  A combination of microchannel plate optics and a 32x32 pixel microcalorimeter
would allow the successor to the Rossi XTE to explore new domains of
spectroscopic timing in a MIDEX class mission. With ~10 times the area and ~100
times the spectral resolution of the PCA (and 10 times that of silicon
detectors) such a mission would be able to explore redshifts and plasma
conditions in weak line features over a wide range of celestial sources. This
would allow several tests of basic physics, both QED/QCD and GR.
