The Hydrostatic Mass of A478: Discrepant Results From Chandra, NuSTAR, and XMM-Newton

TL;DR

This study compares hydrostatic mass estimates of galaxy cluster Abell 478 derived from Chandra, NuSTAR, and XMM-Newton X-ray data, revealing calibration-induced discrepancies in temperature and mass measurements.

Contribution

It provides a direct comparison of temperature and mass estimates from three X-ray observatories, highlighting calibration effects on hydrostatic mass calculations.

Findings

NuSTAR temperatures are ~11% lower than Chandra

XMM-Newton temperatures are ~5% lower than NuSTAR

NuSTAR mass estimate is ~10% lower than Chandra's

Abstract

Galaxy clusters are the most recently formed and most massive, gravitationally bound structures in the universe. The number of galaxy clusters formed is highly dependent on cosmological parameters, such as the dark matter density, $\sigma_8$, and $\Omega_m$. The number density is a function of the cluster mass, which can be estimated from the density and temperature profiles of the intracluster medium (ICM) under the assumption of hydrostatic equilibrium. The temperature of the plasma, hence its mass, is calculated from the X-ray spectra. However, effective area calibration uncertainties in the soft band result in significantly different temperature measurements from various space-based X-ray telescopes. NuSTAR is potentially less susceptible to these issues than Chandra and XMM-Newton, having larger effective area, particularly at higher energies, enabling high precision temperature measurements. In this work, we present analyses of Chandra, NuSTAR, and XMM-Newton data of Abell 478 to investigate the nature of this calibration discrepancy. We find that NuSTAR temperatures are on average $\sim$11% lower than that of Chandra, and XMM-Newton temperatures are on average $\sim$5% lower than that of NuSTAR. This results in a NuSTAR mass at $r_{2500,Chandra}$ of $M_{2500,NuSTAR}=3.39^{+0.07}_{-0.07}\times10^{14}$ $M_{\odot}$, which is $\sim$10% lower than that of $M_{2500,Chandra}$ and $\sim$4% higher than $M_{2500,XMM-Newton}$.