2010ApJ...713..423G

We present high angular resolution (θ_syn \lesssim 0.^′′2) observations of the 23.1 GHz methanol (CH₃OH) transition toward the massive star-forming region NGC 7538 IRS 1. The two velocity components previously reported by Wilson et al. are resolved into distinct spatial features with brightness temperatures (T_B) greater than 10⁴ K, proving their maser nature. Thus, NGC 7538 IRS 1 is the third region confirmed to show methanol maser emission at this frequency. The brighter 23.1 GHz spot coincides in position with a rare formaldehyde (H₂CO) maser, and marginally with a 22.2 GHz water (H₂O) maser, for which we report archival observations. The weaker CH₃ OH spot coincides with an H₂O maser. The ratio of T_B for the 23.1 GHz masers to that of the well-known 12.2 GHz CH₃ OH masers in this region roughly agrees with model predictions. However, the 23.1 GHz spots are offset in position from the CH₃ OH masers at other frequencies. This is difficult to interpret in terms of models that assume that all the masers arise from the same clumps, but it may result from turbulent conditions within the gas or rapid variations in the background radiation field.