A hard look at the X-ray spectral variability of NGC 7582
Inset figure caption: simulated spectra with the best-fitting physical model for the first two slices of the observation of 2016 where short-term variability is observed. The exposure time of each slice is 5 ks. Thanks to the large collecting area of Athena and the high resolution of X-IFU, it will be possible to resolve the 6.4 keV Fe line and constrain the geometry of the absorber in a 5 ks.
By Mehdy Lefkir
Active galactic nuclei (AGN) are luminous sources at the centre of galaxies. Powered by the accretion of matter onto a supermassive black hole, these sources often exhibit variability. In this work, we study the long and short-term X-ray spectral variability of NGC 7582, a well-studied Seyfert-2 galaxy. We use archival observations from XMM-Newton and NuSTAR taken between 2001 and 2016. NGC 7582 exhibits long-term variability between observations and short-term variability in one simultaneous observation. We study the short-term variability by slicing the simultaneous observation in seven slices of duration 15 ks. We find that the variability is consistent with intrinsic variations in the central X-ray source and changes in the absorber located in the broad-line region. We confirm that the absorber is clumpy with possibly various sizes for the clouds or a gradient of density in the clouds.
Thanks to its large effective area, Athena will allow studies of spectral variability on shorter time scales with improved constraints. The high spectral resolution brought by the X-ray Integral Field Unit (X-IFU) will enable distinguishing between possible geometries for the absorber, including better constraints on the density of clouds. In addition, future studies of the short-term variability could help determine the shape of the clouds, for instance, if they have spherical or comet-like structures. Finally, longer exposures will enable similar studies of the variability for low-luminosity obscured AGN.