Search for coronal temperature variation in Seyfert galaxies

Abstract

While the temperature of the X-ray corona ($\mathrm{k}{\mathrm{T}}_{\mathrm{e}}$) in active galactic nuclei (AGN) are 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 $\mathrm{k}{\mathrm{T}}_{\mathrm{e}}$. The data for a total of 52 epochs on these 18 AGN were taken from observations carried out by NuSTAR in the 3$-$79 keV energy band. From phenomenological and physical model fits to the multi-epoch data on these 18 sources from {\it NuSTAR}, we could constrain the cut-off energy ($E_{cut}$) in a large fraction of the sources. Also, from Comptonized model fits, we could obtain $\mathrm{k}{\mathrm{T}}_{\mathrm{e}}$ for our sample. Of the 18 sources, at the 90 per cent confidence level, evidence for variation in $\mathrm{k}{\mathrm{T}}_{\mathrm{e}}$ was found for only one source, namely MCG+08-11-011. For this source, between two epochs, separated by about five years, we found $\mathrm{k}{\mathrm{T}}_{\mathrm{e}}$ to decrease from 57${}_{-16}^{+29}$ keV to 30${}_{-7}^{+11}$ keV. During the same period, the flux decreased from (12.60 to 14.02) $\times$ 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 $\mathrm{k}{\mathrm{T}}_{\mathrm{e}}$ in MCG+08-11-011.

17 pages, 7 figuers, accepted for publication in MNRAS-MJ

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