A search for OH 6 GHz maser emission towards supernova remnants

OH masers at 1720 MHz have proven to be excellent indicators of interactions between supernova remnants and molecular clouds. OH-excitation calculations suggest that the 6049-MHz OH-maser line is excited for higher column densities than for the 1720-MHz line. Previous observations and modelling of 1612-, 1665- and 1667-MHz OH absorption and 1720-MHz OH masers indicated that the column densities in some supernova remnants, ∼10¹⁷ cm^-2, may be high enough for 6049-MHz OH masers to exist. It is therefore a potentially valuable indicator of remnant-cloud interaction. We present excitation calculations predicting the formation of 6049-MHz OH masers and results of a survey, using the Parkes Methanol Multibeam receiver for 6049-, 6035- and 6030-MHz OH masers toward 35 supernova remnants, a star-forming region and four fields in the Large and Small Magellanic Clouds. Two new sites of 6035- and 6030-MHz OH-maser emission associated with star-forming regions have been discovered, but no 6049-MHz masers were detected to a brightness temperature limit of ∼0.3-0.6 K, even though modelling of the OH excitation suggests that maser emission should have been detected. Our upper limits indicate the OH column density for a typical remnant ≲10^16.4 cm^-2, which conflicts with observed and modelled column densities. One possible explanation is that 6049-MHz OH masers may be more sensitive to velocity coherence than 1720-MHz OH masers under some conditions.