Coronal Properties of the Seyfert 1.9 Galaxy MCG -05-23-016 Determined
  from Hard X-ray Spectroscopy with NuSTAR
  Measurements of the high-energy cut-off in the coronal continuum of active
galactic nuclei have long been elusive for all but a small number of the
brightest examples. We present a direct measurement of the cut-off energy in
the nuclear continuum of the nearby Seyfert 1.9 galaxy MCG -05-23-016 with
unprecedented precision. The high sensitivity of NuSTAR up to 79 keV allows us
to clearly disentangle the spectral curvature of the primary continuum from
that of its reflection component. Using a simple phenomenological model for the
hard X-ray spectrum, we constrain the cut-off energy to $116_{-5}^{+6}$ keV
with 90% confidence. Testing for more complex models and nuisance parameters
that could potentially influence the measurement, we find that the cut-off is
detected robustly. We further use simple Comptonized plasma models to provide
independent constraints for both the kinetic temperature of the electrons in
the corona and its optical depth. At the 90% confidence level, we find
$kT_e=29\pm2$ keV and $\tau_e=1.23\pm0.08$ assuming a slab (disk-like)
geometry, and $kT_e=25\pm2$ keV and $\tau_e=3.5\pm0.2$ assuming a spherical
geometry. Both geometries are found to fit the data equally well and their two
principal physical parameters are correlated in both cases. With the optical
depth in the optically thick regime, the data are pushing the currently
available theoretical models of the Comptonized plasma to the limits of their
validity. Since the spectral features and variability arising from the inner
accretion disk have been observed previously in MCG -05-23-016, the inferred
high optical depth implies that a spherical or disk-like corona cannot be
homogeneous.
