The Rare 23.1-GHz Methanol Masers in NGC 7538 IRS 1
  We present high angular resolution (FWHM_beam < 0.2") observations of the
23.1-GHz methanol (CH_3OH) 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 10^4 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
(H_2CO) maser, and marginally with a 22.2-GHz water (H_2O) maser, for which we
report archival observations. The weaker CH_3OH spot coincides with an H_2O
maser. The ratio of T_B for the 23.1-GHz masers to that of the well-known
12.2-GHz CH_3OH masers in this region roughly agrees with model predictions.
However, the 23.1-GHz spots are offset in position from the CH_3OH 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.
