Search for coronal temperature variation in Seyfert galaxies

While the temperature of the X-ray corona (kT_e) in active galactic nuclei (AGNs) is known for many sources, its variation if any is limited to a handful of objects. This is in part due to the requirement of good signal-to-noise X-ray spectra, covering a wide range of energies. We present here results on the X-ray spectral analysis of 18 Seyferts, having more than one epoch of observations to look for variation in kT_e. The data for a total of 52 epochs on these 18 AGNs were taken from observations carried out by NuSTAR in the 3-79 keV energy band. From phenomenological and physical model fits to the multiepoch data on these 18 sources from NuSTAR, we could constrain the cut-off energy (E_cut) in a large fraction of the sources. Also, from Comptonized model fits we could obtain kT_e for our sample. Of the 18 sources, at the 90 per cent confidence level, evidence for variation in kT_e was found for only one source namely MCG + 08-11-011. For this source between two epochs, separated by about 5 yr, we found kT_e to decrease from 57$^{+29}_{-16}$ to 30$^{+11}_{-7}$ keV. During the same period, the flux decreased from (12.60 to 14.02) × 10^-11 erg cm^-2 s^-1 and the optical depth increased from 1.68 to 2.73. We thus found a positive correlation between flux and coronal temperature with a reduction of about 40 per cent in optical depth. Our observations tend to favour the vertically outflowing corona scenario for the observed variation in kT_e in MCG + 08-11-011.