The inverse iron-bias in action in Abell 2028

TL;DR

This paper investigates the inverse iron-bias in galaxy cluster Abell 2028, revealing how spectral fitting biases can lead to overestimated metallicity measurements due to multi-temperature structures in the ICM.

Contribution

It demonstrates the presence of inverse iron-bias in X-ray spectral analysis of galaxy clusters and explains its origin related to spectral fitting of multi-temperature plasmas.

Findings

Naive one-component spectral fits overestimate metallicity.

Inverse iron-bias arises from spectral fitting behavior, not line presence.

Complex cluster structure influences metallicity measurements.

Abstract

Recent work based on a global measurement of the ICM properties find evidence for an increase of the iron abundance in galaxy clusters with temperature around 2-4 keV. We have undertaken a study of the metal distribution in nearby clusters in this temperature range, aiming at resolving spatially the metal content of the ICM. The XMM observation of the first object of the sample, the cluster Abell 2028, reveals a complex structure of the cluster over scale of ~ 300 kpc, showing an interaction between two sub-clusters in a ``cometary'' configuration. We show that a naive one-component fit for the core of Abell 2028 returns a biased high metallicity. This is due to the inverse iron-bias, which is not related to the presence in the spectrum of both Fe-L and Fe-K emission lines but to the behavior of the fitting code in shaping the Fe-L complex of a one temperature component to adjust to the multi-temperature structure of the projected spectrum.