Constraining Properties of AGN Coronae with NuSTAR: the Case of the Obscured Seyfert 1.9 Nucleus MCG -05-23-016

Robust measurements of the high-energy cut-off in the coronal continuum of AGN have long been limited to a small set of the brightest examples and almost exclusively to unobscured nuclei. We report on a direct measurement of the cut-off energy in the nuclear continuum of the obscured Seyfert 1.9 nucleus MCG-05-23-016 with unprecedented precision. The high sensitivity of NuSTAR in the hard X-ray band allows us to clearly disentangle the spectral curvature of the primary continuum from that of the reprocessed component. Using a simple phenomenological spectral model, we measured the cut-off energy to be 116+/-6 keV, while more complex Comptonization models provided independent constraints on the kinetic temperature of the electrons in the corona and its optical depth. Similar to a number of such measurements perfomed with NuSTAR in the past few years, and consistent with analyses of relatively large samples of hard X-ray spectra from the NuSTAR survey of nearby AGN, the optical depth was found to be of order unity for a range of assumed simple geometries. This means that the data are pushing the currently available models to the limits of their validity. In combination with the observations of spectral signatures from the innermost region of the accretion disk, and the observed variability of the high-energy cut-off, these results allow us to constrain the spatial extent of the AGN corona, its inhomogeneity and physical conditions needed to maintain its structure.