David Tytell Sky and Telescope
 This
artist's rendition shows how a black hole manages to "eat"
gas from its companion star. Any material that doesn't pass the
hole's event horizon escapes through superluminous jets. Courtesy
NASA/Honeywell Max-Q Digital Group/Dana Berry.
Using the
Rossi X-ray Timing Explorer satellite, astronomers have found
the best evidence yet for black holes that spin. While analyzing
X-ray emission from the microquasar GRO J1655-40, Tod E. Strohmayer
found a "quasiperiodic oscillation," or QPO, in the
black hole with a frequency of 450 cycles per second — the
fastest signal ever seen from a black hole. The only way something
could orbit that fast would be to circle extremely close to the
black hole's event horizon (edge) with the hole itself spinning,
dragging the surrounding space along with it.
QPOs are often
caused by blobs of hot gas spinning tightly around neutron stars.
The frequency of the oscillation depends on the orbital radius
and the mass of the neutron star. However, the QPOs in J1655-40
have a frequency greater than what should be physically allowed.
To explain the phenomenon, Strohmayer concludes the rotation of
the black hole is speeding up the QPOs. Strohmayer suspects that
the QPOs occur no more than about 30 kilometers of the hole's
event horizon — a number that seems more minute given that
the event horizon itself is only some 20 km in radius.
Interestingly,
spinning black holes have been known for some time. As reported
in the December 1997 issue of Sky & Telescope, Andrew C. Fabian
(Cambridge University) found that the Seyfert galaxy MCG-6-30-15
has a black hole with spectra showing redshifts that require fast
rotation and "frame dragging" of the surrounding space.
Also in 1997, Shuang N. Zhang (NASA/Marshall Space Flight Center)
and his colleagues measured J1655-40 and found circumstantial
evidence for black-hole rotation in the form of superluminal jets.
Scientists believe only spinning black holes can produce such
superfast jets.
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