Heaviest Stellar Black Hole Discovered in Nearby Galaxy (Forwarded)

Jennifer Morcone
Marshall Space Flight Center, Huntsville, Ala.
(Phone: 256/544-7199)

Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
(Phone: 617/496-7998)

For Release: October 17, 2007

Heaviest Stellar Black Hole Discovered in Nearby Galaxy

Astronomers have located an exceptionally massive black hole in orbit
around a huge companion star. This result has intriguing implications for
the evolution and ultimate fate of massive stars.

The black hole is part of a binary system in M33, a nearby galaxy about 3
million light years from Earth. By combining data from NASA's Chandra
X-ray Observatory and the Gemini telescope on Mauna Kea, Hawaii, the mass
of the black hole, known as M33 X-7, was determined to be 15.7 times that
of the Sun. This makes M33 X-7 the most massive stellar black hole known.
A stellar black hole is formed from the collapse of the core of a massive
star at the end of its life.

"This discovery raises all sorts of questions about how such a big black
hole could have been formed," said Jerome Orosz of San Diego State
University, lead author of the paper appearing in the October 18th issue
of the journal Nature.

M33 X-7 orbits a companion star that eclipses the black hole every three
and a half days. The companion star also has an unusually large mass, 70
times that of the Sun. This makes it the most massive companion star in a
binary system containing a black hole.

"This is a huge star that is partnered with a huge black hole," said
coauthor Jeffrey McClintock of the Harvard-Smithsonian Center for
Astrophysics in Cambridge, Mass. "Eventually, the companion will also go
supernova and then we'll have a pair of black holes."

The properties of the M33 X-7 binary system -- a massive black hole in a
close orbit around a massive companion star -- are difficult to explain
using conventional models for the evolution of massive stars. The parent
star for the black hole must have had a mass greater than the existing
companion in order to have formed a black hole before the companion star.

Such a massive star would have had a radius larger than the present
separation between the stars, so the stars must have been brought closer
while sharing a common outer atmosphere. This process typically results in
a large amount of mass being lost from the system, so much that the parent
star should not have been able to form a 15.7 solar-mass black hole.

The black hole's progenitor must have shed gas at a rate about 10 times
less than predicted by models before it exploded. If even more massive
stars also lose very little material, it could explain the incredibly
luminous supernova seen recently as SN 2006gy. The progenitor for SN
2006gy is thought to have been about 150 times the mass of the Sun when it
exploded.

"Massive stars can be much less extravagant than people think by hanging
onto a lot more of their mass toward the end of their lives," said Orosz.
"This can have a big effect on the black holes that these stellar
time-bombs make."

Coauthor Wolfgang Pietsch was also the lead author of an article in the
Astrophysical Journal that used Chandra observations to report that M33
X-7 is the first black hole in a binary system observed to undergo
eclipses. The eclipsing nature enables unusually accurate estimates for
the mass of the black hole and its companion.

"Because it's eclipsing and because it has such extreme properties, this
black hole is an incredible test-bed for studying astrophysics," said
Pietsch.

The length of the eclipse seen by Chandra gives information about the size
of the companion. The scale of the companion's motion, as inferred from
the Gemini observations, gives information about the mass of the black
hole and its companion. Other observed properties of the binary were used
to constrain the mass estimates.

NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra
program for the agency's Science Mission Directorate. The Smithsonian
Astrophysical Observatory controls science and flight operations from the
Chandra X-ray Center in Cambridge, Mass. Gemini is an international
partnership managed by the Association of Universities for Research in
Astronomy under a cooperative agreement with the National Science
Foundation.

Additional information and images are available at:
http://chandra.harvard.edu/photo/2007/m33x7/
and
http://chandra.nasa.gov