The Shock and Extended Remnant around the Crab Nebula

Abstract

The [O III] around the Crab Nebula was shown by Hester et al. to be the Rayleigh-Taylor (R-T)-unstable interface between the synchrotron nebula and swept-up ejecta from the supernova explosion. Unlike the bright photoionized filaments, this skin is most easily understood as the cooling region behind a shock driven into an extended remnant of freely expanding ejecta by the pressure of the synchrotron nebula. Shock models, which are tightly constrained by the physical parameters of the Crab, correctly predict the [O III] brightness of the skin, as well as the strength of C IV λ1549 emission from the skin seen in a Hopkins Ultraviolet Telescope (HUT) spectrum of the Crab published by Blair et al. and the remarkably strong [Ne V] λ3426 seen at two locations by Davidson. While it is possible to match the [O III] brightness with ad hoc photoionization models, such models generally underpredict C IV and [Ne V] by an order of magnitude or more. The existence of the shock in turn requires the existence of an extended remnant surrounding the Crab. The extended remnant could carry the majority of the mass and kinetic energy of the ejecta, yet remain undetected in the recent search by Fesen et al. The shock around the Crab is radiative near the equator of the elongated remnant, which has a symmetry axis running southeast to northwest and tipped into the sky by about 25°. We adopt a fiducial model for the shock at the location of the HUT spectrum having vs = 150 km s-1 and ρ0/mH = 12.7 cm-3. Material behind this shock cools in about 100 years, collects on the surface of the synchrotron nebula, and is concentrated into dense filaments by R-T instabilities. Near the poles of the remnant, however, the cooling time is probably longer than the age of the remnant. In this case, the density at the interface between the synchrotron nebula and shocked ejecta is low, and so the interface is stable. Lack of ongoing filament formation near the poles of the Crab could explain both the open-ended cylindrical filament distribution seen in the Fabry-Perot observations of Lawrence et al. and the apparent breakout of the radio synchrotron nebula to the northwest of the Crab. The walls of the Northern when seen in a deep [O III] image appear to be a continuation of the skin. We suggest that the jet walls result from a shock driven transverse to the jet axis by the pressure of the synchrotron nebula that fills the jet.