Variable X-ray absorption in NGC 4151.
Abstract
The Seyfert galaxy NGC 4151 was monitored over 20 times by EXOSAT in the period 1983 July-1986 March. The very soft spectral component detected by the EXOSAT low-energy telescope, which may be thermal emission from the narrow-line region of NGC 4151, remained constant over the observing period. In contrast, the medium-energy X-ray continuum emanating from the active nucleus of the galaxy varied by a factor of 10 and exhibited marked spectral changes, characterized by a tendency for the spectrum to soften as the X-ray flux increased. A spectral fitting analysis indicated, contrary to a published result, that the photon spectral index generally remained close to 1.55 in the EXOSAT observations and that the observed spectral variations were predominantly associated with changes in the complex absorbing column present in NGC 4151. We consider both `partial covering' and `warm absorber' models but argue that the latter provides the more satisfactory interpretation of the results. The derived value for the ionization parameter of the X-ray-absorbing gas is higher than is normally associated with the inner broad-line emitting clouds. The implications of these results for the structure of the active nucleus in NGC 1151 are discussed.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- January 1989
- DOI:
- 10.1093/mnras/236.2.153
- Bibcode:
- 1989MNRAS.236..153Y
- Keywords:
-
- Galactic Radiation;
- Seyfert Galaxies;
- X Ray Absorption;
- X Ray Sources;
- Active Galactic Nuclei;
- Exosat Satellite;
- Photoionization;
- X Ray Spectra;
- Astrophysics