The Mass of Abell 478: Differing Results From Chandra and NuSTAR

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 and dark energy densities. The number per volume 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 X-ray spectra are ideal for measuring temperature; however, different X-ray telescopes report different temperatures for the same clusters, resulting in the masses differing by ~10%. This is suspected to be a calibration issue. NuSTAR is potentially less susceptible to calibration issues than Chandra, having greater sensitivity at higher energies, where the effective area is relatively constant and the exponential turnover of the continuum is more prominent. In this poster, we present individual and joint analyses of Chandra and NuSTAR data of Abell 478 (A478) to investigate the nature of this calibration discrepancy. The surface brightness profile of A478 was extracted from a mosaic Chandra image. The temperature profile was determined in identical regions from both Chandra and NuSTAR by extracting the spectra individually from concentric annular regions around the center of the cluster and fitting each spectrum isothermally. This was done for both Chandra and NuSTAR separately as well as jointly. The density and temperature profile models were then fit to the data iteratively, and then used to calculate the mass profile. We find the NuSTAR temperatures at all radii to be systematically lower, on average by 14%, than the Chandra temperatures, in better agreement with the previously reported temperature profile measured by XMM-Newton. The resulting difference in calculated mass is ~13%. We discuss the impact of systematic effects that could affect this result, such as NuSTAR’s larger PSF and an imperfect determination of Chandra’s background, as well as the possible implications for cluster cosmology.