  Type Ibn supernovae (SNe) are a rare class of stellar explosions whose
progenitor systems are not yet well determined. We present and analyze
observations of the Type Ibn SN 2019kbj, and model its light curve in order to
constrain its progenitor and explosion parameters. SN 2019kbj shows roughly
constant temperature during the first month after peak, indicating a power
source (likely circumstellar material interaction) that keeps the continuum
emission hot at ~15,000K. Indeed, we find that the radioactive decay of Ni56 is
disfavored as the sole power source of the bolometric light curve. A
radioactive decay + circumstellar-material (CSM) interaction model, on the
other hand, does reproduce the bolometric emission well. The fits prefer a
uniform-density CSM shell rather than CSM due to a steady mass-loss wind,
similar to what is seen in other Type Ibn SNe. The uniform-density CSM shell
model requires ~0.1 solar masses of Ni56 and ~1 solar mass of total ejecta to
reproduce the light curve. SN 2019kbj differs in this manner from another Type
Ibn SN with derived physical parameters, SN 2019uo, for which an order of
magnitude lower Ni56 mass and larger ejecta mass were derived. This points
towards a possible diversity in SN Ibn progenitor systems and explosions.
