ChandraObservations of Two High‐Redshift Quasars

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Abstract
We report the first high-resolution X-ray spectra of two high-redshift quasars, S5 0836+710 and PKS 2149-306, obtained with the Chandra High-Energy Transmission Grating Spectrometer (HETGS). The primary goal of this observation is to use the high spectral resolving power of the HETGS to detect X-ray absorption produced by a hot intergalactic medium (IGM). The continuum of both quasars can be fitted by absorbed power laws. The power-law photon index (Γ) of S5 0836+710 is consistent with that found in previous observations with ROSAT and ASCA, while the power law of PKS 2149-306 is harder than found in a previous observation by ASCA. Excess continuum absorption above the Galactic value is found in S5 0836+710, as evidenced in ASCA and ROSAT observations. No significant emission or absorption feature is detected in either source at the ±3 σ level. Based on the detection limits we constrain the properties of possible emitters and absorbers. The upper limit of the equivalent width of Fe-K emission lines could be as low as ~10 eV. Absorbers with a column density higher than 8 × 1016 cm-2 for O VIII or 5 × 1016 cm-2 for Si XIV would have been detected. We propose a method to constrain the cosmological parameters (namely, Ω0 and Ωb) via the X-ray forest theory, but current data do not give significant constraints. However, we estimate that observing about seven bright quasars should give at least one O VIII and more O VII absorption lines at 95% possibility. We also find that combined with the constraints from the distortion of the cosmic microwave background spectrum, the X-ray Gunn-Peterson test can marginally constrain a uniform, enriched IGM.
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