Discovery of a mid-infrared protostellar outburst of exceptional
  amplitude
  We report the discovery of a mid-infrared outburst in a Young Stellar Object
(YSO) with an amplitude close to 8 mag at $\lambda$$\approx$4.6 $\mu$m. WISEA
J142238.82-611553.7 is one of 23 highly variable WISE sources discovered in a
search of Infrared Dark Clouds (IRDCs). It lies within the small IRDC
G313.671-0.309 (d$\approx$2.6 kpc), seen by the Herschel/HiGal survey as a
compact, massive cloud core that may have been measurably warmed by the event.
Pre-outburst data from Spitzer in 2004 suggest that it is a class I YSO, a view
supported by observation of weak 2.12 $\mu$m H$_2$ emission in an otherwise
featureless red continuum spectrum taken in 2019 (6 mag below the peak in
K$_s$). Spitzer, WISE and VVV data indicate that the outburst began by 2006 and
has a duration $>$13 yr, with a fairly flat peak from 2010--2014. The outburst
luminosity of a few $\times 10^2$ Lsun is consistent with an accretion rate
Mdot $\approx 10^{-4}$ Msun/yr, comparable to a classical FU Orionis event. The
4.6 $\mu$m peak in 2010 implies T = 800-1000 K and a disc radial location
R$\approx$4.5 au for the emitting region. The colour evolution suggests
subsequent progression outward. The apparent absence of the hotter matter
expected in thermal instability or MRI models may be due to complete
obscuration of the innermost disc, e.g. by an edge-on disc view. Alternatively,
disc fragmentation/infalling fragment models might more naturally explain a
mid-infrared peak, though this is not yet clear.
