Study of X-ray variability and coronae of Seyfert galaxies using NuSTAR

The primary X-ray emission in active galactic nuclei (AGNs) originates in a compact region called the corona located very close to the supermassive black hole and the accretion disc. The knowledge of the cut-off energy (E_cut) of the primary X-ray continuum in an AGN is very important as it carries information on the physical characteristics of the hot X-ray emitting corona. We present here the results of our investigation on the spectral properties of a sample of 10 nearby AGN (0.005 < z < 0.037) observed with the Nuclear Spectroscopic Telescope Array (NuSTAR). From fitting of the NuSTAR data of 10 sources, we derived clear E_cut values for the first time in eight sources and a lower limit in one source, thereby, doubling the number of AGN with E_cut measurements from NuSTAR data. Broad Fe K α line was noticed in seven sources, while excess emission in the energy range beyond ∼15 keV arising from Compton reflection was seen in all the sources. We also investigated the correlation of E_cut with various physical characteristics of the AGN such as black hole mass (M_BH), Eddington ratio (λ_Edd.), and X-ray photon index (Γ). We found no correlation between E_cut and M_BH and between E_cut and λ_Edd., however, E_cut correlates with Γ in a complex manner. Also, timing analysis of the 10 sources indicates that they all are variable with indications of more variations in the soft band relative to the hard band in some individual sources, however, considering all the sources together, the variations are indistinguishable between hard and soft bands.