New Dynamics of Pulsar Wind Nebulae: Effects of Mass Loading

We present an extended model of pulsar wind nebulae (PWNe) that incorporates mass injection from the surrounding supernova remnant (SNR) into the steady, spherically symmetric framework of Kennel \& Coroniti (1984). In PWNe, a relativistic magnetized pulsar wind terminates at a shock where particles are accelerated and emit broadband radiation. Theoretically, the flow is expected to be magnetically dominated ($\sigma \gg 1$) near the pulsar, whereas observations suggest that it becomes particle-dominated ($\sigma \ll 1$) in the PWN ($r \sim 2$ pc). However, the mechanism driving this transition remains unclear. The Rayleigh-Taylor instability is expected to mix ambient SNR material into the PWN, yet the dynamical effect of such mass loading has rarely been considered. We incorporate cold, stationary gas injection into the KC model and numerically solve the modified MHD equations. Our results show that, when $\sigma$ is of order unity, the flow can re-accelerate due to mass loading, in contrast to the monotonic deceleration predicted by the original KC model. Remarkably, this behavior occurs even for moderate $\sigma$ and low injection rates. Furthermore, the steady-state solution disappears before reaching the nebula edge under these conditions. These findings indicate that even modest mass loading can significantly alter or destabilize the flow structure.