Reflection Shocked Gas in the Cygnus Loop Supernova Remnant

We performed spectroscopic X-ray observations of the eastern and northern regions of the Cygnus Loop with the ASCA observatory. The X-ray surface brightness of these regions shows a complex structure in the ROSAT all-sky survey image. We carried out a spatially-resolved analysis for both regions and found that $kT_{\rm e}$ did not increase toward the center region, but showed inhomogeneous structures. Such variation cannot be explained by a blast wave model propagating into a homogeneous interstellar medium. We thus investigated the interaction between a blast wave and an interstellar cloud. Two major emission mechanisms are plausible: a cloud evaporation model and a reflection shock model. In both regions, only a reflection shock model qualitatively explains our results. Our results suggest the existence of a large-scale interstellar cloud. We suppose that such a large-scale structure would be produced by a precursor.