Detecting X-ray filaments in the low-redshift Universe with XEUS and Constellation-X

We propose a possible way to detect baryons at low redshifts from the analysis of X-ray absorption spectra of bright AGN pairs. A simple semi-analytical model to simulate the spectra is presented. We model the diffuse warm–hot intergalactic medium (WHIM) component, responsible for the X-ray absorption, using inputs from high-resolution hydrodynamical simulations and analytical prescriptions. We show that the number of O vii absorbers per unit redshift with column density larger than 10^13.5 cm⁻²– corresponding to an equivalent width of ∼1 km s⁻¹– that will possibly be detectable by XEUS, is ≳30 per unit redshift. Constellation-X will detect ∼6 O vii absorptions per unit redshift with an equivalent width of 10 km s⁻¹. Our results show that, in a ΛCDM universe, the characteristic size of these absorbers at z∼ 0.1 is ∼1 h⁻¹ Mpc. The filamentary structure of WHIM can be probed by finding coincident absorption lines in the spectra of background AGN pairs. We estimate that at least 20 AGN pairs at separation ≲20 arcmin are needed to detect this filamentary structure at the 3σ level. Assuming observations of distant sources using XEUS for exposure times of 500 ks, we find that the minimum source flux to probe the filamentary structure is ∼2 × 10⁻¹² erg cm⁻² s⁻¹ in the 0.1–2.4 keV energy band. Thus, most pairs of these extragalactic X-ray bright sources have already been identified in the ROSAT All-Sky Survey. Re-observation of these objects by future missions could be a powerful way to search for baryons in the low-redshift Universe.