Updated at 2:20 p.m. ET.
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 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 smallest 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 there 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.
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