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Measurements of the high-energy cut-off in the coronal continuum of activegalactic nuclei have long been elusive for all but a small number of thebrightest examples. We present a direct measurement of the cut-off energy inthe nuclear continuum of the nearby Seyfert 1.9 galaxy MCG -05-23-016 withunprecedented precision. The high sensitivity of NuSTAR up to 79 keV allows usto clearly disentangle the spectral curvature of the primary continuum fromthat of its reflection component. Using a simple phenomenological model for thehard X-ray spectrum, we constrain the cut-off energy to $116_{-5}^{+6}$ keVwith 90% confidence. Testing for more complex models and nuisance parametersthat could potentially influence the measurement, we find that the cut-off isdetected robustly. We further use simple Comptonized plasma models to provideindependent constraints for both the kinetic temperature of the electrons inthe 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 sphericalgeometry. Both geometries are found to fit the data equally well and their twoprincipal physical parameters are correlated in both cases. With the opticaldepth in the optically thick regime, the data are pushing the currentlyavailable theoretical models of the Comptonized plasma to the limits of theirvalidity. Since the spectral features and variability arising from the inneraccretion disk have been observed previously in MCG -05-23-016, the inferredhigh optical depth implies that a spherical or disk-like corona cannot behomogeneous.

Coronal Properties of the Seyfert 1.9 Galaxy MCG -05-23-016 Determined from Hard X-ray Spectroscopy with NuSTAR