The SRG/eROSITA All-Sky Survey
Authors: Migkas K. , Kox D. , Schellenberger G. , Veronica A. , Pacaud F. , Reiprich T. H. , Bahar Y. E. , Balzer F. , Bulbul E. , Comparat J. , Dennerl K. , Freyberg M. , Garrel C. , Ghirardini V. , Grandis S. , Kluge M. , Liu A. , Ramos-Ceja M. E. , Sanders J. , Zhang X. 
Published: May 2024

Galaxy cluster gas temperatures (T) play a crucial role in many cosmological and astrophysical studies. However, it has been shown thatTmeasurements can significantly vary between different X-ray telescopes. TheseTbiases can propagate to several cluster applications in whichTcan be used, such as measuring hydrostatic cluster masses and constraining the angular variation of cosmological parameters. Thus, it is important to accurately cross-calibrate X-ray instruments to account for systematic biases. In this work, we present the cross-calibration between Spectrum Roentgen Gamma/eROSITA (SRG/eROSITA) andChandra/ACIS and between SRG/eROSITA andXMM-Newton/EPIC using for the first time a large sample of galaxy clusterT. To do so, we used the first eROSITA All-Sky Survey data and the preliminary extremely expanded HIgh FLUx Galaxy Cluster Sample, a large X-ray flux-limited cluster catalog. We spectroscopically measured X-rayTfor 186 independent cluster regions with both SRG/eROSITA andChandra/ACIS in a self-consistent way for three energy bands: 0.7–7 keV (full), 0.5–4 keV (soft), and 1.5–7 keV (hard). We did the same with SRG/eROSITA andXMM-Newton/EPIC for 71 different cluster regions and all three bands. We find that SRG/eROSITA measures systematically lowerTthan the other two instruments, with hotter clusters deviating more than cooler ones. For the full band, SRG/eROSITA returns 20% and 14% lowerTthanChandra/ACIS andXMM-Newton/EPIC, respectively, when the two other instruments each measurekBT ≈ 3 keV. The discrepancy respectively increases to 38% and 32% whenChandra/ACIS andXMM-Newton/EPIC each measurekBT ≈ 10 keV. On the other hand, the discrepancy becomes milder for low-Tgalaxy groups. Moreover, a broken power law fit demonstrated that there is a break at the SRG/eROSITA-Chandra/ACIS scaling relation atkBT ≈ 1.7 − 2.7 keV, depending on the energy band. The soft band shows a marginally lower discrepancy compared to the full band. In the hard band, the cross-calibration of SRG/eROSITA and the other instruments show very strong differences. We tested several possible systematic biases (such as multiphase cluster gas, Galactic absorption, non-Gaussian scatter, and selection effects) to identify the reason behind the cross-calibration discrepancies, but none could significantly alleviate the tension. For now, it is most likely that the systematically lower SRG/eROSITATcan be attributed to systematic effective area calibration uncertainties; however, the exact role of multiphase cluster gas in the observedTdiscrepancies needs to be further investigated. Furthermore, we provide conversion factors between SRG/eROSITA,Chandra/ACIS, andXMM-Newton/EPICTthat will be beneficial for future cluster studies that combine SRG/eROSITATwith data from other X-ray instruments. Finally, we also provide conversion functions between the official eRASS1 cluster catalogTand the equivalent core and core-excisedChandra/ACIS andXMM-Newton/EPICT.