ABSTRACT

We present the first ≥3.5 (conservative) or ≥5.8 σ (sum-of-lines significance) detection of two warm-hot intergalactic medium (WHIM) filaments at , which we find along the line of sight to the blazar Mrk 421. These systems are detected through highly ionized resonant metal absorption in high-quality Chandra ACIS and HRC Low Energy Transmission Grating (LETG) spectra of Mrk 421, obtained following our two Target of Opportunity requests during two outburst phases ( and 60 mcrab; ergs s⁻¹ cm⁻²). Columns of He-like oxygen and H-like nitrogen can be detected in the co-added LETG spectrum of Mrk 421 down to a sensitivity of cm⁻² and cm⁻², respectively, at a significance ≥3 σ. The two intervening WHIM systems that we detect have O vii and N vii columns of cm⁻², cm⁻² and cm⁻², cm⁻², respectively. We identify the closest of these two systems with an intervening WHIM filament at km s⁻¹. The second system, instead, at km s⁻¹, is identified with an intervening WHIM filament located 13 Mpc from the blazar. The filament at lies 5 Mpc from a known H i Lyα system at km s⁻¹ (Shull and coworkers) whose 3 σ maximal H i kinetic temperature, as derived from the observed line FWHM, is K. This temperature is inconsistent with the temperature measured for the X-ray filament, so if the systems are related, a multiphase WHIM is required. Combining UV and far-ultraviolet (FUV) upper limits on the H i Lyα and the O vi_2s→2p transitions with our measurements in the X-rays, we show that, for both filaments, equilibrium collisional ionization (with residual photoionization by both the diffuse UV and X-ray background and the beamed emission of Mrk 421 along our line of sight) provides acceptable solutions. These solutions define ranges of temperatures, metallicity ratios, and equivalent H column densities, which are in good agreement with the predictions of hydrodynamical simulations for the formation of large-scale structures in the universe. From the detected number of WHIM filaments along this line of sight we can estimate the number of O vii filaments per unit redshift with columns larger than cm⁻², , consistent, within the large 1 σ errors, with the hydrodynamical simulation predictions of . Finally, we measure a cosmological mass density of X-ray WHIM filaments , consistent with both model predictions and the estimated number of “missing” baryons at low redshift.