Concordance: In-flight Calibration of X-ray Telescopes without Absolute References

We describe a process for cross-calibrating the effective areas of X-ray telescopes that observe common targets. The targets are not assumed to be "standard candles" in the classic sense, in that the only constraint placed on the source flux is that it is the same for all instruments. We apply a technique developed by Chen et al. (J. Amer. Stat. Association, in press) that involves a popular statistical method called shrinkage estimation, which effectively reduces the noise in disparate measurements by combining information across common observations. We can then determine effective area correction factors for each instrument that brings all observatories into the best agreement, consistent with prior knowledge of their effective areas. We have preliminary values that characterize systematic uncertainties in effective areas for almost all operational (and some past) X-ray astronomy instruments in bands covering factors of two in photon energy from 0.15 keV to 300 keV. We demonstrate the method with several data sets from Chandra and XMM-Newton. Support for this work was provided in part by the National Aeronautics and Space Administration (NASA) through the Smithsonian Astrophysical Observatory (SAO) contract SV3-73016 to MIT for support of the Chandra X-Ray Center (CXC), which is operated by SAO for and on behalf of NASA under contract NAS8-03060.