Constraints on Cosmic-ray Acceleration Efficiency in Balmer Shocks of Two Young Type Ia Supernova Remnants in the Large Magellanic Cloud

Abstract

We present results from an optical study of two young Balmer-dominated remnants of SNIa in the Large Magellanic Cloud, 0509−67.5 and 0519−69.0, in an attempt to search for signatures of efficient cosmic-ray (CR) acceleration. We combine proper motion measurements with corresponding optical spectroscopic measurements of the H α line at multiple rim positions from VLT/FORS2 and SALT/RSS and compare our results to published Balmer shock models. Analyses of the optical spectra result in broad H α widths in the range of 1800–4000 km s −1 for 12 separate Balmer-dominated filaments that show no evidence for forbidden line emission; the corresponding shock speeds from proper motion measurements from HST span a range of 1700–8500 km s −1 . Our measured values of shock speeds and broad H α widths in 0509−67.5 and 0519−69.0 are fit well with a Balmer shock model that does not include effects of efficient CR acceleration. We determine an upper limit of 7%/ χ (95% confidence) on the CR acceleration efficiency for our ensemble of data points, where χ is the ionization fraction of the pre-shock gas. The upper limits on the individual remnants are 6%/ χ (0509−67.5) and 11%/ χ (0519−69.0). These upper limits are below the integrated CR acceleration efficiency in the Tycho supernova remnant, where the shocks predominantly show little H α emission, indicating that Balmer-dominated shocks are not efficient CR accelerators.