EXCITED-STATE OH MASERS AT 6.035 GHZ

We have discovered 52 new masers emitting at the 6.035-GHz transition of excited OH, and have reobserved 20 of the 26 previously known masers of this type. The 72 masers are a complete sample of detections resulting from a search towards several hundred 1.665-GHz ground-state OH masers. The excited-state masers are generally weaker than the ground-state masers, but we find in six of the 72 sources that the peak intensity of excited-state emission is the stronger. Within the sample of 26 previously known masers, there were reports of large and rapid intensity variations. We find that only a few sources in our sample are variable on a 6-month time-scale, and in no case by more than 25 per cent. However, spectra of many sources discovered 20 years ago are now very different, with intensities commonly changed by factors of 2 or more. Many of the excited-state masers exhibit circular polarization indicative of Zeeman splitting. We have used these to measure the direction and intensity of the magnetic field implied by the Zeeman pairs of features. From all sources yet discovered, there appears to be an upper bound to the magnetic field intensities of 10 mG.