The extraordinary outburst in the massive protostellar system
  NGC6334I-MM1: Flaring of the water masers in a north-south bipolar outflow
  driven by MM1B
  We compare multi-epoch sub-arcsecond VLA imaging of the 22 GHz water masers
toward the massive protocluster NGC6334I observed before and after the recent
outburst of MM1B in (sub)millimeter continuum. Since the outburst, the water
maser emission toward MM1 has substantially weakened. Simultaneously, the
strong water masers associated with the synchrotron continuum point source CM2
have flared by a mean factor of 6.5 (to 4.2 kJy) with highly-blueshifted
features (up to 70 km/s from LSR) becoming more prominent. The strongest
flaring water masers reside 3000 au north of MM1B and form a remarkable bow
shock pattern whose vertex coincides with CM2 and tail points back to MM1B.
Excited OH masers trace a secondary bow shock located ~120 au downstream. ALMA
images of CS (6-5) reveal a highly-collimated north-south structure
encompassing the flaring masers to the north and the non-flaring masers to the
south seen in projection toward the MM3-UCHII region. Proper motions of the
southern water masers over 5.3 years indicate a bulk projected motion of 117
km/s southward from MM1B with a dynamical time of 170 yr. We conclude that CM2,
the water masers, and many of the excited OH masers trace the interaction of
the high velocity bipolar outflow from MM1B with ambient molecular gas. The
previously-excavated outflow cavity has apparently allowed the radiative energy
of the current outburst to propagate freely until terminating at the northern
bow shock where it strengthened the masers. Additionally, water masers have
been detected toward MM7 for the first time, and a highly-collimated CS (6-5)
outflow has been detected toward MM4.
