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

Context. For many years, X-ray astronomy missions have used the Crab nebula as a celestial calibration source for the X-ray flux and spectral shape. However, the object is often too bright for current and future missions equipped with instruments with improved sensitivity. Aims. We use G21.5-0.9, a pulsar-wind nebula with a time-constant power-law spectrum and a flux of a few milli-Crab in the X-ray band, as a viable, fainter substitute to the Crab. Using this source, we conduct a cross-calibration study of the instruments onboard currently active observatories: Chandra ACIS, Suzaku XIS, Swift XRT, and XMM-Newton EPIC (MOS and pn) for the soft-band, and INTEGRAL IBIS-ISGRI, RXTE PCA, and Suzaku HXD-PIN for the hard band. Methods. We extract spectra from all instruments and fit under the same astrophysical assumptions. We compare the spectral parameters of the G21.5-0.9 model: power-law photon index, H-equivalent column density of the interstellar photoelectric absorption, and flux in the soft (2-8 keV) or hard (15-50 keV) energy band. Results. We identify systematic differences in the best-fit parameter values unattributable to statistical scatter of the data alone. We interpret these differences as due to residual cross-calibration problems. The differences can be as large as 20% and 9% for the soft-band flux and power-law index, respectively, and 46% for the hard-band flux. The results are plotted and tabulated as a useful reference for future calibration and scientific studies using multiple missions.