ChandraObservations of Two High‐Redshift Quasars

We report the first high-resolution X-ray spectra of two high-redshiftquasars, S5 0836+710 and PKS 2149-306, obtained with the Chandra High EnergyTransmission Grating Spectrometer (HETGS). The primary goal of this observationis to use the high spectral resolving power of the HETGS to detect X-rayabsorption produced by a hot intergalactic medium. The continuum of bothquasars can be fitted by absorbed power laws. The power law photon index(Gamma) of S5 0836+710 is consistent with that found in previous observationswith ROSAT and ASCA, while the power law of PKS 2149-306 is harder than foundin a previous observation by ASCA. Excess continuum absorption above theGalactic value is found in S5 0836+710, as evidenced in ASCA and ROSATobservations. No significant emission or absorption feature is detected ineither source at 3 sigma level. Based on the detection limits we constrain theproperties of possible emitters and absorbers. The upper limit of theequivalent width of Fe K emission lines could be as low as ~ 10 eV. Absorberswith a column density higher than 8e16 cm^{-2} for O VIII or 5e16 cm^{-2} forSi XIV would have been detected. We propose a method to constrain thecosmological parameters (namely Omega_0 and Omega_b) via the X-ray foresttheory, but current data do not give significant constraints. However, weestimate that observing ~ 7 bright quasars should give at least one O VIII andmore O VII absorption lines at 95% possibility. We also find that combined withthe constraints from the distortion of the CMB spectrum, the X-rayGunn-Peterson test can marginally constrain a uniform, enriched IGM.Comment: 25 pages, 6 figures, 3 tables. Accepted for publication in Ap