On the lack of strong O-line excess in the Coma cluster outskirts from Suzaku

TL;DR

This study used Suzaku observations to investigate soft X-ray excess in the Coma cluster outskirts, finding no evidence of the previously reported OVII and OVIII emissions, suggesting they originate from solar wind charge exchange rather than the WHIM.

Contribution

The paper provides the first Suzaku-based upper limits on OVII and OVIII emissions in the Coma outskirts, challenging previous detections and clarifying the origin of soft X-ray excess.

Findings

No OVII or OVIII excess detected in Coma-11 field

Measured upper limits are consistent with other fields and previous XMM-Newton data

Strong oxygen lines previously reported are likely due to solar wind charge exchange

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 million K. Thermal soft excess emission in the spectrum of some cluster outskirts that contains OVII and/or OVIII 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 ever been reported, with Suzaku XIS in order to make clear the origin of the excess. We did not confirm OVII or OVIII excess emission. The OVII and OVIII intensity in Coma-11 is more than 5 sigma below that reported before and we obtained 2 sigma upper limits of 2.8 and 2.9 photons cm^-2 s^-1 sr^-1 for OVII and OVIII, 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 fields 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.