Using two NASA satellites, astronomers
have discovered a black hole that obliterates a record announced
just two weeks ago. The new black hole, with a mass 24 to 33 times
that of our Sun, is the heftiest known black hole that orbits
another star.
The record-breaker 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. "We
now know 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.
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, so a simple
application of Kepler’s Laws show 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 many Suns worth of gas before
the stars explode. Calculations suggest massive stars in our galaxy
leave behind black holes no heavier than about 15 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 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."
This release is being issued jointly with NASA http://www.nasa.gov/centers/goddard/news/topstory/2007/overweight_hole.html