The Refined Shock Velocity of the X-Ray Filaments in the RCW 86 Northeast Rim

TL;DR

This study refines the shock velocity measurement in the RCW 86 supernova remnant's northeastern rim using 11 years of Chandra data, revealing a lower velocity than previously thought and highlighting different velocity components traced by X-ray and H-alpha filaments.

Contribution

The paper provides a more accurate proper motion measurement of X-ray filaments in RCW 86, challenging previous high-velocity estimates and illustrating the complexity of shock dynamics and cosmic-ray acceleration.

Findings

Revised shock velocity of 3000 km/s from 6000 km/s previously reported.

Discrepancy between X-ray and H-alpha filament velocities indicating different shock components.

Evidence of denser ambient material slowing the shock near the thermal X-ray filament.

Abstract

A precise measurement of shock velocities is crucial for constraining the mechanism and efficiency of cosmic-ray (CR) acceleration at supernova remnant (SNR) shock fronts. The northeastern rim of the SNR RCW 86 is thought to be a particularly efficient CR acceleration site, owing to the recent result in which an extremely high shock velocity of ~6000 km/s was claimed (Helder et al. 2009). Here we revisit the same SNR rim with the Chandra X-ray Observatory, 11 years after the first observation. This longer baseline than previously available allows us to determine a more accurate proper motion of the nonthermal X-ray filament, revealing a much lower velocity of 3000 \pm 340 km/s (and even slower at a brighter region). Although the value has dropped to a half of that from the previous X-ray measurement, it is still higher than the mean velocity of the H-alpha filaments in this region (~1200 km/s). This discrepancy implies that the filaments bright in nonthermal X-rays and H-alpha emission trace different velocity components, and thus a CR pressure constrained by combining the X-ray kinematics and the H-alpha spectroscopy can easily be overestimated. We also measure the proper motion of the thermal X-ray filament immediately to the south of the nonthermal one. The inferred velocity (720 \pm 360 km/s) is significantly lower than that of the nonthermal filament, suggesting the presence of denser ambient material, possibly a wall formed by a wind from the progenitor, which has drastically slowed down the shock.