NuSTAR spectral analysis of three Seyfert galaxies: NGC 3227, NGC 5548 and MR 2251$-$178

Abstract

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 situated in the vicinity of the central supermassive black holes (SMBH). The shape of the X-ray continuum, among other factors, depends on the temperature of the corona ($\mathrm{k}{\mathrm{T}}_{\mathrm{e}}$). The launch of the Nuclear Spectroscopic Telescope Array (NuSTAR) has led to the determination of the high energy cut-off (${\mathrm{E}}_{\mathrm{cut}}$; and thereby $\mathrm{k}{\mathrm{T}}_{\mathrm{e}}$) in many AGN. In a handful of sources, multiple observations with NuSTAR have also revealed changes in ${\mathrm{E}}_{\mathrm{cut}}$. In this work we aimed to investigate the variation in $\mathrm{k}{\mathrm{T}}_{\mathrm{e}}$ in three AGN, namely NGC 3227, NGC 5548 and MR 2251$-$178 using more than one epoch of data on a source from NuSTAR. We carried out spectral analysis of multiple epochs of data acquired using NuSTAR on the three sources including a few new observations not published so far. By fitting Comptonization model to the data we determined the temperature of the corona and also investigated changes in $\mathrm{k}{\mathrm{T}}_{\mathrm{e}}$ if any in these sources. In NGC 3227, we found evidence for variation in $\mathrm{k}{\mathrm{T}}_{\mathrm{e}}$. We found no correlation of $\mathrm{k}{\mathrm{T}}_{\mathrm{e}}$, photon index ($\Gamma$), reflection fraction ($R$) and optical depth ($\tau$) with flux, while, $\tau$ is found to anti-correlate with $\mathrm{k}{\mathrm{T}}_{\mathrm{e}}$. These could be due to more than one physical process at work in the source that causes the change in $\mathrm{k}{\mathrm{T}}_{\mathrm{e}}$. Conclusive evidence for the variation in $\mathrm{k}{\mathrm{T}}_{\mathrm{e}}$ is not found in MR 2251$-$178 and NGC 5548.

18 pages, 18 figures, Accepted for publication in Astronomy & Astrophysics (A&A)

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