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X-Ray Timing Explorer

 The X-Ray Timing Explorer will probe the physics of
cosmic x-ray sources by making sensitive measurements of
their rich temporal variability over time scales ranging
from milliseconds to years.  Their time behavior is the
source of important information about processes and
structures in white-dwarf stars, x-ray binaries, neutron
stars, pulsars, and black holes.  For studying known sources
and detecting transient events, observations are required
over an extended period with instruments sensitive to x-ray
energies from 2,000 to 10,000 electron volts.

The objectives of XTE are to investigate:

  (1) periodic, transient, burst, and white-noise
fluctuations in X-ray emissions,
  (2) relaxation times for the interior structure of neutron
stars,
  (3) binary stellar masses, separation distances, and orbit
eccentricities, and
  (4) mass exchange between ordinary and degenerate stars in
binary systems.

     The XTE will be carried into space aboard the space
shuttle and will replace the Extreme Ultraviolet Explorer
(EUVE) after the EUVE's two year operation.  XTE will be
placed on an Explorer Platform through an on-orbit changeout
procedure.

     Mission development for the XTE is expected to begin in
1989.