X-ray eclipse tomography: Resolving the extent of X-ray-emitting region(s) in the Seyfert galaxy ESO362-G18

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Original abstract

X-ray spectral variability in active galactic nuclei (AGNs) can be either intrinsic or caused by external processes, such as variable absorption. Several transient occultation events from individual clouds or X-ray eclipses have been identified in a number of AGNs in the past two decades. We report results from the analysis of two Swift monitoring campaigns of the Seyfert galaxy ESO 362-G18, a changing-look AGN that showed transitions between optical spectral types 1.5 and 1.9 in the past, most likely induced by variable absorption. We identified two X-ray eclipses during the first Swift campaign, one of which we were able to follow almost entirely from ingress to egress. We used time-resolved spectroscopy to follow the evolution of the X-ray spectrum with time in order to derive the properties of the absorbing cloud and the size of the main X-ray-emitting region. The cloud is located towards the innermost dust sublimation zone, intermediate between the dust-free broad-line region and the dusty torus. The X-ray-emitting region is confined within 35 gravitational radii (Rg) from the centre. A deeper analysis indicated that the eclipsing cloud likely comprises a denser core and a more tenuous atmosphere. We were also able to estimate the size of the soft excess and hot corona X-ray-emitting regions separately. These are two of the most relevant continuum components in the X-ray spectra of unobscured AGNs. Our analysis suggests that the soft-excess-emitting region is 50% more extended than the hot corona one. However, it appears to co-exist with and is not replaced by the hot corona one in the innermost 30 Rg. Our results highlight the potential of X-ray eclipse tomography for providing a clearer view of the innermost accretion flow geometry and of the AGN surroundings out to the torus scale.

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