NuSTARspectral analysis of three Seyfert galaxies: NGC 3227, NGC 5548, and MR 2251−178

Abstract

Context.The observed nuclear X-ray emission in the radio-quiet category of active galactic nuclei (AGN) is believed to be from a compact region, the corona, which is situated in the vicinity of central supermassive black holes. The shape of the X-ray continuum depends on, among other factors, the temperature of the corona (kTe). The launch of the Nuclear Spectroscopic Telescope Array (NuSTAR) has led to the determination of the high energy cutoff (Ecut), and therebykTe, in many AGN. In a handful of sources, multiple observations withNuSTARhave also revealed changes inEcut.Aims.In this work we aimed to investigate the variation inkTein three AGN, namely NGC 3227, NGC 5548, and MR 2251−178, using more than one epoch of data on a source fromNuSTAR.Methods.We carried out a spectral analysis of multiple epochs of data acquired usingNuSTARon the three sources, including a few new observations that had not yet been published. By fitting a Comptonization model to the data, we determined the temperature of the corona and investigated changes inkTe, if there were any, in these sources.Results.In NGC 3227, we found evidence for variation inkTe. We found no correlation ofkTe, photon index (Γ), reflection fraction (R), or optical depth (τ) with flux, whileτis found to anti-correlate withkTe. This could be due to more than one physical process at work in the source causing the change inkTe. Conclusive evidence for the variation inkTeis not found in MR 2251−178 or NGC 5548.