Cross-calibrating X-ray detectors with clusters of galaxies: an IACHEC study

Aims. We used a sample of 11 nearby relaxed clusters of galaxies observed with the X-ray instruments XMM-Newton (EPIC) pn and MOS, Chandra ACIS-S and ACIS-I and BeppoSAX MECS to examine the cross-calibration of the energy dependence and normalisation of the effective area of these instruments as of December 2009. We also examined the Fe XXV/XXVI line ratio temperature measurement method for the pn and MOS. Methods. We performed X-ray spectral analysis on the XMM-Newton and Chandra data for a sample of 11 clusters. We obtained the information for BeppoSAX from DeGrandi & Molendi (2002). We compared the spectroscopic results obtained with different instruments for the same clusters in order to examine possible systematic calibration effects between the instruments. Results. We did not detect any significant systematic differences between the temperatures derived in the 2−7 keV band using the different instruments. Also, the EPIC temperatures derived from the bremsstrahlung continuum agreed with those obtained from the Fe XXV/XXVI emission line ratio, implying that the energy dependence of the hard band effective area of the above instruments is accurately calibrated. This also indicates that deviations from ionisation equilibrium and a Maxwellian electron velocity distribution are negligible in the regions studied in the cluster sample. We thus consider the IACHEC sample of clusters of galaxies as standard candles for the calibration of the energy dependence of the hard band (2− 7k eV) effective area of X-ray telescopes. On the other hand, the hard band EPIC/ACIS fluxes disagreed by 5−10% (i.e. at 6−25σ level) which indicates a similar level of uncertainty in the normalisations of the effective areas of these instruments in the 2−7 keV band. In the soft energy band (0.5−2.0 keV) there are greater cross-calibration differences between EPIC and ACIS. We found an energy-dependent increase of ACIS versus pn bias in the cross-calibration of the eff ective area by∼10% in the 0.5−2.0 keV band. This amounts to a systematic difference of (∼20%) in the temperatures measured by the ACIS and the EPIC-pn cameras in this band. Due to the high statistical weight of the soft band data, the 0.5−7.0 keV band temperature measurements of clusters of galaxies with EPIC/XMM-Newton or ACIS/Chandra are uncertain by ∼10−15% on average. These uncertainties will also affect the analysis of the wide band continuum spectra of other types of objects using ACIS or EPIC.