  We present a mission designed for the study of transient phenomena in the
high energy sky, through a wide field X-ray/hard X-ray monitor, and fast (< 1
min) follow up observations with Narrow Field Instrumentation. This is based on
an X-ray telescope with an area of 1000 cm2, equipped with high-resolution
spectroscopy microcalorimeters and X-ray polarimeter. The performances of the
mission on the physics of GRB and their use as cosmological probes are
presented and discussed.
