Very Large Array monitoring of 1720 MHz OH masers toward the Galactic
  Center
  We present the first variability study of the 1720 MHz OH masers located in
the Galactic Center. Most of these masers are associated with the interaction
between the supernova remnant SgrAEast and the interstellar medium, but a few
masers are associated with the Circumnuclear Disk. The monitoring program
covered five epochs and a timescale of 20-195 days, during which no masers
disappeared and no new masers appeared. All masers have previously been
detected in a single epoch observation about one year prior to the start of the
monitoring experiment, implying relatively stable conditions for the 1720 MHz
OH masers. No extreme variability was detected. The masers associated with the
northeastern interaction region between the supernova remnant and the +50km/s
molecular cloud show the highest level of variability. This can be explained
with the +50km/s molecular cloud being located behind the supernova remnant and
with a region of high OH absorbing column density along the line of sight.
Possibly the supernova remnant provides additional turbulence to the gas in
this region, through which the maser emission must travel. The masers in the
southern interaction region are located on the outermost edge of SgrAEast which
line of sight is not covered by either absorbing OH gas or a supernova remnant,
in agreement with the much lower variability level observed. Similarly, the
masers associated with the CND show little variability, consistent with them
arising through collisions between relatively large clumps of gas in the CND
and no significant amount of turbulent gas along the line of sight.
