10-Year Transformation of the Obscuring Wind in NGC 5548

TL;DR

Over a decade, the obscuring wind in NGC 5548 has shown significant variability, with decreased X-ray obscuration and UV absorption linked to changes in the accretion disk's outflowing streams, revealing the dynamic nature of AGN winds.

Contribution

This study provides the first long-term relation between X-ray and UV covering fractions of the disk wind in NGC 5548, demonstrating the wind's variability over ten years.

Findings

X-ray obscuration decreased to its lowest in 2022.

UV broad and narrow absorption lines nearly vanished in 2022.

Strong correlation between X-ray hardening and UV absorption strength.

Abstract

A decade ago the archetypal Seyfert-1 galaxy NGC 5548 was discovered to have undergone major spectral changes. The soft X-ray flux had dropped by a factor of 30 while new broad and blueshifted UV absorption lines appeared. This was explained by the emergence of a new obscuring wind from the accretion disk. Here we report on the striking long-term variability of the obscuring disk wind in NGC 5548 including new observations taken in 2021-2022 with the Swift Observatory and the Hubble Space Telescope's (HST) Cosmic Origins Spectrograph (COS). The X-ray spectral hardening as a result of obscuration has declined over the years, reaching its lowest in 2022 at which point we find the broad C IV UV absorption line to be nearly vanished. The associated narrow low-ionization UV absorption lines, produced previously when shielded from the X-rays, are also remarkably diminished in 2022. We find a highly significant correlation between the variabilities of the X-ray hardening and the equivalent width of the broad C IV absorption line, demonstrating that X-ray obscuration is inherently linked to disk winds. We derive for the first time a relation between the X-ray and UV covering fractions of the obscuring wind using its long-term evolution. The diminished X-ray obscuration and UV absorption are likely caused by an increasingly intermittent supply of outflowing streams from the accretion disk. This results in growing gaps and interstices in the clumpy disk wind, thereby reducing its covering fractions.