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The Crab nebula and pulsar have been widely used as a calibration source forX-ray instruments. The in-flight effective area calibration of the ReflectionGrating Spectrometers (RGS) of XMM-Newton depend upon the availability ofreliable calibration sources. We investigate how the absolute effective areacalibration of RGS can be obtained using Crab as a standard candle. We haveanalysed RGS observations of the Crab using different instrument configurationsand spatial offsets, and made use of previous determinations of the continuumspectrum of the nebula plus pulsar. Due to the high spectral resolution of theRGS, we resolve the main absorption edges and detect the strong 1s-2pabsorption lines of neutral oxygen. We get an excellent fit to the Crabspectrum using this fixed continuum and the absorption spectrum determined byRGS. We get accurate column densities for the neutral atoms of H, N, O, Ne, Mg,and Fe, as well as a clear detection of Fe II and firm upper limits for otherions. Our data are in good agreement with earlier optical and UV spectroscopicmeasurements of some of these ions. We find solar abundances for N and O, whileNe is overabundant by a factor of 1.7 and Fe is underabundant by a factor of0.8. We confirm that there is less dust in the line of sight compared to theprediction based on the absorption column. Our spectra suggest a more prominentrole of ferric iron in the dust compared to ferrous iron. Our high-resolutionobservations confirm that Crab can be used as an X-ray calibration source. RGSspectra have determined the absorption spectrum towards Crab with unprecedenteddetail.

Effective area calibration of the Reflection Grating Spectrometers of XMM-Newton. I. X-ray spectroscopy of the Crab nebula