We present high-resolution long-slit spectroscopy of a Balmer-dominated shock in the northeastern limb of the Cygnus Loop with the Subaru high dispersion spectrograph .
By setting the slit angle along the shock normal , we investigate variations of the flux and profile of the H \alpha line from preshock to postshock regions with a spatial resolution of \sim 4 \times 10 ^ { 15 } cm .
The H \alpha line profile can be represented by a narrow ( 28.9 \pm 0.7 km s ^ { -1 } ) Gaussian in a diffuse region ahead of the shock , i.e. , a photoionization precursor , and narrow ( 33.1 \pm 0.2 km s ^ { -1 } ) plus broad ( 130–230 km s ^ { -1 } ) Gaussians at the shock itself .
We find that the width of the narrow component abruptly increases up to 33.1 \pm 0.2 km s ^ { -1 } , or 38.8 \pm 0.4 km s ^ { -1 } if we eliminate projected emission originating from the photoionization precursor , in an unresolved thin layer ( \lesssim 4 \times 10 ^ { 15 } cm at a distance of 540 pc ) at the shock .
We show that the sudden broadening can be best explained by heating via damping of Alfvén waves in a thin cosmic-ray precursor , although other possibilities are not fully ruled out .
The thickness of the cosmic-ray precursor in the Cygnus Loop ( a soft gamma-ray emitter ) is an order of magnitude thinner than that in Tycho ’ s Knot g ( a hard gamma-ray emitter ) , which may be caused by different energy distribution of accelerated particles between the two sources .
In this context , systematic studies might reveal a positive correlation between the thickness of the cosmic-ray precursor and the hardness of the cosmic-ray energy distribution .