Strong variability of the coronal line region in NGC 5548

Hermine Landt, Martin J. Ward, Katrien C. Steenbrugge, Gary J. Ferland

Research output: Contribution to journalArticlepeer-review

15 Scopus citations

Abstract

We present the second extensive study of the coronal line variability in an active galaxy. Our data set for the well-studied Seyfert galaxy NGC 5548 consists of five epochs of quasisimultaneous optical and near-infrared spectroscopy spanning a period of about five years and three epochs of X-ray spectroscopy overlapping in time with it. Whereas the broad emission lines and hot dust emission varied only moderately, the coronal lines varied strongly. However, the observed high variability is mainly due to a flux decrease. Using the optical [Fe VII] and X-ray OVII emission lines we estimate that the coronal line gas has a relatively low density of ne ~ 103 cm-3 and a relatively high ionisation parameter of log U ~ 1. The resultant distance of the coronal line gas from the ionizing source of about eight light-years places this region well beyond the hot inner face of the dusty torus. These results imply that the coronal line region is an independent entity. We find again support for the X-ray heated wind scenario of Pier & Voit; the increased ionizing radiation that heats the dusty torus also increases the cooling efficiency of the coronal line gas, most likely due to a stronger adiabatic expansion. The much stronger coronal line variability of NGC 5548 relative to that of NGC 4151 can also be explained within this picture. NGC 5548 has much stronger coronal lines relative to the low-ionization lines than NGC 4151 indicating a stronger wind, in which case a stronger adiabatic expansion of the gas and so fading of the line emission is expected.

Original languageEnglish
Pages (from-to)3688-3696
Number of pages9
JournalMonthly Notices of the Royal Astronomical Society
Volume454
Issue number4
DOIs
StatePublished - Dec 21 2015

Bibliographical note

Publisher Copyright:
© 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Keywords

  • Galaxies: Seyfert
  • Infrared: Galaxies
  • Quasars: Emission lines
  • Quasars: Individual: NGC 5548
  • X-rays: Galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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