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

TL;DR

This study analyzes NuSTAR X-ray data of three Seyfert galaxies to investigate changes in the coronal electron temperature, revealing variability in NGC 3227 but not conclusively in the others, enhancing understanding of AGN coronae.

Contribution

It provides the first multi-epoch spectral analysis of these three AGN using NuSTAR data, focusing on coronal temperature variations and their correlations with other spectral parameters.

Findings

Evidence of $ m{kT_{e}}$ variation in NGC 3227.

No clear $ m{kT_{e}}$ variation in MR 2251$-$178 and NGC 5548.

Anti-correlation between optical depth $ au$ and $ m{kT_{e}}$.

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 ($\rm{kT_{e}}$). The launch of the Nuclear Spectroscopic Telescope Array (NuSTAR) has led to the determination of the high energy cut-off ($\rm{E_{cut}}$; and thereby $\rm{kT_{e}}$) in many AGN. In a handful of sources, multiple observations with NuSTAR have also revealed changes in $\rm{E_{cut}}$. In this work we aimed to investigate the variation in $\rm{kT_{e}}$ 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 $\rm{kT_{e}}$ if any in these sources. In NGC 3227, we found evidence for variation in $\rm{kT_{e}}$. We found no correlation of $\rm{kT_{e}}$, photon index ($\Gamma$), reflection fraction ($R$) and optical depth ($\tau$) with flux, while, $\tau$ is found to anti-correlate with $\rm{kT_{e}}$. These could be due to more than one physical process at work in the source that causes the change in $\rm{kT_{e}}$. Conclusive evidence for the variation in $\rm{kT_{e}}$ is not found in MR 2251$-$178 and NGC 5548.