2009A&A...497..167T

We searched for a fast moving Hα shell around the Crab nebula. Such a shell could account for this supernova remnant's missing mass, and carry enough kinetic energy to make SN 1054 a normal type II event. Deep Hα images were obtained with WFI at the 2.2m MPG/ESO telescope and with MOSCA at the 2.56m NOT. The data are compared with theoretical expectations derived from shell models with ballistic gas motion, constant temperature, constant degree of ionisation, and a power law for the density profile. We reach a surface brightness limit of 5x10^–8erg/s/cm2/sr. A halo is detected, but at a much higher surface brightness than our models of recombination emission and dust scattering predict. Only collisional excitation of Lyβ with partial de-excitation to Hα could explain such amplitudes. We show that the halo seen is caused by PSF scattering and thus not related to a real shell. We also investigated the feasibility of a spectroscopic detection of high-velocity Hα gas towards the centre of the Crab nebula. Modelling the emission spectra shows that such gas easily evades detection in the complex spectral environment of the Hα-line. PSF scattering significantly contaminates our data, preventing a detection of the predicted fast shell. A real halo with observed peak flux of about 2x10^–7erg/s/cm2/sr could still be accomodated within our error bars, but our models predict a factor 4 lower surface brightness. Eight meters class telescopes could detect such fluxes unambiguously, provided that a sufficiently accurate PSF model is available. Finally, we note that PSF scattering also affects other research areas where faint haloes are searched for around bright and extended targets.