XEUS

ESA's XEUS, the X-ray Evolving Universe Mission, is a potential successor to XMM-Newton. The current plan envisions two phases for the mission. The mirrors and detectors are planned to be housed in two separate spacecraft assemblies, with both being in close proximity to the International Space Station. The detectors will cover the 0.05-30keV range, with a collecting area of 6m$^2$ (i.e., approximately 10 times the peak effective area of RXTE) at energies of 1keV in the first phase of the mission. This is planned to be expanded to 30 m$^2$ at 1keV and 6m$^2$ at 8keV in the second phase of the mission. The instruments will have energy resolution of 1 to 10 eV (i.e., $\Delta E/E \approx 1000$ at 6keV) and spatial resolution of 2 arcsec. Like Constellation-X, this would be a very powerful combination of large effective area and high spectral resolution, making XEUS ideal for reverberation mapping studies, as we further describe below.