6.7 GHz variability characteristics of new periodic methanol maser
  sources
  Discovery of periodic maser emission was an unexpected result from monitoring
observations of methanol transitions in high-mass young stellar objects. We
report on the detection of five new periodic sources from a monitoring program
with the Torun 32 m telescope. Variability with a period of 149 to 540 d and
different patterns from sinusoidal-like to intermittent was displayed. Three
dimensional structure of G59.633$-$0.192 determined from the time delays of
burst peaks of the spectral features and high angular resolution map implies
that the emission traces a disk. For this source the 6.7 GHz light curve
followed the infrared variability supporting a radiative scheme of pumping. An
unusual time delay of $\sim$80 d occurred in G30.400$-$0.296 could not be
explained by the light travel time and may suggest a strong differentiation of
physical conditions and excitation in this deeply embedded source. Our
observations suggest the intermittent variability may present a simple response
of maser medium to the underlying variability induced by the accretion
luminosity while other variability patterns may reflect more complex changes in
the physical conditions.
