical Journal Letters. "It seems likely
that black holes that form from dying stars can be much larger
than we had realized."
The black hole is located in the
nearby dwarf galaxy IC 10, 1.8 million light-years from Earth
in the constellation Cassiopeia. Prestwich’s team could
measure the black hole’s mass because it has an orbiting
companion: a hot, highly evolved star. The star is ejecting
gas in the form of a wind. Some of this material spirals
toward the black hole, heats up, and gives off powerful X-rays
before crossing the point of no return.
Image left: The galaxy IC 10 is an irregular dwarf
galaxy about 1.8 million light-years from Earth. +
Click for high resolution (1.1 Mb) JPG Credit: Adam
Block/NOAO/AURA/NSF.
In November 2006, Prestwich
and her colleagues observed the dwarf galaxy with NASA’s
Chandra X-ray Observatory. The group discovered that the
galaxy’s brightest X-ray source, IC 10 X-1, exhibits sharp
changes in X-ray brightness. Such behavior suggests a star
periodically passing in front of a companion black hole and
blocking the X-rays, creating an eclipse. In late November,
NASA’s Swift satellite confirmed the eclipses and revealed
details about the star’s orbit. The star in IC 10 X-1 appears
to orbit in a plane that lies nearly edge-on to Earth’s line
of sight, The Swift observations, as well as observations from
the Gemini Telescope in Hawaii, told Prestwich and her group
how fast the two stars go around each other. Calculations
showed that the companion black hole has a mass of at least 24
Suns.
There are still some uncertainties in the black
hole’s mass estimate, but as Prestwich notes, "Future optical
observations will provide a final check. Any refinements in
the IC 10 X-1 measurement are likely to increase the black
hole’s mass rather than reduce it."
The black hole’s
large mass is surprising because massive stars generate
powerful winds that blow off a large fraction of the star’s
mass before it explodes. Calculations suggest massive stars in
our galaxy leave behind black holes no heavier than about 15
to 20 Suns.
The IC 10 X-1 black hole has gained mass
since its birth by gobbling up gas from its companion star,
but the rate is so slow that the black hole would have gained
no more than 1 or 2 solar masses. "This black hole was born
fat; it didn’t grow fat," says astrophysicist Richard
Mushotzky of NASA Goddard Space Flight Center in Greenbelt,
Md., who is not a member of the discovery team.
The
progenitor star probably started its life with 60 or more
solar masses. Like its host galaxy, it was probably deficient
in elements heavier than hydrogen and helium. In massive,
luminous stars with a high fraction of heavy elements, the
extra electrons of elements such as carbon and oxygen "feel"
the outward pressure of light and are thus more susceptible to
being swept away in stellar winds. But with its low fraction
of heavy elements, the IC 10 X-1 progenitor shed comparatively
little mass before it exploded, so it could leave behind a
heavier black hole.
"Massive stars in our galaxy today
are probably not producing very heavy stellar-mass black holes
like this one," says coauthor Roy Kilgard of Wesleyan
University in Middletown, Conn. "But there could be millions
of heavy stellar-mass black holes lurking out there that were
produced early in the Milky Way’s history, before it had a
chance to build up heavy
elements."
Robert Naeye Goddard Space Flight
Center
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