NuSTAR spectral 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 (E꜀ᵤₜ), and thereby kTₑ, in many AGN. In a handful of sources, multiple observations with NuSTAR have also revealed changes in E꜀ᵤₜ. Aims. In this work we aimed to investigate the variation in kTₑ in three AGN, namely NGC 3227, NGC 5548, and MR 2251−178, using more than one epoch of data on a source from NuSTAR. Methods. We carried out a spectral analysis of multiple epochs of data acquired using NuSTAR on 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 in kTₑ, if there were any, in these sources. Results. In NGC 3227, we found evidence for variation in kTₑ. We found no correlation of kTₑ, photon index (Γ), reflection fraction (R), or optical depth (τ) with flux, while τ is found to anti-correlate with kTₑ. This could be due to more than one physical process at work in the source causing the change in kTe. Conclusive evidence for the variation in kTₑ is not found in MR 2251−178 or NGC 5548.

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