HIGH-RESOLUTION SPECTROSCOPY OF BALMER-DOMINATED SHOCKS IN THE LARGE MAGELLANIC CLOUD

We present high-resolution spectra of the four Balmer-dominated supernova remnants (SNRs) in the Large Magellanic Cloud (LMC). These data allow us to measure the width of the narrow component of Ha emission in the nonradiative filaments present in these SNRs. We find that the widths range from 30 to 50 km s-1 FWHM, similar to the only previously measured width of a nonradiative shock filament, that of approximately 30 km s-1 in the NE Cygnus Loop. The line widths do not appear to scale with shock velocity. These observations severely limit the possible broadening mechanisms. We discuss several possibilities, the most promising of which is a precursor associated with cosmic rays. Assuming a cosmic-ray precursor, we are able to place constraints on the cosmic-ray diffusion coefficient. Observations such as these may provide the best method currently available to probe cosmic-ray acceleration in SNR shock waves.