Probing the low-velocity regime of non-radiative shocks with neutron star bow shocks

Abstract

Non-radiative shocks accelerate particles and heat astrophysical plasmas. While supernova remnants are the most well-studied example, neutron star (NS) bow shocks are also non-radiative and Balmer-dominated. NS bow shocks are likely ubiquitous in the interstellar medium due to their large speeds imparted at birth, and they are thought to be a discrete source population contributing to the Galactic cosmic ray spectrum. To date, nine NS bow shocks have been directly observed in H$\alpha$ images. Most of these shocks have been characterized using narrowband H$\alpha$ imaging and slit spectroscopy, which do not resolve the multi-component velocity structure of the shocks and their spatial geometry. Here we present integral field spectroscopy of three NS bow shocks: J0742$-$2822, J1741$-$2054, and J2225$+$6535 (the Guitar Nebula). We measure the shock properties simultaneously in four dimensions: the 2D projected shock morphology, the radial velocity structure, and the H$\alpha$ flux. The broad-to-narrow line ratio ($I_{\rm b}/I_{\rm n}$) is inferred from radial velocity profiles, and for J1741$-$2054 the narrow line is detected in multiple regions of the shock. The inferred line ratios and widths suggest that NS bow shocks represent a low shock velocity regime ($V \lesssim 200$ km/s) in which $I_{\rm b}/I_{\rm n}$ is high, distinct from the shock regime probed by supernova remnants. Our results illustrate a need for non-radiative shock models at velocities lower than previously considered, which will reveal the electron-ion temperature ratios and particle acceleration efficiencies of these bow shocks.