  The NuSTAR observatory, with its high sensitivity in hard X-rays, has enabled
detailed broadband modeling of the X-ray spectra of Active Galactic Nuclei
(AGN), thereby allowing constraints to be placed on the high-energy cutoff of
the X-ray coronal continuum. We investigate the spectral properties of a sample
of 46 NuSTAR-observed Seyfert 1 AGN selected from the Swift/BAT 70-month hard
X-ray survey. Our measurements of the high-energy cutoff of the continuum from
modeling the NuSTAR X-ray spectra are used to map out the temperature -
compactness ($\theta-l$) plane for AGN coronae. We find that most of the
coronae lie clustered near the boundary for runaway pair production, suggesting
that annihilation and pair production act to regulate the temperature of the
corona. We discuss the implications of coronae whose high-energy cutoff may
indicate a low coronal temperature on the heating and thermalization mechanisms
in the corona.
