Observational and theoretical constraints for an H$\alpha$-halo around the Crab Nebula

Abstract

We searched for a fast moving H$\alpha$ 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 H$\alpha$ 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 $5\times 10^{-8}ergs{s}^{-1}c{m}^{-2}s{r}^{-1}$. 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 Ly$\beta$ with partial de-excitation to H$\alpha$ could explain such amplitudes. We show that the halo seen is due to PSF scattering and thus not related to a real shell. We also investigated the feasibility of a spectroscopic detection of high-velocity H$\alpha$ gas towards the centre of the Crab nebula. Modelling of the emission spectra shows that such gas easily evades detection in the complex spectral environment of the H$\alpha$-line. PSF scattering significantly contaminates our data, preventing a detection of the predicted fast shell. A real halo with observed peak flux of about $2\times 10^{-7}ergs{s}^{-1}c{m}^{-2}s{r}^{-1}$ could still be accomodated within our error bars, but our models predict a factor 4 lower surface brightness. 8m 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.

arXiv comment10 pages, 13 figures, accepted for publication in A&A

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• Optical and infrared observations of the Crab Pulsar and its nearby knot
• Spectral evolution and polarization of variable structures in the pulsar wind nebula of PSR B0540-69.3
• Search for Thermal X-ray Features from the Crab nebula with Hitomi Soft X-ray Spectrometer
• A 3D Kinematic Study of the Northern Ejecta "Jet" of the Crab Nebula
• The optical and near-infrared spectrum of the Crab pulsar with X-shooter
• Kinematics, structure and abundances of supernova remnant 0540-69.3
• Detecting the rapidly expanding outer shell of the Crab Nebula: where to look
• The location of the Crab pulsar emission region: Restrictions on synchrotron emission models