RXTE has made some amazing discoveries; many times, though, its observations
create more mysteries than they solve. Such is the way of science.
Here are a
few of the strange and wonderful things RXTE has shown us. For more details
on any of these items, please see our Learning Center feature
RXTE Discoveries
The Rossi X-ray Timing Explorer (RXTE) discovered neutron stars that emit
streams of X-rays that pulse over 1,000 times a second in August of 1996. The
pulses are not strictly periodic (or reoccurring at a constant rate), but
vary slightly from cycle to cycle. Astronomers call them "quasi-periodic
oscillations" or QPOs. This just means that the pulses are almost, but not
quite, periodic. QPOs are significant because they can tell us about how
material falls onto a neutron star or black hole and give us information on
the interaction between accretion disks and the source it surrounds.
Astronomers Dr. Wei Cui of MIT, Dr. Wan Chen of NASA's Goddard Space Flight
Center and Dr. Shuang N. Zhang of NASA's Marshall Space Flight Center used
RXTE in November 1997 to observe a black hole that appears to be dragging
space and time around itself as it rotates! A good analogy is a bowling ball
immersed in molasses. When the bowling ball rotates, it drags the molasses
with it, just like a black hole drags space-time. This effect is called
"frame dragging" and it is something that Einstein's Theory of Relativity
predicts. This is the first time that physical evidence to support this
aspect of Einstein's 1918 theory has been available.
In January of 1998 it was discovered that every half hour or so, the black
hole known as GRS 1915+105 throws off the inner portion its accretion disk,
causing a jet that seems to travel at near light speeds. This disk of
matter re-forms itself after each jet as the black hole pulls in more matter
from its companion star. Even more amazing are the audio files of this
phenomenon, located on our web site. Thanks to Dr. Ed Morgan of MIT, it is
actually possible to listen to this black hole throw off the inner portion
of its disk.
A newly-discovered star that is emitting rapid pulses of X-rays may be
the long-sought missing link between old neutron stars that emit
powerful flashes of X-rays, and older, rapidly spinning neutron stars
that emit mainly radio waves. This star, designated SAX J1808.4-3658,
is located 12,000 light years away towards the constellation
Sagittarius. The discovery, made by two competing teams of scientists
using RXTE, was announced in July 1998. This new pulsar helps
scientists resolve a mystery. Prior to the discovery, two populations
of neutron stars with relatively weak magnetic fields but with
otherwise different characteristics were known. There are old,
accreting neutron stars, which generate X-rays from the material they
are gobbling up from their companions, and a group of radio-wave
emitting millisecond pulsars that are rotating very rapidly and
slowing down gradually. Scientists suspected there was a connection
between the two, and the discovery of this pulsar that is both
emitting X-rays and spinning rapidly provides the link. "This has
sometimes been called the Holy Grail of X-ray astronomy," Dr. van der
Klis, one of the scientists, exclaimed.
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