XTE is a small inexpensive state-of-the-art orbiting observatory which will be launched on a Delta II rocket in August of this year. Its purpose is to explore the broad band X-ray region from 2 to 200 keV on time scales ranging from about a microsecond up to a year.
XTE will provide new astrophysical data on exotic objects such as accreting black holes, active galactic nuclei, jets and relativistic beams, pulsar beaming, and X-ray bursts. For example, XTE will closely examine binary systems containing a compact object (such as a neutron star or black hole) accompanied by a normal star. If the two stars are close enough, material from the normal star will flow onto the compact star attracted by gravity. As the gas falls down the steep gravitational well of the compact star its temperature rises to the point where X-rays are emitted. These X-rays provide detailed information about the conditions in the region of the extremely strong gravitational field and magnetic fields near the compact object.
The same accretion processes apply to Active Galactic Nuclei (AGN) in which the X-ray emission results from accreting material onto a supermassive blackhole. Quasars are one type of AGN. Variations in intensity are a typical characteristic of emission of compact sources and provide crucial information about the dimensions of such sources. Spectral information provides diagnostics of the underlying emission processes. For the first time, studies of X-ray variability ranging from microseconds to several years will be carried out. XTE's design and flexible operating characteristics will allow it to respond rapidly to changes in the X-ray sky (within hours) and will allow multifrequency observations. XTE will be able to study more than a thousand of the brightest X-ray sources in the sky, representing a wide range of high-energy astrophysical phenomenon.