INTEGRAL probes the morphology of the Crab nebula in hard X - rays / soft γ - rays

Aims. We use the IBIS/ISGRI telescope on board INTEGRAL to measure the position of the centroid of the 20-200 keV emission from the Crab region. Methods. We find that the astrometry of the IBIS telescope is affected by the temperature of the IBIS mask during the observation. After correcting for this effect, we show that the systematic errors on the astrometry of the telescope are of the order of 0.5 arcsec. In the case of the Crab nebula and several other bright sources, the very large number of photons renders the level of statistical uncertainty on the centroid smaller or comparable to this value. Results. We find that the centroid of the Crab nebula in hard X-rays (20-40 keV) is shifted by 8.0 arcsec compared to the Crab pulsar in direction of the X-ray centroid of the nebula. A similar shift is also found at higher energies (40-100 and 100-200 keV). We observe a trend of decreasing shift with energy, which can be explained by an increase in the pulsed fraction. To disentangle the contribution of the pulsar and the nebula, we split our data into an on-pulse and off-pulse sample. Surprisingly, the nebular emission (i.e. off-pulse) is found significantly away from the X-ray centroid of the nebula. Conclusions. In all 3 energy bands (20-40, 40-100 and 100-200 keV), we find that the centroid of the nebula is significantly shifted compared to the predicted position. We interpret this shift in terms of a cutoff in the electron spectrum in the outer regions of the nebula, which is probably at the origin of the observed spectral break around 100 keV. From a simple spherically-symmetric model for the nebula, we estimate that the electrons in the external regions of the torus (d ∼ 0.35 pc from the pulsar) reach a maximal energy slightly below 10 14 eV.