  Context. The Crab nebula has been used as a celestial calibration source of
the 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 missions
equipped with instruments with improved sensitivity. Aims. We use G21.5-0.9 as
a viable, fainter substitute to the Crab, which is another pulsar-wind nebula
with a time-constant powerlaw spectrum with a flux of a few milli Crab in the
X-ray band. Using this source, we conduct a cross-calibration study of the
instruments onboard currently active observatories: Chandra ACIS, Suzaku XIS,
Swift XRT, 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 the instruments and fit them 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, flux in the soft (2-8 keV) or hard (15-50 keV) energy
band. Results. We identify the systematic differences in the best-fit parameter
values unattributable to the 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.
