Maser Flares Driven by Variations in Pumping and Background Radiation
  We simulate maser flares by varying either the pump rate or the background
level of radiation in a 3D model of a maser cloud. We investigate the effect of
different cloud shapes, saturation levels and viewpoints. Results are
considered for clouds with both uniform and internally variable unsaturated
inversion. Pumping and background variations are represented by several
different driving functions, some of which are light curves drawn from
observations. We summarise the pumping variability results in terms of three
observable parameters, the maximum flux density achieved, a variability index
and duty cycle. We demonstrate typical ranges of the flux density that may
result from viewing an aspherical object from random viewpoints. The best
object for a flare is a prolate cloud, viewed close to its long axis and driven
from unsaturated conditions to at least modest saturation. Results for
variation of the background level are qualitatively different from the variable
pumping results in that they tend to produce short intervals of low flux
density under conditions of moderate saturation and sufficient variability to
be consistent with strong flaring. Variable background models typically have a
significantly higher duty cycle than those with variable pumping.
