By Shiho Hattori

Abstract: "We present the results from the Suzaku satellite of the surrounding region of a galaxy cluster, A2744, at z = 0.3. To search for oxygen emission lines from the warm-hot intergalactic medium (WHIM), we analyzed X-ray spectra from two northeast 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 (∼ 2r₂₀₀) predicted by hydrodynamical simulations. For the 2.2–3.3 Mpc region, the spectral fit significantly (99.2% significance) improved when we include 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 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 brightness of the O VIII line in 10⁻⁷photons cm⁻² s⁻¹ arcmin⁻² was measured to be 2.7 ± 1.0, 2.1 ± 1.2 for the 2.2–3.3, 3.3– 4.4 Mpc regions, giving the upper limit on the baryon overdensity of δ = 319(< 443), 283(< 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 microcalorimeter is briefly discussed here. In addition, we also derived the ICM temperature distribution of A2744 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."

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