Do we really observe a bow shock in N157B...?

Abstract

I present a model of a pulsar wind interacting with its associated supernova remnant. I will use the model to argue that one can explain the morphology of the pulsar wind nebula inside N157B, a supernova remnant in the Large Magellanic Cloud, without the need for a bow shock interpretation. The model uses a hydrodynamics code which simulates the evolution of a pulsar wind nebula, when the pulsar is moving at a high velocity (1000 km/sec) through the expanding supernova remnant. The simulation shows three different stages of the pulsar wind nebula: the supersonic expansion stage, the subsonic expansion stage and the bow shock stage. The last stage occurs when the head of the pulsar wind nebula, containing the active pulsar, deforms into a bow shock, due to the motion of the pulsar becoming supersonic. I will argue that the pulsar wind nebula inside N157B is currently in the second stage of its evolution, i.e. the expansion of the pulsar wind nebula is subsonic and there is no bow shock around the pulsar wind bubble. The strongly off-centered position of the pulsar with respect to its pulsar wind nebula is naturally explained by the result of the interaction of the reverse shock with the pulsar wind nebula, as the simulation bears out.