Speaker
Description
Galaxy cluster gas temperatures ($T$) are crucial for numerous cosmological and astrophysical applications. Potential $T$ biases can propagate to several such cluster applications. Thus, it is important to accurately cross-calibrate X-ray instruments to account for systematic biases. We present the first cross-calibration between eROSITA-Chandra, and between eROSITA-XMM-Newton, using a large sample of galaxy cluster $T$. To do so, we use the eRASS1 data to spectroscopically measure X-ray $T$ for 186 independent cluster regions with both eROSITA and Chandra for three energy bands; 0.7-7 keV (full), 0.5-4 keV (soft), and 1.5-7 keV (hard). We do the same with eROSITA and XMM-Newton for 71 different cluster regions and all three bands. We find that eROSITA measures systematically lower $T$ than the other two instruments, with hotter clusters deviating more than cooler ones. For the full band, eROSITA returns 20$\%$ and 14$\%$ lower $T$ than Chandra and XMM-Newton respectively, when the two latter instruments measure $k_{\text{B}}T\approx 3$ keV each. The discrepancy increases to 38\% and 32\% when Chandra and XMM-Newton measure $k_{\text{B}}T\approx 10$ keV respectively. Moreover, a broken power law fit demonstrates a break at the eROSITA-Chandra scaling relation at $k_{\text{B}}T\approx 1.7-2.7$ keV. The soft band shows a marginally lower discrepancy than the full band. TIn the hard band, the cross-calibration of eROSITA and the other instruments exhibits substantial differences. We tested several possible systematic biases to identify the reason behind the $T$ discrepancies but none could significantly alleviate the tension. Most importantly, we simulated several clusters with different 3D profiles for gas density and $T$ and fitted their projected spectra with all three instruments. We found no expected $T$ discrepancies due to the presence of multitemperature gas and the different effective areas of the telescopes. For now, it is most likely that the systematically lower eROSITA $T$ can be attributed to systematic effective area calibration uncertainties.
