Further probing of the X-ray source in NGC 4151 - New constraints on the nuclear geometry

Details Further probing of the X-ray source in NGC 4151 - New constraints on the nuclear geometry Further probing of the X-ray source in NGC 4151 - New constraints on the nuclear geometry

May 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993mnras.262..435y&link_type=abstract

Monthly Notices of the Royal Astronomical Society (ISSN 0035-8711), vol. 262, no. 2, p. 435-448.

Astronomy and Astrophysics

Astronomy

90

Galactic Nuclei, Seyfert Galaxies, X Ray Sources, Accretion Disks, X Ray Spectra

Scientific paper

We report on the results of new Ginga observations of the archetypal Seyfert galaxy, NGC 4151, performed over an 18-month period between early 1990 and mid-1991. The present program complements earlier extensive monitoring of this source in the medium-energy X-ray band by both EXOSAT and Ginga to give a combined data set of some 36 separate observations made over a period of 8 years. The new observations bring to light a remarkable transient change in the low-energy extinction and soft X-ray excess apparent in the X-ray spectrum of NGC 4151. The spectrum became systematically softer below about 4 keV from mid-1988, with the spectral softness peaking in mid-1990 and then reverting to its former state by late 1990. Interpreting this in terms of the partial covering model, the covered fraction of the source, which normally takes a value of 80-90 percent, underwent a sharp decline to about 20 percent over a 2-year period and then returned to the canonical level over the following six months. This observation favors the presence of a normally stable 'bulk' structure such as an edge-on accretion disk. A transitory change of azimuthal structure in the disk could then account for the apparent dip in the covered fraction. If the central X-ray source radiates isotropically then the medium producing the bulk of the iron-line emission must deviate from a spherically symmetric distribution.

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