Particle acceleration controlled by ambient density in the southwestern rim of RCW 86

Particle acceleration physics at supernova remnant (SNR) shocks is one of the most intriguing problems in astrophysics. SNR RCW~86 provides a suitable environment for understanding the particle acceleration physics because one can extract the information of both accelerated particles and acceleration environment at the same regions through the bright X-ray emission. In this work, we study X-ray proper motions and spectral properties of the southwestern region of RCW~86. The proper motion velocities are found to be $\sim 300$--2000~km~s$^{-1}$ at a distance of 2.8~kpc. We find two inward-moving filaments, which are more likely reflected shocks rather than reverse shocks. Based on the X-ray spectroscopy, we evaluate thermal parameters such as the ambient density and temperature, and non-thermal parameters such as the power-law flux and index. From the flux decrease in time of several non-thermal filaments, we estimate the magnetic field amplitudes to be $\sim 30$--100~$\mu$G. Gathering the physical parameters, we then investigate parameter correlations. We find that the synchrotron emission from thermal-dominated filaments is correlated with the ambient density $n_{\rm e}$ as $\text{(power-law flux)} \propto n_{\rm e}^{1.0 \pm 0.2}$ and $\text{(power-law index)} \propto n_{\rm e}^{0.38 \pm 0.10}$, not or only weakly with the shock velocity and shock obliquity. As an interpretation, we propose a shock-cloud interaction scenario, where locally enhanced magnetic turbulence levels have a great influence on local acceleration conditions.