The Extraordinary Outburst in the Massive Protostellar System
  NGC6334I-MM1: Strong Increase in Mid-Infrared Continuum Emission
  In recent years, dramatic outbursts have been identified toward massive
protostars via infrared and millimeter dust continuum and molecular maser
emission. The longest lived outburst ($>6$ yr) persists in NGC6334I-MM1, a
deeply-embedded object with no near-IR counterpart. Using FORCAST and HAWC+ on
SOFIA, we have obtained the first mid-infrared images of this field since the
outburst began. Despite being undetected in pre-outburst ground-based 18 $\mu$m
images, MM1 is now the brightest region at all three wavelengths (25, 37, and
53 $\mu$m), exceeding the ultracompact HII region MM3 (NGC6334F). Combining the
SOFIA data with ALMA imaging at four wavelengths, we construct a spectral
energy distribution of the combination of MM1 and the nearby hot core MM2. The
best-fit Robitaille radiative transfer model yields a luminosity of
$(4.9\pm0.8)\times10^4 L_\odot$. Accounting for an estimated pre-outburst
luminosity ratio MM1:MM2 = $2.1\pm0.4$, the luminosity of MM1 has increased by
a factor of $16.3\pm4.4$. The pre-outburst luminosity implies a protostar of
mass 6.7 $M_\odot$, which can produce the ionizing photon rate required to
power the pre-outburst hypercompact HII region surrounding the likely
outbursting protostar MM1B. The total energy and duration of the outburst
exceed the S255IR-NIRS3 outburst by a factor of $\gtrsim3$, suggesting a
different scale of event involving expansion of the protostellar photosphere
(to $\gtrsim$ 20 $R_\odot$), thereby supporting a higher accretion rate
($\gtrsim$0.0023 $M_\odot$ yr$^{-1}$) and reducing the ionizing photon rate. In
the grid of hydrodynamic models of Meyer et al. 2021, the combination of
outburst luminosity and magnitude (3) places the NGC6334I-MM1 event in the
region of moderate total accretion ($\sim$0.1-0.3 $M_\odot$) and hence long
duration ($\sim$40-130 yr).
