Friday, May 22, 2009

Black Hole Gives Up Some Secrets

Black hole


separate teams of researchers have unlocked some longstanding secrets of two stars that have puzzled astronomers for more than 20 years.

The studies focus on the pair of objects known as binary system SS 433, some 16,000 light-years from Earth. The pair consists of an old, faint star locked in a tight orbit with a stellar corpse -- either a black hole or dense neutron star. The presumed black hole constantly strips gas from its companion, channels it into a flat "accretion disk" and then later spits the material out in opposing, polar jets that shoot out at 90-degree angles to the disk.

Astronomers have seen SS 433 do some strange things since the system was first discovered in the 1960s. Not only are its jets detectable in infrared and X-rays, but also visibly too. And the light spectrum seen by astronomers seems to change over time.

The system "remains unique and hard to understand 25 years after its discovery," said Bruce Margon, associate director for the Space Telescope Science Institute, which runs the Hubble Space Telescope. "For example, why is this the only star system we see with these relativistic jets?"

Relativistic is a term used to describe material moving at a significant fraction of the speed of light.

The jets were the target of researchers at Massachusetts Institute of Technology (MIT), who used the Chandra X-ray Observatory to study SS 433s emissions, which stream from the object at a roughly one-fourth of light's speed. Their results may help develop new ways to measure masses of black holes.

The system resembles the output of giant galaxies in the making, objects called quasars that exist mostly far away and in the early universe.

"We believe its very much like a quasar jet," MITs Herman Marshall, lead investigator for his team, said of SS 433. "Its a jet laboratory thats closer to home."

Another effort by astronomers with the National Radio Astronomy Observatory (NRAO) made daily observations of SS 433s apparent black hole, compiling the images into a movie that allowed them to track individual ejections of jet material as they spewed forth. The movie helped researchers determine the source of brightness variations in the jets.

A separate team led by astronomer Todd Hillwig of Georgia State University made the first observations the companion star in SS 433.

The results of all three studies were presented here Monday during the 203rd meeting of the American Astronomical Society.

Honing in on a microquasar

SS 433 is what astronomers call a microquasar, a system where the central massive object -- a neutron star or black hole -- mimics the behavior of quasars, which are thought to consist of gigantic black holes at the center of distant, developing galaxies and which consistently spit bright jets of materials from their poles.

But since microquasars are closer to Earth than their quasar cousins -- which can sit 10 billion light-years in the distance -- they are easier to observe and changes occur more noticeably over time.

"If you were trying to [see] this with an extragalactic jet, it would take years, even decades" said Amy Mioduszeweski, who led one study for the NRAO.

NRAO astronomers used the Very Large Baseline Array, a network of 10 radio telescopes arranged across 5,000 miles (8,046 kilometers) from Hawaiis Mauna Kea to St. Croix in the U.S. Virgin Islands to track SS 433s jets.

Observations of past microquasars have shown that their jets typically get fainter as they taper out into space. But SS 433 doesnt. As the jets shoot out, they sometimes flare up to become even brighter.

The NRAO study also detected other, non-jet related radio waves emanating from SS 433s microquasar core, leading researchers to believe the objects accretion disk forms a wind that interacts with a denser wind from the black holes companion star, causing the added emissions. The disk wind may also interact with jet material to cause the belated jet flare-ups.

"The most obvious place for it to come from is from the center of this system," Mioduszeweski said. "But we dont know for sure where this outflow is coming from."

The system is in the constellation Aquila, the Eagle.

A matter of timing

Hillwigs team had a hard time catching a glimpse of the SS 433s companion star. The accretion disk and bright jets of the black hole are so bright they blot out any chance of seeing the companion star, except during a time of eclipse, when the star crosses in front of the black hole as seen by observers on Earth.

"We think its a matter of timing," explained Douglas Gies, a Georgia State astrophysicist and team member who presented the study. "The best time to see to see the mass-donor star is when the accretion disk wobbles in precession during an eclipse. This set of requirements exists just twice each year."

Although the star passes in front of the black hole every 13 days, only twice a year does it do so at an angle high enough above the black hole to be noted by ground-based instruments.

After a failed first attempt, Hillwigs team successfully identified the star from its companion using the 13-foot (four-meter) telescope at Arizonas Kitt Peak National Observatory. What the team found was a light pattern suggesting SS 433s star is an old, swollen supergiant with a surface temperature of about 13,000 Fahrenheit. It has a mass 11 times greater than that of Earths Sun. The large mass provides a feast for SS 433s much more compact black hole.

"Apparently, the black hole cant digest the overwhelming amount of gas its companion star is giving," Gies said. "So it may be funneling the excess into these powerful jets."

How much less massive the black hole is than its neighbor is up for debate. Hillwigs team claimed the black hole was roughly three solar masses while the MIT study originally found it significantly larger, some 16 solar masses. MIT researchers later reduced that estimate to about eight solar masses. The two teams plan to work together in the future to hone in on a more definitive mass for the black hole.

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