Temperature measurements of galaxy clusters are used to determine their masses, which in turn are used to determine cosmological parameters. However, there are systematic differences between temperatures measured with different X-ray telescopes, limiting the utility of clusters as cosmological probes. This project uses NuSTAR’s unprecedented ability to focus high energy X-rays on a sample of 8 galaxy clusters to quantify a cross-calibration of cluster temperature between NuSTAR and Chandra. The analysis includes an extensive investigation into the systematic uncertainties on the NuSTAR temperature measurement due to post-launch problems affecting its observations. The spectra were fit to a single temperature, absorbed APEC model, between the energy band 3-10 keV. The Chandra spectra were also fit between 0.6-9 keV due to Chandra’s better energy sensitivity at lower energies, improving the signal-to-noise ratio. It was found that for a cluster measured to be 10 keV by Chandra, on average, it would be measured 11.9±4.5% lower by NuSTAR when the Chandra spectra were fit between 0.6-9 keV and 16.1±6.6% lower when fit between 3-10 keV. The results tell us that Chandra systematically measures the temperature of clusters to be higher than NuSTAR, even when fit in a lower energy band. As Chandra’s detector is more sensitive to lower energy photons and less sensitive to higher energy photons than NuSTAR, it suggests that the difference in temperature measured is not caused by the different energy sensitivity of the two telescopes and is most likely caused by the calibration. Furthermore, as the difference is still present when the spectra are fit between 3-10 keV, ignoring the soft energy band, it tells us that the problem isn’t localised to the soft energy band. This work is the first cross-calibration of cluster temperatures between NuSTAR and another X-ray telescope, giving further insight into the calibration of X-ray telescopes.
NuSTAR and Chandra Cross-Calibration using Galaxy Clusters
Wallbank, G. (Author). 25 Jan 2022
Student thesis: Master's Thesis › Master of Science by Research (MScR)