The Cygnus Loop: Shock Precursors and Electron–Ion Equilibration

Abstract

The outermost edges of some supernova remnants are marked by filaments of pure Balmer line emission produced by nonradiative shock fronts. The Hα profiles of these filaments provide the shock speed and electron–proton temperature ratio in the shock. We have used the Hectochelle multiobject spectrograph on the MMT telescope to observe nine positions along the eastern nonradiative filaments of the Cygnus Loop, thereby extending the studies of Medina et al. to lower shock speeds. We complement the spectra with images from 1993 to 2020 to measure proper motions to further constrain the parameters. We also present observations of the photoionization precursor to show that the gas is strongly heated, and that the Hα emission from the precursor is dominated by collisional excitation rather than recombination.