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

Abstract

The primary X-ray emission in active galactic nuclei (AGN) originates in a compact region called the corona located very close to the super-massive black hole and the accretion disk. 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 {\it Nuclear Spectroscopic Telescope Array (NuSTAR)} . From fitting of the {\it NuSTAR} data of 10 sources, we derived clear $E_{cut}$ values for the first time in 8 sources and a lower limit in one source, thereby, doubling the number of AGN with $E_{cut}$ measurements from {\it NuSTAR} data. Broad Fe K$\alpha$ line was noticed in 7 sources, while, excess emission in the energy range beyond $\sim$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 ($\lambda_{Edd.}$) and X-ray photon index ($\Gamma$). We found no correlation between $E_{cut}$ and $M_{BH}$ and between $E_{cut}$ and $\lambda_{Edd.}$, however, $E_{cut}$ correlates with $\Gamma$ 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.