Medina, Amber A., Raymond, John C., Edgar, Richard J., Caldwell, Nelson, Fesen, Robert A., and Milisavljevic, Dan. 2014. "Electron-Ion Equilibrium and Shock Precursors in the Northeast Limb of the Cygnus Loop." The Astrophysical Journal 791:30. https://doi.org/10.1088/0004-637X/791/1/30
We present an observational study using high-resolution echelle spectroscopy of collisionless shocks in the Cygnus Loop supernova remnant. Measured Hα line profiles constrain pre-shock heating processes, shock speeds, and electron-ion equilibration (Te /Ti ). The shocks produce faint Hα emission line profiles, which are characterized by narrow and broad components. The narrow component is representative of the pre-shock conditions, while the broad component is produced after charge transfer between neutrals entering the shock and protons in the post-shock gas, thus reflecting the properties of the post-shock gas. We observe a diffuse Hα region extending about 2.'5 ahead of the shock with line width ~29 km s-1, while the Hα profile of the shock itself consists of broader than expected narrow (36 km s-1) and broad (250 km s-1) components. The observed diffuse emission arises in a photoionization precursor heated to about 18,000 K by He I and He II emission from the shock, with additional narrow component broadening originating from a thin cosmic-ray precursor. Broad to narrow component intensity ratios of ~1.0 imply full electron-ion temperature equilibration Te ~= Ti in the post-shock region. Broad component line widths indicate shock velocities of about 400 km s-1. Combining the shock velocities with proper motions suggests that the distance to the Cygnus Loop is ~890 pc, significantly greater than the generally accepted upper limit of 637 pc.