2012ApJ...746..166Y

The warm-hot intergalactic medium (WHIM) at temperatures 10⁵-10⁷ K is believed to contain 30%-50% of the baryons in the local universe. However, all current X-ray detections of the WHIM at redshifts z > 0 are of low statistical significance ( ≲ 3σ) and/or controversial. In this work, we aim to establish the detection limits of current X-ray observatories and explore requirements for next-generation X-ray telescopes for studying the WHIM through X-ray absorption lines. We analyze all available grating observations of Mrk 421 and obtain spectra with signal-to-noise ratios (S/Ns) of ∼90 and 190 per 50 mÅ spectral bin from Chandra and XMM-Newton observations, respectively. Although these spectra are two of the best ever collected with Chandra and XMM-Newton, we cannot confirm the two WHIM systems reported by Nicastro et al. in 2005. Our bootstrap simulations indicate that spectra with such high S/N cannot constrain the WHIM with O VII column densities N_OVII≈10¹⁵/cm2 (corresponding to an equivalent width of 2.5 mÅ for a Doppler velocity of 50 km/s) at ≳ 3σ significance level. The simulation results also suggest that it would take >60 Ms for Chandra and 140 Ms for XMM-Newton to measure the N_OVII at ≥ 4σ from a spectrum of a background QSO with flux of ∼0.2 mCrab (1 Crab = 2x10^–8 erg/s/cm2 at 0.5-2 keV). Future X-ray spectrographs need to be equipped with spectral resolution R ∼ 4000 and effective area A ≥ 100 cm² to accomplish the similar constraints with an exposure time of ∼2 Ms and would require ∼11 Ms to survey the 15 QSOs with flux ≳ 0.2 mCrab along which clear intergalactic O VI absorbers have been detected.