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Context. The Crab nebula has been used as a celestial calibration source ofthe X-ray flux and spectral shape for many years by X-ray astronomy missions.However, the object is often too bright for current and future missionsequipped with instruments with improved sensitivity. Aims. We use G21.5-0.9 asa viable, fainter substitute to the Crab, which is another pulsar-wind nebulawith a time-constant powerlaw spectrum with a flux of a few milli Crab in theX-ray band. Using this source, we conduct a cross-calibration study of theinstruments onboard currently active observatories: Chandra ACIS, Suzaku XIS,Swift XRT, XMM-Newton EPIC (MOS and pn) for the soft-band, and INTEGRALIBIS-ISGRI, RXTE PCA, and Suzaku HXD-PIN for the hard band. Methods. We extractspectra from all the instruments and fit them under the same astrophysicalassumptions. We compare the spectral parameters of the G21.5-0.9 model:power-law photon index, H-equivalent column density of the interstellarphotoelectric absorption, flux in the soft (2-8 keV) or hard (15-50 keV) energyband. Results. We identify the systematic differences in the best-fit parametervalues unattributable to the statistical scatter of the data alone. Weinterpret these differences as due to residual cross-calibration problems. Thedifferences can be as large as 20% and 9% for the soft-band flux and power-lawindex, respectively, and 46% for the hard-band flux. The results are plottedand tabulated as a useful reference for future calibration and scientificstudies using multiple missions.

Cross-calibration of the X-ray Instruments onboard the Chandra, INTEGRAL, RXTE, Suzaku, Swift, and XMM-Newton Observatories using G21.5-0.9