  High-mass protostars (M$_{\star} >$ 8 M$_{\odot}$) are thought to gain the
majority of their mass via short, intense bursts of growth. This episodic
accretion is thought to be facilitated by gravitationally unstable and
subsequently inhomogeneous accretion disks. Limitations of observational
capabilities, paired with a lack of observed accretion burst events has
withheld affirmative confirmation of the association between disk accretion,
instability and the accretion burst phenomenon in high-mass protostars.
Following its 2019 accretion burst, a heat-wave driven by a burst of radiation
propagated outward from the high-mass protostar G358.93-0.03-MM1. Six VLBI
(very long baseline interferometry) observations of the raditively pumped 6.7
GHz methanol maser were conducted during this period, tracing ever increasing
disk radii as the heat-wave propagated outward. Concatenating the VLBI maps
provided a sparsely sampled, milliarcsecond view of the spatio-kinematics of
the accretion disk covering a physical range of $\sim$ 50 - 900 AU. We term
this observational approach `heat-wave mapping'. We report the discovery of a
Keplerian accretion disk with a spatially resolved four-arm spiral pattern
around G358.93-0.03-MM1. This result positively implicates disk accretion and
spiral arm instabilities into the episodic accretion high-mass star formation
paradigm.
