Absorption at the dust sublimation radius and the dichotomy between X-ray and optical classification in the Seyfert galaxy H0557-385

TL;DR

This study analyzes 15 years of X-ray data of Seyfert galaxy H0557-385, revealing dramatic spectral variability caused by a clumpy, neutral absorber outside the broad line region, affecting optical and X-ray classifications.

Contribution

It provides the first detailed multi-epoch analysis linking X-ray spectral variability to a clumpy absorber at the dust sublimation radius in H0557-385.

Findings

Spectral states vary from unabsorbed to Compton-thin absorbed over ~5 years.

A clumpy neutral absorber with NH ~7x10^{23} cm^{-2} causes the variability.

Optical broad emission lines are detected during X-ray absorption events.

Abstract

In this work, the analysis of multi-epoch (1995-2010) X-ray observations of the Seyfert 1 galaxy H0557-385 is presented. The wealth of data presented in this analysis show that the source exhibits dramatic spectral variability, from a typical unabsorbed Seyfert 1 type spectrum to a Compton-thin absorbed state, on time scales of ~5 years. This extreme change in spectral shape can be attributed to variations in the column density and covering fraction of a neutral absorbing medium attenuating the emission from the central continuum source. Evidence for Compton reflection of the intrinsic nuclear emission is present in each of the spectra, though this feature is most prominent in the low-state spectra, where the associated Fe emission line complex is clearly visible. In addition to the variable absorbing medium, a warm absorber component has been detected in each spectral state. Optical spectroscopy concurrent with the 2010 XMM-Newton observation campaign have detected the presence of broad optical emission lines during an X-ray absorption event.From the analysis of both X-ray and optical spectroscopic data, it has been inferred that the X-ray spectral variability is a result of obscuration of the central emission region by a clumpy absorber covering >80 per cent of the source with an average column density of NH ~7x10^{23} cm^{-2}, and which is located outside the broad line region at a distance from the central source consistent with the dust sublimation radius of the AGN.