The Lack of Strong O‐Line Excess in the Coma Cluster Outskirts fromSuzaku

Abstract

About half of the baryons in the local universe are thought to reside in the so-called warm-hot intergalactic medium (WHIM) at temperatures of 0.1-10 MK. Thermal soft excess emission in the spectrum of some cluster outskirts that contains O VII and/or O VIII emission lines is regarded as evidence of the WHIM, although the origin of the lines is controversial due to strong Galactic and solar system foreground emission. We observed the Coma-11 field, where the most prominent thermal soft excess has been reported, with Suzaku XIS in order to clarify the origin of the excess. We did not confirm O VII or O VIII excess emission. The O VII and O VIII intensity in Coma-11 is more than 5 σ below that reported before, and we obtained 2 σ upper limits of 2.8 and 2.9 photons cm−2 s−1 sr−1 for O VII and O VIII, respectively. The intensities are consistent with those in another field (Coma-7) that we measured, and with other measurements in the Coma outskirts (Coma-7 and X Com field with XMM-Newton). We did not confirm the spatial variation within Coma outskirts. The strong oxygen emission lines previously reported are likely due to solar wind charge exchange.