temperature, e.g., a factor of 2 variation in temperature causes 10–20% in density. We adopt the average value $6,600\pm 900$ cm$^{-3}$ at $T=5,000$ K as the characteristic electron density of the filaments.

We also detected line toward -pk1. The dereddened ratio of 164 to line is $77^{+14}_{-10}$, which is much greater than that ($\simlt 0.1$) of HII regions but comparable to the ratios observed in other SNRs (see § 4.2).

### Proper Motion during 2003–2005

We have two 164 images taken in 2.2 years apart, i.e., in 2003 June and 2005 August. The time interval is not long enough to notice the proper motion of the filaments in the difference image obtained by subtracting one from the other. We instead inspect one-dimensional intensity profiles of the bright -SE filament to search for its proper motion associated with an expansion.

Fig. 7 shows the intensity profiles across the two bright segments of the -SE filament along the cuts (dashed lines) in Fig. 3. The cuts are made to point to the central pulsar which is very close to the geometrical center of the SNR shell [@kas01]. The distance in the abscissa is measured from the upper right end of the cuts, 