Correlation between relativistic reflection fraction and photon index in NuSTAR sample of Seyfert 1 AGN

Abstract

The primary X-ray emission from AGN, described by a power law, irradiates the accretion disc producing reflection features in the spectrum. The reflection features arising from the inner regions of the disc can be significantly modified by the relativistic effects near the black hole. We investigate the relationship between the relativistic reflection fraction $R_f$, defined as the ratio of the coronal intensity that illuminates the accretion disc to the coronal intensity observed directly, and the hard X-ray photon index $\Gamma$ of a NuSTAR sample of Seyfert 1 galaxies. The X-ray spectra are modelled using relxill code which helps to directly obtain the reflection fraction of a relativistically smeared reflection component. The parameter $R_f$ depends on the amount of Comptonised X-ray emission intercepted by the inner accretion disc. We found a positive correlation between $\Gamma$ and $R_f$ in our sample. Seed photons from a larger area of an accretion disc entering the corona will result in increased cooling of the coronal plasma, giving rise to steeper X-ray spectrum. The corona irradiating the larger area of the disc will result in higher reflection fraction. Thus, the observed $R_f-\Gamma$ relation is most likely related to the variations in the disc-corona geometry of AGN.

14 pages, 7 figures, 6 tables. Accepted for publication in MNRAS

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