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6.11 57.16 $\mu $m SW2 Feature in the Fabry-Pérot Spectra

An unidentified feature has been observed on several sources (Orion, 30 Dor, G 298.228$-$0.331) at 57.16 $\mu $m, about 0.17 $\mu $m shortwards of the [N III] 57.33 $\mu $m line on detector SW2. This line cannot be real because it is not seen on detector SW3. It is believed to be related to the line asymmetry seen on the grating spectrum for the $57\, \mu m$ line on SW2 only, however the wavelength shift needed to make the asymmetry observed on the grating in the cases of 30 Dor and G 298.228$-$0.331 is higher (0.35 $\mu $m) than that observed on the FP spectrum. What causes this feature is not known.

ISO Handbook Volume III (LWS), Version 2.1, SAI/1999-057/Dc