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

TL;DR

This study searched for a fast-moving Hα shell around the Crab Nebula to explain its missing mass, but found that observed halos are due to PSF scattering, not a real shell, highlighting challenges in detecting faint structures.

Contribution

The paper provides observational constraints and models for an Hα halo around the Crab Nebula, demonstrating that previous detections are likely due to PSF scattering rather than a true shell.

Findings

Detected a halo at higher brightness than models predict, attributed to PSF scattering.

Spectroscopic detection of high-velocity Hα gas is complicated by PSF contamination.

A real faint halo could still exist within current observational limits, but is difficult to confirm.

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 $5\times10^{-8} ergs s^{-1} cm^{-2} sr^{-1}$. 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\times10^{-7} ergs s^{-1} cm^{-2} sr^{-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.