Periodic class II methanol masers in G9.62+0.20E
  We present the light curves of the 6.7 and 12.2 GHz methanol masers in the
star forming region G9.62+0.20E for a time span of more than 2600 days. The
earlier reported period of 244 days is confirmed. The results of monitoring the
107 GHz methanol maser for two flares are also presented. The results show that
flaring occurs in all three masing transitions. It is shown that the average
flare profiles of the three masing transitions are similar. The 12.2 GHz masers
are the most variable of the three masers with the largest relative amplitude
having a value of 2.4. The flux densities for the different masing transitions
are found to return to the same level during the low phase of the masers,
suggesting that the source of the periodic flaring is situated outside the
masing region, and that the physical conditions in the masing region are
relatively stable. On the basis of the shape of the light curve we excluded
stellar pulsations as the underlying mechanism for the periodicity. It is
argued that a colliding wind binary can account for the observed periodicity
and provide a mechanism to qualitatively explain periodicity in the seed photon
flux and/or the pumping radiation field. It is also argued that the dust
cooling time is too short to explain the decay time of about 100 days of the
maser flare. A further analysis has shown that for the intervals from days 48
to 66 and from days 67 to 135 the decay of the maser light curve can be
interpreted as due to the recombination of a thermal hydrogen plasma with
densities of approximately $1.6 \times 10^6 \mathrm{cm^{-3}}$ and $6.0 \times
10^5 \mathrm{cm^{-3}}$ respectively.
