Abstract Approximately 30% to 50% of the total baryons of the present universe are considered to take a form of warm/hot intergalactic medium (WHIM) and to have evaded direct detection. The WHIM of T=106-107K is most likely detected through absorption and emission lines of and . The equivalent widths of the absorption lines are typically 0.4eV, which is consistent with the absorption lines having redshifts indistinguishable from zero observed in the spectra of a few bright active galactic nuclei (AGN). However, from the midplane density estimated from absorption line in the Galactic X-ray source, , we consider that a significant fraction of warm/hot plasma responsible for the AGN absorption lines is located in our Galaxy rather than in the local group. For a systematic study of the WHIM, survey-type observations detecting emission lines are necessary. While the typical surface brightness of and emission lines is below the detection limit of present and proposed future missions, an unambiguous detection will be feasible with a small X-ray mission dedicated for this purpose. Our proposed mission, DIOS (Diffuse Intergalactic Oxygen Surveyor), is sensitive to the WHIM of temperatures T=106-107K and can survey about 0.1sr area to the depth of cosmological redshift z<0.3 in two years.

Keywords X-rays: ISM - cosmology: miscellaneous

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