  We study the association between active galactic nuclei (AGN) and environment
at scales of $0.01-1\ h^{-1}$Mpc in the IllustrisTNG (TNG100) simulated
universe. We identify supermassive black hole (BH) pairs and multiples within
scales of 0.01, 0.1, & 1 $h^{-1}$Mpc and examine their AGN activity in relation
to randomly-selected pairs and multiples. The number density of BHs in TNG100
is $n=0.06\,h^3$Mpc$^{-3}$ at $z\lesssim1.5$ ($n=0.02\,h^3$ Mpc$^{-3}$ at
$z=3$). About $\sim10$% and $\sim1$% of them live in pairs and multiples,
respectively, within 0.1 $h^{-1}$Mpc scales. We find that BH systems have
enhanced likelihood (up to factors of 3-6) of containing high Eddington ratio
($\eta\gtrsim0.7$) AGN compared to random pairs and multiples. Conversely, the
likelihood of an AGN to live in 0.1$h^{-1}$Mpc scale systems is also higher (by
factors $\sim4$ for $\eta\gtrsim0.7$) compared to random pairs and multiples.
We also estimate that $\sim10$% of ultra-hard X-ray selected AGN in TNG100 have
detectable 2-10 keV AGN companions on $0.1\ h^{-1}$Mpc scales, in agreement
with observations. On larger spatial scales ($\sim 1$ $h^{-1}$Mpc), however, no
significant enhancement is associated with BH pairs and multiples, even at high
Eddington ratios. The enhancement of AGN activity in rich, small-scale
($\lesssim0.1$ $h^{-1}$Mpc) environments is therefore likely to be driven by
galaxy interactions and mergers. Nonetheless, the overall percentage of AGN
that live in $\lesssim0.1$ $h^{-1}$Mpc scale multiples is still subdominant (at
most $\sim40$% for the highest Eddington ratio AGN). Furthermore, the
enhancement in Eddington ratios of BH systems(as well as merging BHs) is only
up to factors of $\sim2-3$. Thus, our results support the existence of a
merger-AGN connection, but they also suggest that mergers and interactions play
a relatively minor role in fueling the AGN population as a whole.
