Average energy of the X-ray spectrum as a model-independent proxy for the mass of galaxy clusters

TL;DR

This paper proposes using the average energy of X-ray spectra as a model-independent proxy for galaxy cluster mass, calibrated with simulations, offering a robust alternative for shallow surveys where spectral fitting is challenging.

Contribution

It introduces and calibrates a novel, model-independent method to estimate galaxy cluster mass from average X-ray energy, validated with simulations and applicable to wide-area surveys.

Findings

Average X-ray energy correlates with ICM temperature with ~10% scatter.

Cluster mass estimates from average X-ray energy have ~20% scatter.

Method is robust against unresolved ICM structure and projection effects.

Abstract

Temperature of the hot gas in galaxy clusters is known to be a reliable proxy for their total gravitating mass, allowing one to use spectroscopic X-ray observations for halo mass function measurements. Data of shallow wide area surveys, however, often precludes direct fitting of the X-ray spectra, given possible biases arising due to unresolved (multi-temperature) inner structure of the intracluster medium (ICM), projection effects and necessity of certain model assumptions to be made to allow for robust spectral fitting. We consider using a simple observable value - the average energy of the observed cluster X-ray spectrum - as a model-independent proxy for the ICM temperature, and consequently cluster's mass. We calibrate relation of this proxy to the cluster parameters using mock observations for a sample of 84 massive galaxy clusters extracted from the \textit{Magneticum} cosmological hydro simulations. We consider observational parameters corresponding to the all-sky survey observations by \textit{SRG/eROSITA}. Taking into account contributions of various background and foreground signals, average energy of the simulated X-ray spectra in the $0.4-7.0$ keV band is shown to be a stable indicator of the ICM temperature with $\sim10\%$ scatter and cluster's mass $M_{500}$ with a $\sim 20\%$ scatter. A database containing simulated X-ray images and their spectra (subtracted in several concentric rings) is publicly available.