Shedding a New Light on the Universe
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
Quasi-Periodic Oscillations (QPOs)
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.
Black Hole Drags Space-Time
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.
Black Hole Sheds Accretion Disk
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.
The Missing Link Pulsar
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.