  Over the last decade, the evidence is mounting that several aspects of black
hole accretion physics proceed in a mass-invariant way. One of the best
examples of this scaling is the empirical "Fundamental Plane of Black Hole
Accretion" relation linking mass, radio and X-ray luminosity over eight orders
of magnitude in black hole mass. The currently favored theoretical
interpretation of this relation is that the physics governing power output in
weakly accreting black holes depends more on relative accretion rate than on
mass. In order to test this theory, we explore whether a mass-invariant
approach can simultaneously explain the broadband spectral energy distributions
from two black holes at opposite ends of the mass scale but at similar
Eddington accretion fractions. We find that the same model, with the same value
of several fitted physical parameters expressed in mass-scaling units to
enforce self-similarity, can provide a good description of two datasets from
V404 Cyg and M81*, a stellar and supermassive black hole, respectively.
Furthermore, only one of several potential emission scenarios for the X-ray
band is successful, suggesting it is the dominant process driving the
Fundamental Plane relation at this accretion rate. This approach thus holds
promise for breaking current degeneracies in the interpretation of black hole
high-energy spectra, and for constructing better prescriptions of black hole
accretion for use in various local and cosmological feedback applications.
