Search for the warm–hot intergalactic medium around A 2744 using Suzaku

Abstract

Abstract We present the results from Suzaku satellite observations of the surrounding region of a galaxy cluster, A 2744, at z = 0.3. To search for oxygen emission lines from the warm–hot intergalactic medium (WHIM), we analyzed X-ray spectra from two northeastern regions 2.2–3.3 and 3.3–4.4 Mpc from the center of the cluster, which offers the first test on the presence of a WHIM near the typical accretion shock radius (∼2 r200) predicted by hydrodynamical simulations. For the 2.2–3.3 Mpc region, the spectral fit significantly (99.2% significance) improved when we included O vii and O viii lines in the spectral model. A comparable WHIM surface brightness was obtained in the 3.3–4.4 Mpc region and the redshift of the O viii line is consistent with z = 0.3 within errors. The present results support that the observed soft X-ray emission originated from the WHIM. However, considering both statistical and systematic uncertainties, O viii detection in the northeast regions was marginal. The surface brightnesses of the O viii line in 10−7 photons cm−2 s−1 arcmin−2 for the 2.2–3.3 and 3.3–4.4 Mpc regions were measured to be 2.7 ± 1.0 and 2.1 ± 1.2, giving upper limits on the baryon overdensity of δ = 319(< 442) and 284(< 446), respectively. This is comparable with previous observations of cluster outskirts and their theoretical predictions. The future prospect for WHIM detection using the Athena X-IFU micro-calorimeter is briefly discussed here. In addition, we also derived the intracluster medium temperature distribution of A 2744 to detect a clear discontinuity at the location of the radio relic. This suggests that the cluster has undergone strong shock heating by mass accretion along the filament.