NuSTAR spectral analysis of two bright Seyfert 1 galaxies: MCG +8-11-11 and NGC 6814

Abstract

We report on the NuSTAR observations of two bright Seyfert 1 galaxies, namely MCG +8-11-11 (100 ks) and NGC 6814 (150 ks). The main goal of these observations was to investigate the Comptonization mechanisms acting in the innermost regions of AGN which are believed to be responsible for the UV/X-ray emission. The spectroscopic analysis of the NuSTAR spectra of these two sources revealed that although they had different properties overall (black hole masses, luminosity and Eddington ratios) they had very similar coronal properties. Both presented a power law spectrum with a high-energy cutoff at $\sim 150-200$ keV, a relativistically broadened Fe K$\alpha$ line and the associated disk reflection component, plus a narrow iron line likely emitted in Compton thin and distant matter. The intrinsic continuum was well described by Comptonization models that show for MCG +8-11-11 a temperature of the coronal plasma of kT$_{\rm e} \sim$ 60 keV and an extrapolated optical depth $\tau$=1.8; for NGC 6814 the coronal temperature was kT$_{\rm e} \sim$ 45 keV with an extrapolated optical depth of $\tau$=2.5. We compare and discuss these values to some most common Comptonization models which aim at explaining the energy production and stability of coronae in active galactic nuclei.