2007ApJ...659L.133L

We present an Hα spectral observation of a Balmer-dominated shock on the eastern side of Tycho's supernova remnant using the Subaru Telescope. Utilizing the High Dispersion Spectrograph (HDS), we measure the spatial variation of the line profile between preshock and postshock gas. Our observation clearly shows a broadening and centroid shift of the narrow-component postshock Hα line relative to the Hα emission from the preshock gas. The observation supports the existence of a thin precursor where gas is heated and accelerated ahead of the shock. Furthermore, the spatial profile of the emission ahead of the Balmer filament shows a gradual gradient in the Hα intensity and line width ahead of the shock. We propose that this region (∼10¹⁶ cm) is likely to be the spatially resolved precursor. The line width increases from ∼30 up to ∼45 km/s, and its central velocity shows a redshift of ∼5 km/s across the shock front. The characteristics of the precursor are consistent with a cosmic-ray precursor, although the possibility of a fast neutral precursor is not ruled out.