Soft X-ray Spectroscopy of NGC 1068 with XMM-Newton RGS and Chandra LETGS

TL;DR

This study uses high-resolution X-ray spectra from XMM-Newton and Chandra to analyze the warm, photoionized gas in NGC 1068, revealing its large spatial extent and complex density distribution.

Contribution

It provides a detailed, self-consistent model of the warm, photoionized gas in NGC 1068, showing its large scale and variable density distribution, and links emission to the warm absorber in Seyfert galaxies.

Findings

Emission originates from a large (~300 pc) warm absorber.

Radial ionic column densities match Seyfert 1 absorption data.

A broad density distribution explains spectral differences.

Abstract

We present high-resolution soft-X-ray spectra of the prototypical Seyfert 2 galaxy, NGC 1068, taken with XMM-Newton RGS and Chandra LETGS. Its rich emission-line spectrum is dominated by recombination in a warm plasma (bright, narrow radiative recombination continua provide the ``smoking gun''), which is photoionized by the inferred nuclear power-law continuum. Radiative decay following photoexcitation of resonant transitions is also significant. A self-consistent model of an irradiated cone of gas is capable of reproducing the hydrogenic/heliumlike ionic line series in detail. The radial ionic column densities we infer are consistent with absorption measurements (the warm absorber) in Seyfert 1 galaxies. This strongly suggests that the emission spectrum we observe from NGC 1068 emanates from its warm absorber. The observed extent of the ionization-cone/warm absorber in NGC 1068 of about 300 pc implies that a large fraction of the gas associated with generic warm absorbers may typically exist on the hundreds-of-parsec scale rather than much closer to the nucleus (e.g., less than a parsec). Spatially-resolved spectroscopy using the LETGS of two distinct emission regions yields two noticeably different spectra. We show that these differences are solely due to differing radial column densities. A fairly flat distribution in ionization parameter is necessary to explain the inferred radial ionic column densities of all spectra. This must primarily be due to a broad density distribution at each radius, spanning roughly 0.1-100 cm$^{-3}$. (Abridged)