A New X-Ray Spectral Observation of NGC 1068
Abstract
The first moderate-resolution (FWHM ~ 100 eV) X-ray spectrum of a Seyfert 2 galaxy was obtained with the BBXRT experiment aboard Astro-1. The observed X-ray continuum of NGC 1068 from 0.3 to 10 keV is well fitted as the sum of two power-law spectra with no evidence for absorption intrinsic to the source. Strong Fe K emission lines with a total equivalent width of 2700 eV were detected due to iron less ionized than Fe XX and to iron more ionized than Fe XXIII. Unresolved Fe L lines with a total equivalent width of 340 eV were also seen. No evidence was seen for lines due to the recombination of highly ionized oxygen with an upper limit for the O Lyα emission line of 40 eV. The discovery of multiple Fe K and Fe L emission lines clearly indicates a broad range of ionization states for this gas. The X-ray emission from NGC 1068 is thought to be due to radiation from the obscured nucleus of the galaxy that is Compton-scattered by warm (T ~2 x 10^5^ K) electrons above the obscuring material. We associate the cooler, less ionized Fe with the warm electrons producing the polarized optical light, and the hotter (T ~ 4 x 10^6^ K), highly ionized Fe with a previously unseen component. The two components may share the same volume, or the hot component may be located closer to the nucleus. The X-ray emission from the two components is modeled for various geometries using a photoionization code that calculates the temperature and ionization state of the gas. We present models consistent with the X-ray, UV, and optical observations. Typical model parameters are a total Compton depth of a few percent, an inner boundary of the hot component of about 1 pc, and an inner boundary of the warm component of about 20 pc. All the successful models require that oxygen be under-abundant compared with solar by a factor of at least 5, while Fe is most likely overabundant. We show that the observed optical and UV line strengths from the NLR are consistent with these abundances.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- March 1993
- DOI:
- 10.1086/172350
- Bibcode:
- 1993ApJ...405..168M
- Keywords:
-
- Seyfert Galaxies;
- X Ray Astronomy;
- X Ray Spectra;
- Astronomical Models;
- Compton Effect;
- Line Spectra;
- Astrophysics;
- GALAXIES: INDIVIDUAL NGC NUMBER: NGC 1068;
- GALAXIES: SEYFERT;
- RADIATION MECHANISMS: MISCELLANEOUS;
- X-RAYS: GALAXIES