sr$^{-1}$, and the dereddened total flux of the SE filament is $1.9\pm 0.2 \times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$.

The interstellar ultraviolet (UV) photons in principle could excite and heat the gas to produce similar excitation temperature if the gas is dense enough for collisions to dominate deexcitation [@ste89; @bur90]. However, the expected 212 surface brightness by UV photon excitation is low unless the density is high and the radiation field is very strong, e.g., $\nh\ge 10^5$ cm$^{-3}$ and $G_0\ge 10^4$ for $\ge 1\times 10^{-4}$ erg cm$^{-2}$ s$^{-1}$ sr$^{-1}$ where $\nh$ is the number density of H nuclei and $G_0$ is far UV (FUV) intensity relative to the interstellar radiation field in the solar neighborhood [@bur90]. Note that $G_0=10^4$ corresponds to an O4-type star at a distance of $\sim 1$ pc [@tie05]. No such strong FUV source exists around the filament.

X-ray emission from the remnant is another source that could possibly excite and heat the filament. We may consider a molecular clump situated at some distance from an SN explosion. As the SN explodes and the SNR evolves, the X-ray flux increases and, in principle, an ionization-dissociation front may develop and propagate into the clump. If the density is sufficiently 