Constraining the complexities in Seyfert X-ray spectra – an analysis of simultaneous observations with Ginga and ROSAT

Abstract

Absorption and reprocessing of X-ray photons in the nuclei of active galaxies have recently been found to alter significantly the appearance of the emergent spectrum. The limited bandwidth and resolution of most contemporary X-ray spectral data often leave open differing interpretations of the nature and magnitude of these effects and, consequently, the precise form of the underlying continuum spectrum. A unique opportunity to examine, over some two decades in energy, the X-ray spectra of five bright Seyfert galaxies, based on simultaneous observations with the Ginga and ROSAT satellites, has now provided data to clarify the form of this reprocessing. In every case, the underlying continuum has a power-law form, with a preferred photon index consistent with the revised canonical value of 1.9 α 0.1. A somewhat surprising result, however, is the range and variety of spectral features superimposed on this continuum. For NGC 4051, the dominant feature is absorption from a substantial column density of ionized gas in the line of sight to the continuum source. Division of these data into low-and high-flux states confirms spectral vari ability for NGC 4051, with an indication of a flux-linked change in the ionization parameter and the emergence, in the high state, of a significant soft emission feature. Mrk 841 and 509 both exhibit features attributable to reflection from cold matter, plus a soft excess. In addition, Mrk 509 has a spectral feature near ˜ 1 keV that is possibly due to FeL emission. Finally, for NGC 7213 and Fairall-9, which both show Fe K emission at an unusually high (rest) energy and a soft excess, a good overall spectral fit is obtained with an ionized-disc model.