The RXTE Story
RXTE Sees the Violent Side of the Universe
A marshmallow striking the surface of a neutron star would have the impact of a thousand hydrogen bombs. Of course, marshmallows really don't rain down onto neutron stars. But gas from neighboring stars does -- at a speed of millions of miles an hour.
Gas funnels into black holes at such a speed too, and it creates quite a fireworks display in X-ray light as it does. RXTE is a satellite built to observe these kinds of fireworks that sometimes flicker in intensity thousands of time per second or perhaps burst only once every few years.
For RXTE, it's all in the timing. The timescale of flickering X-rays, called oscillations, reveals the underlying physics of the violent environment around objects such as neutron stars and black holes. Here's how it works.
X-rays Light the Way to Black Holes
Black holes and neutron stars can live alone, but often they have a companion. This companion is a bright, active, hydrogen-burning star like our Sun, sometimes bigger or sometimes smaller. The active star orbits the "dead" neutron star or black hole in what we call a binary star system.
On their own, black holes and neutron stars would be difficult to spot. By definition, a black hole emits no light. And a neutron star is tiny tiny tiny. Picture the sphere of planet Earth. Now picture a dot on the sphere representing New York City. This is the size of a neutron star. Imagine looking for that dot when it is a billion miles away. We can often see the black hole and neutron star, though, with a little help from the companion star.
Both the neutron star and black hole have intense gravitational fields. Matter doesn't just fall toward these objects like an apple plopping down on the soft grass of Earth. No, matter plows into these objects. This matter, for the most part, is gas from the companion star.
The black hole and neutron star actually pull blobs of gas from the companion star. As the stolen gas nears these objects, it gets hotter and hotter and emits X-ray light. RXTE and other X-ray telescopes detect these X-rays, thereby allowing us to see what was once invisible. Yet as other X-ray telescopes zoom in on the fingerprints of the X-ray emitting gas (something called spectroscopy, the study of spectrum), RXTE takes a different approach and records the timing of the flashes of X-ray light.
It's All in the Timing
The flow of gas and the X-ray fireworks that accompany the flow varies with time. Sometimes the active star may orbit more closely to a neutron star, allowing greater blobs of its gas to escape toward the neutron star. Sometimes there might be a back up in the accretion disk, that path of gas that funnels into the neutron star or black hole. Back-ups might cause a sudden flare of X-ray light from within the accretion disk. Sometimes the flow of gas and production of X-rays are dictated by the rapidly spinning neutron star or pulsar.
RXTE measures these variations in X-ray brightness, often occurring in a fraction of a second. The variations are called oscillations. Sometimes they are periodic, appearing at specific dependable rates. Sometimes they are almost periodic (quasiperiodic oscillations). Regardless, the oscillations reveal the nature of the physical environment of the star system. By studying these oscillations and tracking the same X-ray sources for years, RXTE scientists form a picture of the events that are taking place.
Among satellites, nothing like RXTE has come before, and as of yet, there is no replacement.