Probing the Low-velocity Regime of Nonradiative Shocks with Neutron Star Bow Shocks

Abstract

Nonradiative shocks accelerate particles and heat astrophysical plasmas. While supernova remnants are the most well-studied example, neutron star (NS) bow shocks are also nonradiative 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 Hα images. Most of these shocks have been characterized using narrowband Hα imaging and slit spectroscopy, which do not resolve the multicomponent 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 Hα flux. The broad-to-narrow line ratio (Ib/In) 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 ≲ 200 km s−1) in which Ib/In is high, distinct from the shock regime probed by supernova remnants. Our results illustrate a need for nonradiative shock models at velocities lower than previously considered, which will reveal the electron–ion temperature ratios and particle acceleration efficiencies of these bow shocks.