Supermirror design for the XEUS X-ray telescope

X-ray telescopes have been providing high sensitivity in X-ray observations in various missions. Recently XMM-Newton achieved an effective area of about 2000 cm² at 8 keV and a very high sensitivity. For X-ray telescopes in the future, one of the key technologies is to expand the energy band beyond 10 keV. In our previous work, we designed depth-graded multilayers, so called supermirror, for a hard X-ray telescope and it was successfully flown in a balloon flight. This technology is promising to extend the energy band up to 40 keV. We would like to propose supermirror coatings on the mirrors of the XEUS X-ray telescope, which has a focal length of 50 m, a diameter of 4.5 m and a mirror length of 1 m. In this configuration, incident angle is ranging from 0.17 to 0.65 degrees and a large effective area of 30000 cm² at 8 keV can be achieved. To extend the energy band up to 80 keV, platinum-carbon supermirrors are effective for the incident angle below 0.4 degrees. If the inner part of the XEUS mirrors are coated with multilayers, the effective area will be enhanced above 10 keV and be kept over 100 cm² up to 80 keV. In this paper, we propose the supermirror design and discuss the performance of the XEUS hard X-ray telescope with supermirrors.