Search for coronal temperature variation in Seyfert galaxies

Abstract

While the temperature of the X-ray corona (kTe) 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 kTe. 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 (Ecut) in a large fraction of the sources. Also, from Comptonized model fits we could obtain kTe for our sample. Of the 18 sources, at the 90 per cent confidence level, evidence for variation in kTe was found for only one source namely MCG + 08-11-011. For this source between two epochs, separated by about 5 yr, we found kTe 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 kTe in MCG + 08-11-011.