Anatomy of the AGN in NGC 5548 IV. The short-term variability of the outflows

TL;DR

This study analyzes the short-term variability of outflows in NGC 5548 during a 2013-14 multiwavelength campaign, revealing changes in the obscurer and warm absorber linked to the source’s brightness and geometry.

Contribution

It provides the first detailed analysis of the variability of the obscurer and warm absorber in NGC 5548 over days to months, highlighting the impact of geometric and luminosity changes.

Findings

Obscurer varies in column density and covering fraction over days to months.

Spectral features of the warm absorber are detectable during high soft X-ray flux.

Warm absorber responds to changes in ionizing luminosity caused by the obscurer.

Abstract

During an extensive multiwavelength campaign that we performed in 2013-14 the prototypical Seyfert 1 galaxy NGC 5548 has been found in an unusual condition of heavy and persistent obscuration. The newly discovered "obscurer" absorbs most of the soft X-ray continuum along our line of sight and lowers the ionizing luminosity received by the classical warm absorber. Here we present the analysis of the high resolution X-ray spectra collected with XMM-Newton and Chandra throughout the campaign, which are suitable to investigate the variability of both the obscurer and the classical warm absorber. The time separation between these X-ray observations range from 2 days to 8 months. On these timescales the obscurer is variable both in column density and in covering fraction. This is consistent with the picture of a patchy wind. The most significant variation occurred in September 2013 when the source brightened for two weeks. A higher and steeper intrinsic continuum and a lower obscurer covering fraction are both required to explain the spectral shape during the flare. We suggest that a geometrical change of the soft X-ray source behind the obscurer cause the observed drop in the covering fraction. Due to the higher soft X-ray continuum level the September 2013 Chandra spectrum is the only X ray spectrum of the campaign where individual features of the warm absorber could be detected. The spectrum shows absorption from Fe-UTA, \ion{O}{iv}, and \ion{O}{v}, consistent to belong to the lower-ionization counterpart of the historical NGC 5548 warm absorber. Hence, we confirm that the warm absorber has responded to the drop in the ionizing luminosity caused by the obscurer.