X-ray absorption lines in the Seyfert 1 galaxy NGC 5548 discovered with Chandra-LETGS

TL;DR

This paper presents the first high-resolution X-ray spectrum of the Seyfert galaxy NGC 5548, revealing narrow absorption lines from highly ionized species indicating an outflowing warm shell, with implications for understanding AGN outflows.

Contribution

First high-resolution X-ray spectrum of NGC 5548 showing detailed absorption features and modeling the outflowing medium as a photoionized shell.

Findings

Detection of blueshifted absorption lines from highly ionized species.

Absorbing medium modeled as an outflowing, warm, photoionized shell.

Emission lines consistent with the absorbing medium's properties.

Abstract

We present for the first time a high-resolution X-ray spectrum of a Seyfert galaxy. The Chandra-LETGS spectrum of NGC 5548 shows strong, narrow absorption lines from highly ionised species (the H-like and He-like ions of C, N, O, Ne, Na, Mg, Si, as well as Fe XIV--XXI). The lines are blueshifted by a few hundred km/s. The corresponding continuum absorption edges are weak or absent. The absorbing medium can be modelled by an outflowing, thin and warm shell in photoionization equilibrium. The absorption lines are similar to lower ionization absorption lines observed in the UV, although these UV lines originate from a different location or phase of the absorbing medium. Redshifted with respect to the absorption lines, emission from the O VIII Lyman alpha line as well as the O VII triplet is visible. The flux of these lines is consistent with emission from the absorbing medium. The O VII triplet intensity ratios demonstrate that photoionization dominates and yield an upper limit to the electron density.