2022A&A...662A..78P

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 (kT_e). The launch of the Nuclear Spectroscopic Telescope Array (NuSTAR) has led to the determination of the high energy cutoff (E_cut), and thereby kT_e, in many AGN. In a handful of sources, multiple observations with NuSTAR have also revealed changes in E_cut.
Aims. In this work we aimed to investigate the variation in 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.
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 kT_e, if there were any, in these sources.
Results. In NGC 3227, we found evidence for variation in kT_e. We found no correlation of kT_e, photon index (Γ), reflection fraction (R), or optical depth (τ) with flux, while τ is found to anti-correlate with kT_e. This could be due to more than one physical process at work in the source causing the change in kT_e. Conclusive evidence for the variation in kT_e is not found in MR 2251-178 or NGC 5548.