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

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, σ _8 , and Ω _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 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 A478 to investigate the nature of this calibration discrepancy. We find that NuSTAR temperatures are on average ∼11% lower than those of Chandra, and XMM-Newton temperatures are on average ∼5% lower than those of NuSTAR. This results in a NuSTAR mass at r _2500,Chandra of ${M}_{2500,\mathrm{NuSTAR}}\,=\,{3.39}_{-0.07}^{+0.07}\times {10}^{14}$ M _⊙ , which is ∼10% lower than that of M _2500,Chandra and ∼4% higher than M _2500,XMM−Newton .

Keywords

• Galaxy clusters
• X-ray astronomy