ar X iv : a st ro - p h / 02 03 02 1 v 1 2 M ar 2 00 2 1 SOFT X - RAY SPECTROSCOPY OF NGC 1068 WITH XMM - NEWTON RGS AND CHANDRA

We present high-resolution soft-X-ray spectra of the prototypical Seyfert 2 galaxy, NGC 1068, taken with the XMM-Newton Reflection Grating Spectrometer (RGS) and the Chandra Low Energy Transmission Grating Spectrometer (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 photoex-citation of resonant transitions also provides an important contribution to the spectrum. A self-consistent model of a photoionized and photoexcited cone of gas is capable of reproducing the hydrogenic/heliumlike ionic line series in detail. The ratio of photoexcitation to photoionization in the cone provides important geometric information such as the radial ionic column densities, which 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 500 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 ξ = L X /n e r 2 (over at least log ξ ∼ 0–3) is necessary to explain the inferred radial ionic column densities of all spectra. We show that this must primarily be due to a broad density distribution f (n e) ∝ n −1 e at each radius, spanning roughly n e ∼ 0.1– 100 cm −3. Any additional contribution to the soft-X-ray regime from hot, collisionally-ionized gas is negligible.