  AXTAR is a NASA MIDEX mission concept for X-ray timing of compact objects
that combines very large collecting area, broadband spectral coverage, high
time resolution, highly flexible scheduling, and an ability to respond promptly
to time-critical targets of opportunity. It is optimized for submillisecond
timing of bright Galactic X-ray sources in order to study phenomena at the
natural time scales of neutron star surfaces and black hole event horizons,
thus probing the physics of ultradense matter, strongly curved spacetimes, and
intense magnetic fields. AXTAR's main instrument is a collimated, thick Si
pixel detector with 2-50 keV coverage and over 3 square meters effective area.
For timing observations of accreting neutron stars and black holes, AXTAR
provides at least a factor of five improvement in sensitivity over the RXTE
PCA. AXTAR also carries a sensitive sky monitor that acts as a trigger for
pointed observations of X-ray transients in addition to providing high duty
cycle monitoring of the X-ray sky. We review the science goals and design
choices that face a next generation timing mission. We then describe the
technical concept for AXTAR and summarize a preliminary mission design study at
the NASA/MSFC Advanced Concepts Office.
