Cross Calibration of Galaxy Cluster Temperatures Measured with NuSTAR, XMM-Newton, and Chandra

TL;DR

This study compares galaxy cluster temperature measurements from NuSTAR, XMM-Newton, and Chandra, revealing systematic differences and demonstrating NuSTAR's potential to resolve longstanding calibration discrepancies in X-ray astronomy.

Contribution

The paper provides new NuSTAR measurements for galaxy clusters and analyzes calibration differences, offering insights into resolving temperature measurement discrepancies among X-ray observatories.

Findings

NuSTAR temperatures are ~15% higher than XMM-Newton.

Chandra and NuSTAR agree for clusters with kT < 7 keV.

Chandra's effective area calibration explains temperature trends.

Abstract

The use of galaxy clusters to constrain cosmology is limited in part due to uncertainties in derived cluster masses, which often depend on the gas temperature. Unfortunately, there exists a longstanding discrepancy in temperature measurements of the same galaxy clusters made by the two most sensitive X-ray observatories, Chandra and XMM-Newton. The NuSTAR X-ray Observatory's greater sensitivity to the exponential turnover in the bremsstrahlung continuum allows for more precise and potentially more accurate galaxy cluster temperature estimates, especially given its unique ability to independently calibrate its optics in orbit. We present new NuSTAR spectra of 10 relaxed clusters ($5~\mathrm{keV} < kT < 10~\mathrm{keV}$), extracted from identical regions as previous spectra from Chandra and XMM-Newton. The 3--20~keV spectra are well fit by single temperature models, and fits done in narrower bandpasses provide no clear evidence in support of the existence of multi temperature gas. We find NuSTAR temperatures are typically $\sim 15\%$ higher than XMM-Newton temperatures. In contrast, good agreement is found between NuSTAR and Chandra temperatures for clusters with $kT \lesssim 7~\mathrm{keV}$, with Chandra measurements exceeding NuSTAR's in hotter systems. When more clusters are included, the trend is reinforced and can be extended to higher temperatures. A generic increase to Chandra's $E > 2~\mathrm{keV}$ effective area ($\sim 5\%$ at 5~keV) is found to explain the trend reasonably well. These results demonstrate the potential for NuSTAR data to address the two-decade-old temperature discrepancy between Chandra and XMM-Newton.