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Deep Hα , N II, S II, and O III Schmidt images of CTB 1 reveal substantially more emission than previous images of this galactic supernova remnant (SNR). A complete =~ 35' diameter shell is observed except for a large gap in the northeast with a morphology suggestive of a shell rupture. Faint Hα and S II filaments and diffuse Hα emission are detected as far as 30' eastward from the shell rupture point. These optical emissions, together with coincident x-ray emission extending eastward from the SNR interior, suggest that the CTB 1 shock has broken out into a large H I cavity off CTB 1's northeastern limb like that proposed by Fich ApJ, 303, 465 (1986). CTB 1's optical morphology shows large variations when imaged in various emission lines. Particularly striking is the O III-bright western limb where spectra indicate relatively high O III/Hβ ratios (>= 25), implying shock velocities gtrsim 100 km s(-1) and large-scale, incomplete shock cooling possibly due to a strong interstellar magnetic field. Spectra taken along CTB 1's southeast and western rims indicate significant reddening variations, E(B-V) = 0.70\ to 1.0, consistent with the presence of several dust clouds visible around and in front of the SNR. Deep Hα and S II images of the neighboring SNR G116.5+1.1 reveal a 30' long filament along this remnant's NW rim, and spectra show S II/Hα = 1.02, confirming it as shock emission. An Hα image of a second nearby SNR, G114.3+0.3, shows several extremely faint filaments which may be associated with the SNR's radio-bright western limb.
Fesen et al. (Sat,) studied this question.