Long-term variability of Class I methanol masers in the high mass star
  forming region DR21(OH)
  High mass stars play an important role in the Interstellar Medium, but much
remains to be known about their formation. Class I methanol masers may be
unique tracers of an early stage of high mass star formation, and a better
understanding of such masers will allow them to be used as more effective
probes of the high mass star forming process. We present an investigation of
the long-term variability of Class I methanol masers at 44 GHz toward the high
mass star forming region DR21(OH). We compare observations taken in 2017 to
2012 and also to 2001 data from the literature. A total of 57 maser spots were
found in the 2017 data, with center velocities ranging between -8.65 km/s to
+2.56 km/s. The masers are arranged in a western and an eastern lobe with two
arcs in each lobe that look like bowshocks, consistent with previous
observations. The general trend is an increase in intensity from 2001 to 2012,
and a decrease from 2012 to 2017. Variability appears to be more prevalent in
the inner arc of the western lobe than in the outer arc. We speculate that this
may be a consequence of episodic accretion, in which a later accretion event
has resulted in ejection of material whose shock reached the inner arc at some
point in time after 2001. We conclude that class I methanol masers are variable
on long timescales (of the order of 5-10 years).
