  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. 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. 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. On the other hand, the hard band EPIC/ACIS fluxes
disagreed by 5-10% (i.e. at 6-25 sigma 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. 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 or ACIS are uncertain by ~10-15%
on average.
