Variations of the X-ray INTEGRAL spectrum of the active galactic nucleus of NGC 4945

Abstract

Results of 11-year-long X-ray INTEGRAL observations of the nucleus of Seyfert galaxy NGC 4945 in the 3–500 keV range were processed. A two-component spectrum model, which includes strong radiation absorption in the Compton-thick torus around the AGN “central engine” and secondary radiation reflected from the torus walls, was used in the analysis. The following primary spectrum parameters were determined based on the data accumulated throughout the entire exposure period: photon index Γ = 1.60 ± 0.07, exponential cutoff energy E_c =157_-22⁺²⁹ keV, and column density of the medium that absorbs primary radiation N_H,1 =5.0_-0.9^+1.0 × 10²⁴ cm^–2. The column density of the medium absorbing reflected radiation is two orders of magnitude lower. Both the X-ray flux in the ranges of 20–40, 40–60, and 60–100 keV and the shape of the X-ray spectrum of NGC 4945 vary. The spectrum shape variations may be induced by inhomogeneities of the absorbing medium surrounding the AGN. At the same time, there is some evidence for moderate spectrum variations in the highenergy region, which may be associated with changes in the “central engine.”