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/s) through the expanding supernova remnant. The evolution of the pulsar wind nebula can roughly be divided into three stages. In the first stage the pulsar wind nebula is expanding supersonically through the freely expanding ejecta of the progenitor star (∼1000 years). In the next stage the expansion of the pulsar wind nebula is not steady, due to the interaction with the reverse shock of the supernova remnant; the pulsar wind nebula oscillates violently between contraction and expansion, but will ultimately relax towards a steady subsonic expansion (∼1000–10,000 years). The last stage occurs when the head of the pulsar wind nebula, containing the active pulsar, deforms into a bow shock (>10,000 years), due to the motion of the pulsar becoming supersonic. Ultimately it is this bow shock structure bounding the pulsar, which directly interacts with the shell structure of the supernova remnant, just before the pulsar breaks out of the supernova remnant. 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.