DIOS: Diffuse Intergalactic Oxgen Surveyor

DIOS will carry out a survey of cosmic missing baryons by measuring Oxygen K emission lines with superior energy resolution. The instrument consists of an array of TES microcalorimeters with an energy resolution of about 2 eV, cooled by mechanical coolers. We introduce a new type of X-ray optics employing 4-reflection mirrors with a focal length as short as 70 cm. With the superior energy resolution and the large SΩ of 100 cm^2 deg^2, DIOS will identify extragalactic warm-hot gas with gas temperature T=10^6-7 K and overdensity δ=10-100 located at z<0.3 through emission lines of OVII and OVIII. The mission is planned to be launched around 2008, and a sky area of 10° × 10° will be surveyed in 1--2 years.