VARIABILITY OF INTERSTELLAR HYDROXYL MASERS

The spectra of 10 interstellar OH maser complexes were monitored in the 1665 MHz transition for variability on short (minutes-hours) and long (days-years) time scales. Eight of the sources exhibit short time scale variability in the relative amplitudes of the spectral features above that expected due to radiometer noise, and all sources for which multiepoch observations were made show variability on long time scales. Amplitudes of the short time scale fluctuations are typically 5%-10% of the peak intensity and are as large as 20%. Over long time scales, the fluctuations range from 0 to nearly 100% in amplitude. The OH emission spectrum from a main-line OH/IR star was also monitored. Long time scale variability is noted, distinct from that caused by pulsations of the stellar pumping source, while no short time scale variability (above 5%) is apparent. The origin of the short time scale fluctuations of the interstellar masers is not clear. Instrumental effects are unlikely. The variability is either propagation-induced (interstellar scintillation), or intrinsic to the masers. Scintillation requires angularly compact sources with exceedingly large brightness temperature approaching 10(24) K. Alternatively, the variations are the response of the masers to a series of pump disturbances, or fluctuations of the masing level populations about their steady-state values. Brightness temperatures of 10(10) - 10(15) K are implied, depending on the true nature of the maser light curves for which we have only a small time sample. The long time scale variability is likely an intrinsic property of the masers.