NASA scientists have identified the smallest, lightest black hole yet
found.
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 pu****ng 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 mut be in that confined
region to generate all the surrounding activity.
More specifically, astronomers weigh black holes by using a
relation****p between the apparent size of the black hole and the X-
rays emitted by the torrent of gas that swirls in the black hole's
disk before taking its fatal plunge.
As the hot gas piles up near the black hole, it 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 smallest
star that will produce a black hole is.
Astronomers know that some unknown critical threshold, possibly
between 1.7 and 2.7 solar m*****, marks the boundary between a star
that generates a black hole upon its death and one that produces a
neutron star.
Knowing this boundary would help scientists understand the behavior of
matter when it is scrunched to extraordinarily high densities.
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