Observational Signatures of the Warm-Hot Intergalactic Medium and X-ray Absorption Lines by the Halo of our Galaxy

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 O VII and O VIII. 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 O VII density estimated from O VII absorption line in the Galactic X-ray source, 4U1820-303, 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 O VII and O VIII 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.