Observational and theoretical constraints for an Hα-halo around the Crab nebula ^*,**

¹ Department of Astronomy, AlbaNova University Center, Stockholm University, 106 91 Stockholm, Sweden e-mail: anestis@astro.su.se
² Isaac Newton Group of Telescopes, Calle Alvarez Abreu 68 2, 38700 Santa Cruz de La Palma, Spain
³ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 20 November 2008
Accepted: 5 February 2009

Abstract

Aims. 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.

Methods. Deep Hα images were obtained with WFI at the 2.2 m MPG/ESO telescope and with MOSCA at the 2.56 m 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.

Results. We reach a surface brightness limit of 5 10^-8 erg 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 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.

Conclusions. PSF scattering significantly contaminates our data, preventing a detection of the predicted fast shell. A real halo with observed peak flux of about 2 10^-7 erg s^-1 cm^-2 sr^-1 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.