Most Massive Stellar Black Hole Found in Binary System

Sara Goudarzi
for National Geographic News
October 17, 2007
A strange black hole locked in a tight orbit with a huge star in a nearby galaxy could be the most massive stellar black hole known, astronomers say.

Stellar black holes form when a weighty, dying star's core collapses, creating a region with such a strong gravitational pull that even light can't escape.

Astronomers also believe that supermassive black holes with masses equal to millions of suns exist at the centers of many galaxies, including our own Milky Way.

The newly described stellar black hole has a mass almost 16 times that of our sun, said lead study author Jerome A. Orosz, a professor of astronomy at San Diego State University.

"Before this, the most massive known stellar black hole was GRS 1915+105, with a black-hole mass estimated to be 14 plus or minus 4 solar masses," Orosz said. But the mass of GRS 1915+105 has recently come into question, he added.

Orosz's team also found that the black hole's companion star has a mass about 70 times that of the sun—making it the most massive star in a binary system containing a black hole.

"The present-day companion is no more than about three million years old," Orosz said. "The model suggests that it will die in another two to three million years, forming another black hole."

Orosz and colleagues report their puzzling new observations in this week's issue of the journal Nature.

X-Ray Vision

Since black holes can't be seen directly, astronomers look for the intense radiation released as hot matter spirals into a singularity.

Researchers can then detect the existence of a black hole in a binary system by observing a visible star in orbit around its invisible companion.

The mass of the black hole in such a system can then be deduced by measuring its gravitational tug on the star.

NASA's Einstein X-Ray Observatory Satellite first spotted the black hole and its companion described in the new study in 1981.

The binary system—dubbed M33 X-7—lies in Meisser 33, a spiral galaxy about three million light-years away.

Astronomers soon realized that the companion star passes directly in front of the black hole on its three-day orbit, eclipsing the black hole's x-ray emissions.

This arrangement allowed the team to combine data from NASAs orbiting Chandra X-Ray Observatory and the Gemini North Telescope on Mauna Kea, Hawaii, to calculate the two object's masses more accurately than usual.

(Related images: views from the Chandra X-Ray Observatory.)

Tight Orbit

Orosz and his team don't fully understand how a massive black hole formed in close orbit around a massive companion star.

But experts think the formation of close binaries that contain the remains of dead stars—such as white dwarfs, neutron stars, and black holes—might involve a process called the common-envelope phase.

During this phase, a dying star in a binary system expells its outer layers while sucking the other star closer.

This leads to either a merger of the two objects or the formation of a tight binary system.

"During the common-envelope phase, the progenitor of the M33 X-7 black hole must have lost a large amount of mass for the two objects to be so close," Tomasz Bulik, of the University of Warsaw in Poland, writes in a commentary article also appearing in Nature.

"In this system we know that quite a lot of mass was expelled, and on the other hand that the remaining mass formed the black hole," Bulik told National Geographic News.

"Thus we have an upper and lower bound on the amount of mass expelled."

According to lead study author Orosz, "the reason M33 X-7 is so hard to understand is that both the black hole and the companion star have a very high mass.

"The predicted mass loss [of the star as it died before becoming a black hole] is so great that M33 X-7 should not have been able to enter a common-envelope phase."

Researchers hope that by understanding systems like M33 X-7, they can gain better insight into the formation of tight binary systems.

"The common-envelope stage is poorly known," Bulik said, "and this system will help us understand it."

Also this week, an independent team led by Andrea H. Prestwich of the Harvard-Smithsonian Center for Astrophysics described what it says could be an even more massive stellar black hole weighing 24 to 33 times as much as the sun.

The object is also part of a binary system that includes a massive star, and it lies 2.2 million light-years away in the starburst galaxy IC 10.

The researchers, who published their findings online this week in the Astrophysical Journal Letters, say more data are needed to confirm that the object is actually a black hole.

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