Soft X-ray excess of clusters: a thermal filament model, and the strong lensing of background galaxy groups

TL;DR

This paper explores the thermal filament model to explain the soft X-ray excess in galaxy clusters, constrains WHIM parameters through observations, and proposes strong lensing of background WHIM as a potential source of the excess.

Contribution

It introduces an analytic filament model for soft X-ray excess, constrains WHIM parameters observationally, and suggests strong lensing of background WHIM as a novel explanation.

Findings

Derived an analytic radial profile of soft excess surface brightness.

Constrained the total mass of warm gas in clusters.

Proposed strong lensing of background WHIM as a source of soft excess.

Abstract

The observational and theoretical status of the search for missing cosmological baryons is summarized, with a discussion of some indirect methods of detection. The thermal interpretation of the cluster soft X-ray and EUV excess phenomenon is examined in the context of emission filaments, which are the higher density part of the warm hot intergalactic medium (WHIM) residing at the outskirt of clusters. We derived an analytic radial profile of the soft excess surface brightness using a simple filament model, which provided us a means of observationally constraining the WHIM parameters, especially the total mass budget of warm gas associated with a cluster. We then pointed out a new scenario for soft excess emission, viz. a cluster that can strongly lens the soft X-rays from background WHIM knots. If, as seems quite likely, the missing baryons are mostly in the WHIM halos of galaxy groups, the lensing probability will be quite high ($\sim$ 10 %). This way of accounting for at least part of a cluster's soft excess may also explain the absence of O VII absorption at the redshift of the cluster.