Re: Yahoo! News Story - Smallest Black Hole Found - Yahoo! News
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Mark A. Holmes
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> > Smallest Black Hole Found - Yahoo! News
> > http://news.yahoo.com/s/space/20080401/sc_space/smallestblackholefound
> Mark A. Holmes
> Begin story.--
> Smallest Black Hole Found
> Andrea Thompson
> Staff Writer
> SPACE.com Tue Apr 1, 2:46 PM ET
> NASA scientists have identified the smallest, lightest black hole yet
> The new lightweight record-holder weighs in at about 3.8 times the
> mass of our sun and is only 15 miles (24 kilometers) in diameter.
> "This black hole is really pushing the limits," said study team leader
> Nikolai Shaposhnikov of NASA's Goddard Space Flight Center in
> Greenbelt, Md. "For many years astronomers have wanted to know the
> smallest possible size of a black hole, and this little guy is a big
> step toward answering that question."
> The low-mass black hole sits in a binary system in our galaxy known as
> XTE J1650-500 in the southern hemisphere constellation Ara. NASA's
> Rossi X-ray Timing Explorer (RXTE) satellite discovered the system in
> 2001, and astronomers soon realized that the system harbored a
> relatively lightweight black hole. But the black hole's mass had never
> been precisely measured.
> Black holes can't be seen, but they're identified by the activity
> around them, which also helps astronomers estimate a size of the
> region inside the activity, and how much mass must be in that confined
> region to generate all the surrounding activity. More specifically,
> astronomers can weigh black holes by using a relationship between the
> apparent size of the black hole and the X-rays emitted by the torrent
> of gas that swirls into the black hole's disk from its companion star.
> As the gas piles up near the black hole, it "becomes very dense and
> congested," like a traffic jam, Shaposhnikov said at a press
> conference announcing the find. "So matter has to literally squeeze
> into the black hole."
> As it is squeezed, the gas heats up and radiates X-rays. The intensity
> of the X-rays varies in a pattern repeated over a nearly regular
> interval. Astronomers have long suspected that the frequency of this
> signal, called the quasi-periodic oscillation, or QPO, depends on the
> mass of the black hole.
> As the black hole gets bigger, the zone of swirling gas is pushed
> farther out, so the QPO ticks away slowly. But for smaller black
> holes, the gas sits closer in and the QPO ticks rapidly.
> Shaposhnikov and his colleague Lev Titarchuk of George Mason
> University used this method to "weigh" XTE J1650-500 and found a mass
> of 3.8 suns. This value is well below the previous record holder GRO
> 1655-40, which tips the scales at about 6.3 suns.
> This new �mass measurement could help shed light on what the
> star that will produce a black hole is. Astronomers know that some
> unknown critical threshold, possibly between 1.7 and 2.7 solar masses,
> marks the boundary between a star that generates a black hole upon its
> death and one that produces a neutron star.
> "This new result brings us much closer to the theoretically predicted
> limit," Shaposhnikov said.
> Knowing this boundary would help scientists understand the behavior of
> matter when it is scrunched to extraordinarily high densities.
> "The question of black hole masses has concerned us for more than a
> decade now," said astrophysicist Vicky Kalogera of Northwestern
> University, who was not involved with the study, during the press
> conference. Scientists had predicted that their should be more black
> holes at the lower end of the mass range than astronomers had
> identified, so this study helps clear up some confusion as to where
> these lightweight black holes were, she added.
> Kalogera did caution that the method used by Shaposhnikov and
> Titarchuk is not the main way that black hole masses are measured, but
> noted that their measurements of the masses of other black holes
> agreed well with the results from the standard method.
> Shaposhnikov and Titarchuk presented their findings on March 31 at the
> American Astronomical Society's High-Energy Astrophysics Division
> meeting in Los Angeles.
> --End story.